Full Paper(1) Graduate Engineer, Connell Wagner, Adelaide, South Australia. (2) Senior Associate, Connell Wagner, Adelaide, South Australia.
ABSTRACT
An environmental impact assessment was conducted for a Biomass Power Station to be situated near a rural town-ship. The proposed facility is to consist of a plantation waste fuelled bubbling fluidized bed boiler with emissions control achieved through an array of baghouse fabric filters to generate 40 MW of power.. This paper will describe the motivation and sustainability characteristics of this renewable energy infrastructure development and the veracity of the use of TAPM version 3 and version 4 (The Air Pollution Model, prognostic numerical model developed by the CSIRO) generated meteorological data to assist in the prediction of noise propagation using standard models that account for meteorological effects on outdoor noise propogation.
(1) Day Design Pty Ltd, Peakhurst, NSW (2) Benbow Environmental, Parramatta, NSW
ABSTRACT
This paper discusses the directional properties of sound emitted from a large inclined metal-clad roof surface.
An opportunity was recently presented to measure the sound emission from a particularly large ribbed galvanised steel roof deck with a very loud broadband noise below from many noise sources. In 37 years of practice, the author has not been presented with a better opportunity to quantify the directivity of sound from a flat roof surface.
Measurements were taken at 0.5 metres above the roof and at distances of 10, 50 and 150 metres from the lower edge of the roof. The sound power level of the roof was established from the close proximity measurements and simple surface area calculations. The level of noise emission from the roof was calculated at three noise measurement
locations to determine the horizontal "directivity" of the roof noise emission.
Its directivity was found to be related to the energy vector in the direction of sound propagation.
University of Adelaide
ABSTRACT
Theory will tell us that it is 0, but measurements by the Australian Standard AS1191-2002 give different results! What is the sound absorption of an open window? Theory will tell us that it is unity, but measurements by the Australian Standard AS ISO 354-2006 give different results! This paper is about two "silly" measurements, but I think it uncovers deficiencies in our standards and measurement techniques, and in our application of the theoretical properties of sound in enclosures and sound transmission.
(1)The University of Adelaide, School of Mechanical Engineering, Adelaide SA5005 (2)BASSETT, Level 6, 100 Pirie St, Adelaide SA5000
ABSTRACT
Wind induced noise is a problem that affects most outdoors acoustic measurement campaigns. The interaction between the local wind and turbulence with the fixed surfaces of the microphone generates non-acoustic pressure fluctuations at the diaphragm that significantly affect the microphone output. Various types of wind shields are used to overcome this problem, but the benefit of this measure is generally not well quantified. This issue is particularly relevant in the context wind farm assessments, where the dependency of ambient noise as a function of the local wind speed is of primary importance when determining the noise criteria and undertaking the compliance noise measurements at the site. This paper presents the results of wind generated noise testing for a range of commercially available wind shields. It is demonstrated that the principal variable in wind screen performance is the diameter of the wind-screen.
Parsons Brinckerhoff Australia Pty Ltd
ABSTRACT
Acoustic design for new industrial plant must often consider a combination of design goals, achieving not only environmental and safety noise level targets, but must also contribute to the project's value management efforts and aim for the best possible sustainability outcome. These requirements are often contradictory and sometimes mutually exclusive, yet the acoustic designer must attempt to balance the overall design with the best possible compromise. A method is proposed which provides acoustic designers with a systematic procedure for adopting the most appropriate noise control strategy solution within the design parameters.
(1) Bassett Acoustics, Sydney, NSW 2000, Australia (2) Wilson, Ihrig & Associates, Oakland, CA 94618, USA
ABSTRACT
A vibration isolated rail trackform, or Floating Slab Track (FST) was designed using empirical, numerical and experimental techniques. This paper focuses on the correlation between modal testing and Finite Element Analysis (FEA) of the FST. The modeshapes of the FST in the frequency range of 5 Hz to 250 Hz are discussed and the effects of adding the unsprung mass of a rail vehicle are explored within the framework of the FE method. Vibration levels measured at a slab during a train pass-by are presented. Characteristics of the pass-by spectra are explained with the developed model.
(1) gfai tech, Rudower Chaussee 30, 12489 Berlin, Germany (2) GFaI, Society for the promotion of applied computer science, Rudower Chaussee 30, 12489 Berlin, Germany (3) GFaI, Society for the promotion of applied computer science, Rudower Chaussee 30, 12489 Berlin, Germany
ABSTRACT
Traditional beam forming systems with planar microphone arrays do not give comprehensive information about the sound sources inside a cavity such as car interior or an office room. Acoustic measurements inside any cavity are mostly conducted with a few number of microphones. By this means it is possible to gain information about frequencies, orders, sound pressures. However, a space selective analysis is nearly impossible and it is not feasible to find multiple sound sources positions in space in a practical way. Therefore, the constituents of the Acoustic Camera of the GFaI were extended by a spherical, acoustically transparent and omni directional array. A new option is to map onto a common 3D-CAD-models of the object of interest, for instance an office or class room. First of all the advantages and disadvantages of 2D- and 3D-mappings will be discussed in the paper. Furthermore, the important issue of positioning an array at first in the coordinate system of the 3D-model and second in the actual cavity which is to be measured will be addressed. The paper discusses the geometric and acoustic properties of microphone arrays which are applicable for complete 3D-measurements and mappings of cavities. A practicable way of determining the array's position and direction related to the measurement object will be proposed. Several practical experiments will be displayed to underline the methods capability and reliability.
School of Applied Sciences, RMIT University, GPO Box 2476V Melbourne, Victoria 3163, Australia CSIRO Materials Science and Engineering, PO Box 56 Highett Victoria 3190, Australia
ABSTRACT
A theoretical method has been developed for predicting the directivity of the sound that is radiated from one side of a panel, or an opening, which is excited by sound incident from the other side of the panel, or the opening, from a room or duct. This directivity needs to be known when one is predicting the sound level at an external position which is due to the radiation of sound from the roof, wall, ventilating duct or chimney flue of a factory. The theoretical method is essentially a two dimensional method, although it does include some three dimensional considerations. This paper compares this theoretical method with published experimental data. The theory presented in this paper agrees with the average trend of the experimental measurements. However the experimental results show significant variability about the theoretical predictions. This is believed to be due to both experimental and theoretical difficulties.
Faculty of Architecture, Design and Planning, University of Sydney, NSW 2006
ABSTRACT
Source directivity has the potential to affect speech transmission index (STI) measurements. When the source is a model of a human talker, there is a question of how accurate that model needs to be. However, in many acoustic environments source directivity has little effect on STI because other factors dominate. One instance is soundfields in which the direct sound is dominant (e.g., in the quiet outdoors, in the nearfield, or in an anechoic room): directivity will have no effect on STI so long as the direct sound is strong enough. Another instance is soundfields in which the reverberant soundfield is dominant (e.g., in the far field in a room with moderate or more reverberation). This paper examines theoretical situations where source directivity has a substantial influence on STI because of the balance between direct (or early) and reverberant soundfields, as well as the role that background noise can have in increasing the importance of source directivity.
(1) Building and Engineering Services, Jurong International, Singapore (2) School of Mechanical and Manufacturing Engineering, The University of New South Wales, Sydney, Australia
ABSTRACT
Wind-induced noise often contaminates noise measurements in windy environments despite the use of wind-shields. Techniques have been developed by various researchers to reduce the influences of background wind induced micro-phone noises during measurement. However these techniques have not yet yielded accurate results. An attempt was undertaken in this study to improve on existing design methods and understand the mechanisms associated with wind noise measurements. This study aims to lay the foundation for further experiments to be conducted to investigate the performance and characteristics of wind noise in a measuring microphone. Qualitative study of the design and construction of a suitable experimental rig was carried out to investigate the effects and characteristics of wind noise on a measuring microphone in a controlled environment.
Faculty of Architecture, Design and Planning, University of Sydney, NSW 2006, Australia
ABSTRACT
The acoustic environment of an auditorium stage can enhance the ability of musicians or actors to hear each other clearly, and this is assessed through a set of parameters called ‘stage support'. Stage support parameters are conventionally measured using an omnidirectional source and microphone separated by 1 m (with an elevation of 1.2 -1.5 m) on the stage. Various stage support parameters are derived from energy ratios of the direct and reflected (or reverberant) sound in the impulse response from source to microphone. The present paper examines the possibility of assessing the directional characteristics of the reflected soundfield by using a multi-directional microphone. Measurements were made in the Sydney Theatre Company's ‘Sydney Theatre' with and without a stage set. The microphone used allows first order spherical harmonic decomposition of the soundfield, which was transformed to cardioid components for orthogonal directions (upstage, downstage, left, right, up and down). Results demonstrate the potential of this simple extension of standard stage support measurement by showing the acoustical effect of the set.
(1) Technology & Marketing, Bruel & Kjaer Sound & Vibration Measurement A/S, Naerum, Denmark (2) Technology & Marketing, Bruel & Kjaer Sound & Vibration Measurement A/S, Naerum, Denmark (3) Technology & Marketing, Bruel & Kjaer Sound & Vibration Measurement A/S, Naerum, Denmark (4) Sales Area South ASPAC, Bruel & Kjaer Australia, Melbourne, Australia
ABSTRACT
Mapping techniques with respect to Noise Source Identification have been with us for many years. They have been based on techniques such as Intensity Mapping, Near-field Acoustical Holography, (NAH), and Beamforming. They typically produce results that are mapped over flat planes. An obvious improvement would be to be able to map over a conformal surface corresponding to that of the measurement object, to give a much better visualisation of the noise sources. A method investigated for this has been Inverse Boundary Element Modelling, (IBEM), but the practical application of this technique has proved complex. This paper describes an alternative method for conformal mapping based on NAH, which proves to be much simpler to apply. The method is made possible through the SONAH algorithm that allows the use of a small, hand-held microphone array, and positional detectors in the array that allows the use of so-called "patch" holography. The method is illustrated with measurements on a household vacuum cleaner.
Faculty of Architecture, Design and Planning, University of Sydney, NSW 2006, Australia
ABSTRACT
Room impulse responses can be analysed to yield several room acoustical parameters, such as reverberation time, early decay time, clarity index and strength factor. Despite the considerable noise immunity of modern impulse response measurement techniques (such as swept sinusoids), it is common for room impulse responses to have sufficient noise to have some effect on the derived room acoustical parameters. The effect of noise is typically seen by a change in the slope of the late part of the decay function (derived from Schroeder's reverse integration method), that can be detected using a series of short term correlation coefficients over the course of the decay function. By describing a room acoustical parameter error with a correlation coefficient value, a mathematical model can be established to correct errors from a calculated correlation coefficient over the 20 or 30 dB decay range used to calculate reverberation time. Such a technique is ideally suited for room soundfields that exhibit exponential decay, and needs to be used carefully in real room level decay functions. The method is evaluated using real room impulse responses with artificially introduced noise at various levels.
School of Mechanical and Manufacturing Engineering, The University of New South Wales (UNSW), Sydney, NSW, 2052, Australia
ABSTRACT
The structural and acoustic responses of a submarine hull to fluctuating propeller forces are investigated in the low frequency range. The fluctuating forces are caused by operation of the propeller in a non-uniform wake, and are transmitted to the submarine hull through the shaft and also via the fluid. Numerical models have been developed to simulate the strongly coupled fluid-structure interaction of the submerged vessel in the frequency domain. The hull is modelled using the finite element method (FEM), which allows for structural complexities such as the ring-stiffeners, bulkheads, end closures and the propeller/shafting system. A simple passive vibration attenuation system known as a resonance changer (RC) is included in the model of the propeller/shafting system. The fluid is modelled using the boundary element method (BEM) in which the radiation damping and added mass effect of the water are taken into account. It is shown that the performance of the RC is influenced greatly by the effects of forces transmitted to the hull via the fluid. Propeller vibration is taken into account in determination of the fluid and structural excitation of the hull. The effect of changing the propeller mass is observed.
(1) Student of Architecture Design and Planning, University of Sydney, Sydney, Australia (2) Department of Architecture Design and Planning, University of Sydney, Sydney, Australia (3) Department of Architecture Design and Planning, University of Sydney, Sydney, Australia
ABSTRACT
An expansion chamber functions as a silencer due to a change of acoustic impedance at each end of the silencer. A common disadvantage of this type of silencer is that the air flow is impeded, which results in a pressure drop between inlet and outlet. Porous absorptive material can improve the acoustic performance of an expansion chamber silencer (making it function as a combined dissipative and reactive silencer), but has limited practical applications (due to potential clogging of the absorber with particles, and entrainment of absorber fibbers in the airflow) and does not solve the problem of pressure loss. This study examines the performance of a lined expansion chamber that has a thin membrane arranged so that the free air flow cross section does not change. The advantage of this arrangement is that there is little pressure drop compared to the simple expansion chamber, and acoustic performance is similar to that of the chamber with porous absorber, but without the associated problems. It also allows the use of more effective linings such as graded density ones which are easily damaged by airflows.
Maritime Platforms Division, Defence Science and Technology Organisation, 506 Lorimer Street, Fishermans Bend, Victoria 3207, Australia
ABSTRACT
A theoretical analysis of active control of low-frequency radiated noise from a pressure hull due to a fluctuating propeller load is presented. The model consists of a large water-loaded cylinder, where one end is excited by an axial force due to the fluctuating propeller load while the other end is free. The active control system is implemented through a circumferential line moment applied to a ring stiffener, which could be achieved with a T-sectioned circumferential stiffener driven by pairs of PZT stack actuators. These actuators are located under the flange of the stiffener and driven out of phase to produce a control moment. The effect of the feedback of the active control action on the axial displacement of the propeller is investigated. In general, it was found that the control system was capable of reducing more than 85% of the total radiated pressure from the pressure hull at the first three axial ("concertina") modes.
Bassett Acoustics, part of AECOM, Melbourne, Australia
ABSTRACT
This paper presents some of Bassett Acoustics' recent experiences with acoustic design implications associated with current building services initiatives for sustainable buildings. Acoustic design practices are explored with respect to various sustainable heating, ventilating and air conditioning (HVAC) methods, with comparisons between the impact on the acoustic design of a building for sustainable and traditional HVAC systems presented. A case study of two re-cent sustainable buildings projects with alternative HVAC methods is presented, in which alternative acoustic design strategies and practices are required for each of the different sustainable HVAC methods that are implemented.
of Architecture, Design and Planning, University of Sydney, NSW 2006, Australia
ABSTRACT
Room impulse responses (RIRs) are used to characterise the acoustical conditions inside sound-critical rooms such as auditoria. The analysis of RIRs typically involves octave-band analysis, with parameters such as reverberation time, early decay time, temporal energy ratios and spatial parameters derived from this. This paper explores the potential for applying auditory models for the analysis of RIRs - incorporating auditory temporal integration (and masking), auditory filterbank analysis, and loudness calculation. The purpose of this is to produce analysis results that are closely related to the sound experienced by listeners. A preliminary step for such analysis is to filter RIRs so that their power spectrum is similar to that of typical material that would be listened to in the rooms (e.g. music or speech), and this paper proposes a music filter suitable for orchestral music, derived from long term power spectra of anechoic music recordings. Dynamic loudness analysis of RIRs yield a loudness decay functions that are approximately exponential, which should provide a useful analogy with conventional analysis methods applied to RIRs.
School of Applied Sciences, RMIT University, GPO Box 2476V Melbourne, Victoria 3163, Australia CSIRO Materials Science and Engineering, PO Box 56 Highett Victoria 3190, Australia
ABSTRACT
A theoretical method has been developed for predicting the directivity of the sound that is radiated from one side of a panel, or an opening, which is excited by sound incident from the other side of the panel, or the opening, from a room or duct. This directivity needs to be known when one is predicting the sound level at an external position which is due to the radiation of sound from the roof, wall, ventilating duct or chimney flue of a factory. The theoretical method is essentially a two dimensional method, although it does include some three dimensional considerations. This paper compares this theoretical method with published experimental data. The theory presented in this paper agrees with the average trend of the experimental measurements. However the experimental results show significant variability about the theoretical predictions. This is believed to be due to both experimental and theoretical difficulties.
(1) Principal, Sinclair Knight Merz Pty Ltd, Sydney, Australia (2) Senior Consultant, Sinclair Knight Merz Pty Ltd, Sydney, Australia
ABSTRACT
Minimising the impact of music practice and "Garage Band" performances upon neighbours in a residential area is challenging in terms of noise emissions from musical instruments, but particularly so when drums and percussion instruments are involved. Normal residential building facades and roofing designs offer limited low frequency noise attenuation and domestic building construction methods can severely compromise the performance of seemingly adequate partition construction details.
This paper presents the results of design, construction and testing activities for a private drum studio that was required to meet stringent boundary noise emission targets in order to comply with local council Development Application requirements. High transmission loss light-weight partition test data is provided for the as-built final installation, along with details of cavity absorption, panel damping and vibration isolation treatments that contributed to maximising façade sound reduction performance. A range of room internal absorption treatments, including low frequency "tube traps", corner traps and diffusers were successfully employed to achieve compliance with BBC recommended reverberation times for small recording studios.
Marshall Day Acoustics Pty Ltd, Melbourne, Australia
ABSTRACT
Acoustic assessment methods for wind turbines and wind farms in rural areas of Australia have grown to be well established in recent years. Guidance documents such as New Zealand Standard NZS6808 and Environmental Noise Guidelines: Wind Farms published by the South Australian EPA provide clear guidance on how to assess noise from wind turbines and the noise limits which should apply to them. But these guidance documents have generally been developed for application to rural areas where the turbines are located several hundred metres, if not kilometres away from the nearest residential dwelling. The assessment of wind turbines in urban and city environments involves several factors which are absent in the rural context such as a higher background noise level, potentially more variability of background noise level with time of day and day of the week, closer proximity of residential dwellings and the potential for residential dwellings to share common structural elements with the wind turbine. Given these factors, this paper investigates the suitability of existing wind turbine noise guidance documents and other applicable guidance documents.
(1) Acoustics and Vibration Unit University of NSW at Australian Defence Force Academy Canberra, Australia (2) Bassett Acoustics, part of AECOM, Adelaide, South Australia
ABSTRACT
There is a high demand in Australia for new staff for acoustical consulting companies, as evidenced by the "positions available" listings. Companies would ideally like to hire staff with a good engineering, physics or building background plus an understanding of acoustics. Programs of study at University may deal with the principles of acoustics and vibration but there is usually a need for supplementary education and training for new staff in practical applications of those principles. In this paper we will discuss the development of the requirements for sustainable acoustics education in Australia. In particular we will explain the development and implementation of the flexible Professional Education in Acoustics program aimed at providing for ongoing acoustics education. This program is based upon the UK Institute of Acoustics Diploma program and designed to meet the criteria identified by the profession. This paper will provide the basis for further discussions during a workshop on acoustics education in Australia.
Parsons Brinckerhoff Australia Pty Ltd
ABSTRACT
When the acoustic amenity of a region changes, either by the addition or modification of a transportation noise source, the overall community noise impact depends on the number of affected receivers as well as the change in noise level at each receiver. This overall impact could be quantified as an arithmetic average of the change in noise level at the affected receiver. However, a simple arithmetic average does not adequately consider the relative significance of the change at the receivers which experience substantial increase or decrease of noise level. For instance, a change of 2 dB(A) is usually regarded as being a barely perceptible change for environmental noise and a change of 10 dB(A) is usually regarded as approximately doubling or halving the subjective sound loudness. A method is pro-posed whereby a change in noise amenity is quantified in terms of the significance of the change in noise level as a function of the number of affected receivers and the severity to which they are affected. The proposed method is particularly useful in cases where the noise impact from several alternative proposed transportation route options need to be compared as a single-number rating in terms of overall noise impact upon the community. The result would be useful for input into a Multi-Criteria Decision Matrix [MCDM]. Two methods are presented for comparing the over-all community noise impact from alternative transportation route options.
Department of Applied Physics, Faculty of Arts and Sciences, University of Sharjah PO Box: 27272 Sharjah, United Arab Emirates. Email: hmelmehdi@sharjah.ac.ae
ABSTRACT
According to ACI traffic statistics for 2002, Dubai International Airport is the second fastest growing airport in the world. In 2007, it catered to more than 36 million passengers, with approximetly 1000 daily flight events, including take off and landing. In this paper we report the first assessment of community annoyance caused by civil aircraft noise exposure in 9 sites around Dubai International Airport (United Arab Emirates). Our aim is to assess the relationship between aircraft noise levels and annoyance responses in Dubai. To accomplish our goal, we have adapted the WECPNL and Ldn as the aircraft noise indices in Dubai, and the percentage of %HA have been used to assess the dose-response of aircraft noise. The %HA was obtained using social surveys that were carried out in 9 sites within close proximity of the airport using the ISO/TS 15666, 2003 questionnaire. Subjects were randomly selected ranging between 20 and 65 years in age. Results show that 41% of the respondents said that they are Highly Annoyed; a percentage considered high, but in agreement with similar studies done near major airports in other countries.
Noise and Sound Services, St Ives, NSW 2075, Australia.
ABSTRACT
Whether they are small independent stores or larger multi faceted complexes, the potential for noise impacts from supermarket developments is considerable. A thorough assessment of all potential noise sources associated with the development should be undertaken. Ideally such an assessment should be conducted at the design stage to assist in minimising the noise impact on the surrounding community, as far as reasonably practicable. The aim of this paper is to discuss the various issues encountered when assessing proposed supermarket and associated developments, the variety of noise sources and mitigation measures.
(1) Environmental Acoustics Team, Renzo Tonin & Associates Pty Ltd, Australia (2) Major Projects Acoustics Team, Renzo Tonin & Associates Pty Ltd, Australia (3) Environmental Acoustics Team, Renzo Tonin & Associates Pty Ltd, Australia (4) Environmental Acoustics Team, Renzo Tonin & Associates Pty Ltd, Australia
ABSTRACT
The use of Cadna-A is widely accepted in Europe as a tool for predicting noise from various types of sources, including traffic noise. However, traffic noise modeling using Cadna-A is still in the early stages of acceptance in Australia and as such the appropriateness and accuracy of Cadna-A for Australian conditions is currently being verified. Unlike Cadna-A, the SoundPLAN noise prediction model is extensively used in Australia, particularly for road traffic noise predictions, and has been recognised and accepted nationally by various regulatory authorities including the major road authorities and environmental agencies.
The aim of this paper is to compare predicted traffic noise levels using the Calculation of Road Traffic Noise (CoRTN) algorithms as implemented by SoundPLAN and the Cadna-A noise models for a proposed arterial road and an existing arterial road. A validation of both noise modeling packages is also conducted based on actual measurements of traffic noise from an existing arterial road and compared to one another.
Results from this study show that the Cadna-A noise modeling software package is as accurate and effective as the SoundPLAN model in modeling arterial road traffic noise.
Marshall Day Acoustics Pty Ltd, Adelaide, Australia
ABSTRACT
Calculating the attenuation provided by barriers and other screening elements is an essential part of practical noise control engineering. There are well-documented approaches to this problem, with noise propagation standards often specifying which approach to take. A noticeable exception is CONCAWE: the principal prediction method used in Australia. CONCAWE provides complex calculation methods for ground and meteorological effects, but not for barrier attenuation. The sound prediction software SoundPLAN uses the barrier attenuation algorithm from the General Prediction Method (GPM) in its implementation of CONCAWE. In Victoria, noise assessments are performed under neutral meteorological conditions, but the GPM assumes downwind propagation conditions. SoundPLAN therefore can under-predict the performance of the barrier. While predicting noise propagation under favourable conditions has merit, this paper discusses several barrier-attenuation algorithms and their suitability for modelling noise propagation in neutral meteorological conditions.
(1) Acoustics, Environmental Management Group Australia, 77 Marion Street., Harris Park, NSW, 2150 Australia (2) Social Consulting, Environmental Resources Management Australia Pty Ltd, 33 Saunders St., Pyrmont, NSW, 2009 Australia
ABSTRACT
Acoustic criteria work on the premise that there are acceptable levels of sound in particular areas and industries. These limits are enshrined in our regulatory codes providing the basis for monitoring and assessing acceptable sound generated by industry, transport, commercial development and entertainment events for example. Such limits are typically annoyance based criteria, derived from community expectations determined through social research and surveys. As our cities increase in density noise is increasingly becoming one of the key factors in how people measure quality of life. Determining acceptable noise limits is changing as the focus of noise intensifies. Increasingly how we manage noise is expected to consider the balance between regulatory limits and the views and experience of people in localities surrounding sites where industry or events generate high levels of sound. While this is not entirely new it does require that we collect and consider additional data from affected communities as well as sound levels generated. Using project case studies this paper explores how our approach to Noise Management Planning is changing to enable us to consider the social context of noise management planning as part of the technical assessment. Identifying the increasing requirement that noise management strategies meet public demand for greater input, transparency in monitoring and recording and in some cases review of regulation. The case studies presented demonstrate the effectiveness of various forms of noise management techniques based on measured noise levels, assessment against goals and importantly the community's response.
(1) Acoustic Engineer, Connell Wagner, Sydney, Australia (2) Senior Associate, Connell Wagner, Adelaide, Australia
ABSTRACT
The South Australian EPA "Wind Farm Noise Guidelines" (2003) have been used extensively in Australia, and was based on the New Zealand Standard NZS 6808-1998 "Acoustics-The Assessment and Measurement of Sound from Wind Turbine Generators" which in turn was based on the report ETSU-97-R "The Assessment and Rating of Noise from Wind Farms" by the United Kingdom's Energy Technology Support Unit. The New Zealand Wind Energy Association (NZWEA) and Energy Efficiency and Conservation Authority (EECA) jointly commissioned an unofficial review of NZS 6808 given recent research findings, while the draft Australian Standard DR 07153 CP "Acoustics-Measurement, prediction and assessment of noise from wind turbine generators" has been in preparation for the past few years. Both standards attempt to account for the effect of atmospheric stability on the wind speed profile with height (commonly referred to the van de Berg effect). This paper describes a unique method to accurately assess the effects of atmospheric stability on the definition of noise criteria (established from noise logging) and the prediction of noise levels from wind turbines.
Palmer Acoustics (Aust) Pty Ltd, Park Ridge, QLD. Australia
ABSTRACT
In the desire to control noise, regulators are setting their sights on a form of quasi inaudibility, "Background Plus". If an acoustic environment is reduced to a single variable and the disturbance to that environment is controlled to be indiscernibly louder then noise nuisance will be controlled. There is a certainty and a proving about the calculation and the result. The approach works for both the regulators and the acoustician seeking to 'get a project up' within tight time constraints and desperately seeking to avoid an information request and a 20 day extension to an evaluation period. Gone are the discussions, queries and amendments about what measured values may actually mean and what effect the new variable will have on the environment. This paper sets out to explore where this universal criteria works and where it doesn't and the vast continuum between the 'certainties'.
Science & Sustainability Division, Environment Protection Authority of SA, GPO Box 2670 Adelaide SA 5001, Australia
ABSTRACT
Many governmental strategic plans encourage the use of renewable energy sources. Responding to industry needs, the Environment Protection Authority of SA introduced Wind Farms - Environmental Noise Guidelines (Interim) 2007. The core objective of the Guidelines is to balance developments in the wind farms sector with the level of amenity expected by the surrounding community. Situations where the existing background level is comparable, or exceeds, wind farm noise are common. The submitted paper aims to consider a few methods that might be incorporated into the Guidelines as a compliance checking procedure and can be used to provide an adequate measure of the wind turbines noise contribution against background and other sources within a vary quiet rural environment. The methods do not require employment of complex techniques, special instruments or advanced data post-processing. International and national practice and results of wind farm noise monitoring are taken into account to develop the compliance checking procedures.
Day Design Pty Ltd, Peakhurst, NSW
ABSTRACT
This paper discusses the directional properties of sound emitted from the open end of a ventilation duct. When de-signing a duct silencer to reduce noise from a vertical discharge duct, it is useful to note that the first 10 to 15 dBA noise reduction may result from directivity losses at 90 degrees and can be accurately predicted. On two occasions in 1971, sound directivity tests were conducted by the author with 300 and 600 mm diameter ducts and the results made into a rough chart of Duct Directivity Losses that ultimately found its way into the EPA Environmental Noise Control Manual (5 June 1985, page 207.1).
Over the last 13 years further sound directivity testing has been undertaken on ducts of 305, 400, 610, 915 and 1220 mm diameter and re-analysed to produced a more useful Duct Directivity Chart. The directivity data has been related to the sound power level of noise emitted from the duct and the spherical dispersion of sound energy. While Strouhal numbers have been used to correlate the test data, the Duct Directivity Chart allows the directivity gain or loss to be obtained for any diameter from 100 mm to 10 metres, at angles from zero to 135 degrees without the need for complex calculations.
Energy Research Group, School of Science and Engineering, The University of Waikato, Private Bag 3105, Hamilton 3240, New Zealand.
ABSTRACT
With increasing focus on climate change and greenhouse gas emissions there is renewed interest in minimising inefficiencies in compressed gas systems that result from leaks. Airborne ultrasound has been used for more than 30 years to locate such leaks, however traditional methods have had limited success in accurately quantifying the actual leak rate for a given leak and therefore the actual true cost. This paper will outline the in-depth study used to characterise a range of compressed gas leaks using air as the fluid medium across a range of pressures, leak types, shape and size.
(1) Vipac Engineers and Scientists Ltd., PO Box 2419, Kent Town, SA 5071, Australia (2) School of Mechanical Engineering, The University of Adelaide, SA 5000, Australia
ABSTRACT
The transfer matrix method has been extensively explored by many researchers for analysing acoustic duct systems. However, there does not exist a comprehensive source that elaborates the use of transfer matrix method for evaluating the performance of acoustic filters. The current approach describes a detailed step-by-step method for evaluating in-duct net acoustic power transmission for a harmonic plane wave travelling in a circular duct with an attached cylindrical Helmholtz resonator using the transfer matrix method. The net acoustic power transmission is evaluated using two different methods: (1) the product of acoustic pressure and acoustic volume velocity at the duct exit, and (2) estimates of the in-duct sound field using the two-microphone technique.
(1) School of Mechanical Engineering, University of Adelaide, SA, Australia (2) Polytec GmbH, Polytec-Platz 1-7, 76337 Waldbronn, Germany
ABSTRACT
The recent advent of three-dimensional scanning laser vibrometers has enabled extremely accurate non-contact measurement of the three-dimensional displacements of structures. This paper looks at the feasibility of using a scanning laser vibrometer for the non-contact measurement of dynamic strain fields across the surface of a planar structure. Is-sues such as laser head alignment and choice of finite-difference scheme are discussed. Finally, experimental results of a test specimen are presented which clearly demonstrate the significant potential of this new experimental technique.
(1) Director, Wilkinson Murray Pty Ltd, Sydney, Australia. (2) Former Manager Noise Strategies, NSW Dept. of Environment and Climate Change, Australia.
ABSTRACT
Since Roman times, there have been continual attempts to mitigate the impacts of noise, and it is the assessment, understanding, communication and management of noise impacts that has provided the challenge for all involved in the process of reducing the impacts of noise. It is however, the communication of noise impacts to those who are, or potentially will be impacted that is often given the least consideration and is done in an ad hoc and inconsistent way.
The following paper examines the key aspects of noise communication and attempts to provide a summary of current practice and suggest strategies for improving the communication of information on noise so that it achieves the best outcomes. There is an examination of the statement by Paul Schomer (2005) that while good public relations may result in a 5 dB bonus (US EPA, 1974) the converse is also true, with bad public relations equivalent to a 5 dB penalty. The authors also discuss the merit of adopting multiple assessment criteria and introduce the concept of a risk based approach to communicating the noise impact message.
Music Acoustics, School of Physics, University of New South Wales, Australia
ABSTRACT
To play the glissando opening Gershwin's Rhapsody in Blue, expert clarinettists combine unusual fingerings with even more unusual vocal tract configurations to achieve a nearly continuous rise in pitch. To study directly the player's vocal tract when performing the glissando, we incorporated an acoustic impedance measurement head within a clarinet mouthpiece. The player's vocal tract impedance spectra were measured and compared with the corresponding clarinet impedance spectra for the fingering used at that pitch. Partially uncovering an open finger-hole raises the frequency of clarinet impedance peaks, allowing smooth increases in resonance frequency of the downstream bore. In the clarinet's second register, however, upstream resonances in the player's vocal tract are manipulated to be comparable in magnitude with those in the clarinet. Thus, by skillfully coordinating their fingers and simultaneously coupling strong vocal tract resonances to the continuously changing pitch, experienced players facilitate a smoothly rising pitch, particularly across the glissando's final octave.
School of Engineering and Mathematical Sciences La Trobe University, Australia
ABSTRACT
Modern western bells are cast with basic axial symmetry and the same general ‘bell' shape. The size of the bell determines the fundamental frequency. The exact profile of the bell, however, varies between founders and has usually been designed using empirical techniques to produce a bell with a clear recognisable pitch. This generally means that the fundamental and significant partials are tuned to particular values and radiate clearly. In this paper, we take a more modern approach and use numerical methods to design bell profiles with appropriately tuned modes of vibration.
School of Physics, University of New South Wales Sydney NSW 2052
ABSTRACT
Traditional didjeridus have an irregular bore produced largely by termites eating the interiors of small eucalypt trees. The recent popularity of the instrument, both in Australia and abroad, creates demands upon the number of suitable eucalypts available for harvest, and also the number of indigenous craftsmen who can produce an authentic traditional instrument. Cylindrical plastic tubes are often used as cheap, widely available and easily tuned substitutes. However, their quality as musical instruments is generally perceived by players as being substantially inferior.
In this paper we report methods that might improve the playing quality of these plastic instruments. We found in an earlier study that the ranked 'overall quality' of a didjeridu was correlated negatively with the magnitude of its acoustic input impedance, particularly in the frequency range from 1 to 2 kHz. This is consistent with our observations that players used peaks in the impedance of their vocal tracts to modify the spectral envelope to produce the changes in timbre that are characteristic of didjeridu performance. Instruments with lower impedance in the critical band 1 to 2 kHz are therefore favoured. We consequently report on three modifications that reduce the acoustic impedance of a plastic instrument in the 1 to 2 kHz frequency range. The first involved increasing the internal damping in the instrument bore. The second involved adding multiple, suitably tuned, Helmholtz resonators along the instrument. The third involved the addition of a flared section or horn at the far end of the instrument. All modifications successfully reduced the impedance in the required range, but the addition of a final flared section had the additional advantage of increasing the output sound level in this range.
School of Mechanical Engineering, The University of Adelaide, Adelaide, South Australia, Australia
ABSTRACT
This paper presents a numerical model for the calculation of the transmission loss of a panel with an array of tuned vibration absorbers attached. The transmission loss of the panel is calculated for the case of the bare panel, with tuned vibration absorbers attached, and equivalent blocking masses. The theoretical predictions of transmission loss are compared with experimental measurements.
Sinclair Knight Merz, Australia
ABSTRACT
Brake squeal relating to the application of tread block brakes on rail cars can generate significant noise annoyance is-sues and presents a challenging problem due to the associated technical complexities as well as inherent design constraints. In this paper the phenomenon of brake squeal occurring on a rail vehicle is investigated using experimental techniques. The operational vibration response of the brake head is examined as is the modal characteristics of both the brake head and the wheel. A combination of analytical and finite element methods is then used to design a con-strained layer damping treatment for the rail wheel to reduce the vibration response and hence reduce brake squeal noise. The design of the constrained layer damping treatment is validated by testing.
School of Mechanical Engineering, The University of Western Australia
ABSTRACT
Significant vortex-induced vibration (VIV) occurs when the vortex shedding frequency about a structure coincides with its natural frequency. This is a cause of major problems in the offshore industry that account for large maintenance costs. One method to reduce VIV is by the use of splitter plates to reduce vortex shedding behind bluff bodies such as marine risers. The function of splitter plates was traditionally regarded as prohibiting lift force and amplitude response of VIV by the reduction of vortex interaction from either side of the structure. By allowing the cylindrical structure to vibrate transversely in a towing tank, interesting observations were made as a result of changing splitter plate length. Contrary to existing literature and belief, lock-in region displacement amplitudes of cylinders with splitter plates of lengths 0.5 to 1.5 diameters are up to 4 times larger than bare cylinder displacements. This paper discusses the significance of this experimental result and details on experimental set-up, procedure and vortex induced forces and displacements of cylinders in sub critical flows of reduced velocities ranging from 0 to 32.5 and Reynolds Numbers of 1.0x104 < Re < 7.5x104 with splitter plates of lengths 0 to 3 diameters.
(1) LMS International NV, Interleuvenlaan 68, B-3001 Leuven, Belgium
ABSTRACT
Fifteen years of NVH applications make Transfer Path Analysis (TPA) appear a commodity tool. Despite the fact that TPA is used in a large variety of applications today, the method requires an expert in both the NVH problem and all the related signal and system analysis constraints. TPA is proven to be reliable, but the main bottleneck remains the huge measurement time to build the full data model.
For this reason, industry is constantly seeking for simpler and faster methods. One such method is Operational Path Analysis (OPA), which was introduced about two years ago. OPA is a fully-operational method, requiring only operational measurements of the path references (body-side mount accelerations, pressures close to vibrating surfaces, nozzles and apertures, etc.) and target response(s). The OPA method is indeed very time-efficient, but suffers from several limitations leading to false path contributions and wrong engineering decisions. Its major limitations are: (i) cross-coupling effects between path references which may lead to faulty interpretations, (ii) potential errors due to missing paths and (iii) numerical ill-conditioning problems related to the estimation of transmissibilities from operational data. So, despite the fact that OPA is a very time-efficient approach, its benefit is limited in most application cases.
This paper introduces a novel path contribution method which combines the advantages of classical TPA and OPA. The method is based on simplifications that allow balancing path accuracy and speed of execution. The principles of the method are first outlined. Then, the method is compared with the existing TPA and OPA methods using an auto-motive example.
Vipac Engineers & Scientists
ABSTRACT
The Burj Dubai "Dubai Tower" is a skyscraper currently under construction in Dubai, United Arab Emirates. Concrete is being pumped to the top of the building using high-pressure hydraulic pumps. The hydraulic hammer action of the pumps, located on the buildings podium ground level, produces high levels of vibration. The vibration transmits to the lower basement levels and radiate as structure-borne noise. Noise and vibration measurements were con-ducted when concrete was pumped to level 140 at a height of 500m from the ground. The measurements were con-ducted at a number of points on the Concourse Level, Basement Level 1 and Basement Level 2. Reductions of vibration level as it transmitted away from the source and to other floors were measured. Structure-borne noise was estimated from measured vibration levels and compared to measured values where reasonable verification has been achieved.
Acoustics & Vibration Unit, School of Aerospace & Mechanical Engineering, University College, The University of New South Wales, Australian Defence Force Academy, Canberra, ACT 2600, Australia
ABSTRACT
Brake squeal has become an increasing concern for the automotive industry because of associated warranty costs and the requirement for the continued reduction of interior vehicle noise. Low and high frequency noises in car brakes, often referred to as brake squeal, are known to be a result of nonlinearity, unstable behaviour and bifurcations leading to limit cycle behaviour. By using the data from a separate experimental study designed to determine the influence of the geometric parameters of brake pads (such as the number and location of slots) on brake squeal noise, we examine two new approaches for providing improved understanding of the brake squeal phenomenon: statistical and nonlinear dynamics analyses. Results of the statistical analysis indicate that the performances of certain pad designs correlate with their levels of nonlinearity. The nonlinear time series analysis reveals that, in the experimental data, not only are limit cycle behaviours present but also a route to chaotic solutions can be observed.
Faculty of Architecture, Design and Planning, University of Sydney, NSW 2006.
ABSTRACT
The effectiveness of three binaural loudness calculation procedures using dummy head recordings was tested to predict the results obtained from a listening test in which the sounds were broad noise bands with different degrees of diffusivity. The loudness calculation results obtained with the binaural recordings were also compared with the loudness calculation results obtained from an omnidirecitonal microphone. Using rms error as an indicator, one loudness calculation procedure proved to be more accurate than the other two for the binaural recordings while the loudness calculation results obtained with the omnidirecitonal microphone unexpectedly proved to be more reliable in predicting the results from the listening test than any of the binaural loudness calculation procedures tested.
Faculty of Architecture, Design and Planning, University of Sydney, NSW 2006 Australia
ABSTRACT
Recent studies using subwoofers have provided consistent evidence that localisation along the left-right axis can occur for sound in the frequency range below 100 Hz, and even includes signals in the lowest octave of human hearing (however, front-back localisation fails for low frequency sound). If such left-right localisation is possible, the most likely explanation is a surprisingly acute sensitivity to interaural time or phase difference in the very low frequency range. The present study investigates this hypothesis using stimuli presented via headphones in a quiet anechoic room. Stimulus signals consisted of 1/3-octave noise bands centred on frequencies from 20 Hz -100 Hz with interaural time differences ranging between ±650 microseconds. The stimulus duration was 800 ms and was multiplied by a hanning window resulting in a smooth fade-in and fade-out (with the two channels faded together, regardless of the interaural time difference - hence this might be thought of as a frequency-dependent linear phase shift rather than a simple time difference). Tested on a head and torso simulator, the presentation sound pressure level was 40 dB(A), and distortion and background noise were both negligible. The subjects' task was to identify, on a scale from left to right, the location of the auditory image (i.e. the task was 'lateralisation' rather than localisation). Results show mild lateralisation for frequencies at and above 31.5 Hz with the lateralization of the image becoming clearer in the higher frequencies and the higher time delays across the frequency range tested, and so support the hypothesis.
(1) Faculty of Architecture, Design and Planning, University of Sydney, NSW 2006, Australia (2) Empirical Musicology Lab, School of Music and Music Education, University of New South Wales, NSW 2052, Australia
ABSTRACT
This paper presents possibilities offered by a computer program for analysing features of sound recordings, PsySound3. A wide variety of spectral and sound level analysis methods are implemented, together with models of loudness, roughness, pitch and binaural spatial analysis. In addition to providing access to these analysis methods, this analysis environment provides a context for easy comparison between analysis methods, which is very useful both for teaching and for the testing and development of models for research applications. The paper shows some of the potential for this by way of example. The software is structured so as to be easily extensible (using the Matlab programming environment), and many extensions are envisaged. Written by the authors and colleagues, PsySound3 is freely available via www.psysound.org.
(1) School of Aerospace, Mechanical and Mechatronic Engineering, University of Sydney, NSW, Australia (2) School of Aerospace, Mechanical and Mechatronic Engineering, University of Sydney, NSW, Australia (3) School of Aerospace, Mechanical and Mechatronic Engineering, University of Sydney, NSW, Australia
ABSTRACT
The reduction of radiated noise from mAV (miniature Aerial Vehicle) and small UAV (Unmanned Aerial Vehicle) platforms is of significant consideration as they start to perform more diverse roles in close proximity to populated areas. This paper examines a possible noise reduction technique for a small size propeller with a short chord length (15~30mm) operating at low Reynolds numbers. A series of static and wind tunnel tests have been conducted to investigate the mechanism behind a large broadband noise reduction achieved in static test conditions. It has been found that a leading edge boundary layer trip can produce a large reduction in broadband noise in simulated cruise conditions, with no evidence of performance detriments. Preliminary conclusions suggest that both a strong source of laminar boundary layer noise along with turbulent boundary layer noise mechanisms are present on the suction surface of the propeller blade, along with the existence of a laminar separation bubble on the blade surface during simulated cruise conditions. The introduction of a boundary layer trip appears to eliminate the laminar separation bubble and result in a large broadband noise reduction. It is believed that the elimination of the laminar separation bubble removes the strong laminar boundary layer noise source, and reduces the noise generated by the turbulent boundary layer. The leading edge trip has proved successful in achieving a broadband reduction which results in a repeatable 6-7dB(A) reduction at the sample location, which corresponds to an overall reduction in SWL by up to 4dB in static tests.
(1) Network Systems & Capability, QR Network Pty Ltd, Brisbane, Australia (2) Bassett Acoustics, part of AECOM, Brisbane, Australia (3) Heggies Pty Ltd, Brisbane, Australia
ABSTRACT
Rail noise predictions in Queensland have historically been undertaken using a DOS-based implementation of the SoundPLAN program. Rail noise levels modelled using this DOS program have been validated with measured noise levels throughout the history of its use. This DOS package has been superseded by Windows-based implementations of this program. Queensland Rail Network has commissioned a study to compare the modelling results between the currently accepted DOS-based version of SoundPLAN and the latest Windows-based implementation. The outcomes of the study contained in this paper demonstrate why QR Network is now able to accept Windows SoundPLAN results for rail noise prediction projects within Queensland. Equivalent confidence in the modelled noise levels reduces the amount of noise monitoring required at affected properties, leading to a more efficient and sustainable use of available acoustical resources in Queensland. Such reliable noise modelling results therefore enable more efficient delivery of mitigation measures to sensitive areas than can be achieved with reliance upon measurement results.
Research and Development, Bassett Acoustics | AECOM, Sydney, Australia
ABSTRACT
This paper investigates from theoretical analysis and field data, sources of error in using the Calculation of Road Traffic Noise (CoRTN) procedure to predict Leq noise levels. Particular attention has also been given to providing a possible theoretical explanation for the Lden conversion factors obtained by Transport Research Laboratory (TRL). The equations in CoRTN have been compared with established relationships for Leq noise levels. The investigation focused on equations involving traffic volume, distance and speed which theoretically contribute to a change in the relationship between L10 and Leq. It was found that most of the equations in CoRTN are valid for predicting Leq and that the correction equations derived by TRL for predicting Lden introduce additional complications and the potential for significant error. Revised equations are proposed.
(1), (2) & (3) ASK Consulting Engineers, Brisbane, Queensland, Australia
ABSTRACT
Tyre/road noise measurements conducted using the close proximity (CPX) method provide a useful means of assessing road surfaces and tyres, and particularly the variation of noise along the length of a road surface. In this paper we present results of recent CPX tyre/road noise measurements undertaken with the ROAD-EAR trailer in and around Brisbane and Townsville, Queensland. Testing was performed over short (100 m) distances and a long (5000m) distance. Measurements were conducted on 22 road surfaces including chip seal, OGA, DGA, SMA & PCC, and used three tyres, including two candidate tyres for ISO 11819-2, as well as the Australian made (passenger car) tyre Bridgestone RE92. Most of the SMA and OGA surfaces showed the lowest overall noise levels, although some of these road surfaces provided relatively high emissions of noise. The range in CPXI noise levels over the 100m test sections was 5 dB(A). The local Bridgestone RE92 tyre was found to be substantially quieter than both the Continental and BFGoodrich candidate tyres for all road surface types tested. Long distance measurements using the ROAD-EAR trailer showed considerable variability in concrete (PCC) and asphalt surfaces to the degree that some transverse tyned PCC sections proved equal in overall noise level emissions to the normally quieter asphalt (OGA & SMA) surfaces. This paper presents an assessment of these variations, and suggests explanations for the results.
(1) Principal, TEF Consulting and Visiting Research Fellow, UNSW, Sydney, Australia (2) Design, Environment and Stewardship Division, Queensland Department of Main Roads, Brisbane, Australia
ABSTRACT
This paper presents an investigation into the road traffic noise attributes of five types of pavement surface currently in service in Queensland. The investigation, perhaps the most extensive of its kind ever undertaken in Australia, was configured to determine the acoustic performance of the set of pavement surfaces and how these performances varied over time. To do this, a considerable set of roadside noise data, measured according to the statistical passby technique, was collected at 29 sites in 2002, 2003, 2005, 2006 and 2007. These data were collected at 21 sites in South East Queensland and at 8 sites in North Queensland in the Townsville environs. Analyses of the data produced values of a parameter known as the Statistical Passby Index which are presented in the paper and which quantify the acoustic performance of the pavement surfaces. Moreover, some of the pavement surfaces exhibited very stable acoustical performance over the 2002 to 2007 period while the acoustical performance of others varied somewhat. From there the paper presents a new method developed by the authors for applying the outcomes of the investigation to the prediction of road traffic noise on the five types of pavement surface over a five year period, taking into account the age of each of the pavement surfaces involved.
(1) Defence Science and Technology Organisation P.O. Box 1500, Edinburgh, SA 5111, Australia (2) Thales Australia 274 Victoria Road, Rydalmere, NSW 2116, Australia (3) Centre for Marine Science & Technology, Curtin University of Technology GPO Box U1987, Perth WA 6845, Australia
ABSTRACT
The reflection of underwater sound from the ocean surface boundary is one of the critical phenomena which must be described within models of sonar transmission and reverberation at mid-frequencies (up to about 5 kHz), yet it has not been studied within Australian organisations for a considerable time. Some of the algorithms which are frequently referenced are based on simplistic theoretical assumptions of roughness, whereas others are based on studies which are quite old. In order to address the issue in the Australian context, and with reference to modern sonar models, commonly referenced models of surface reflection loss are compared with each other and with at-sea time series sonar data obtained by DSTO within the Australian region.
Centre for Marine Science and Technology, Curtin University of Technology, Perth, Western Australia
ABSTRACT
Until very recently, underwater acoustics has been the exclusive domain of marine animals, researchers, the military, and a few companies that make their living from building sonar and related equipment. However, new Government regulations aimed at reducing detrimental impacts on marine animals have greatly broadened interest in this field by placing limits on underwater sound levels produced as a by-product of human activities of considerable commercial importance - particularly offshore seismic exploration and pile-driving. As a consequence, many acoustic consulting companies are finding themselves involved in this field for the first time. This paper looks at some of the issues involved in modelling and measuring sound levels in the underwater environment and in translating these levels into some sort of estimate of likely environmental impacts. There is a lot more to it than simply exchanging your micro-phone for a hydrophone!
Maritime Operations Division, Defence Science and Technology Organisation, Edinburgh, SA, 5111, Australia
ABSTRACT
A plane two-dimensional acoustic waveguide with an elastic infinite cylinder as a scattering object is considered. Based on the solution of the scattering problem obtained by the author previously, acoustic pressure distribution and energy streamlines are calculated. It is shown that, with some specific configurations of acoustic sources on the surface of the cylinder, the scattered field will contain evanescent waveguide modes only, thus being absent at large distances from the cylinder and making the cylinder acoustically invisible. It is also shown that such sources consume zero power on average without taking into consideration viscous losses. It is shown that, for a specific frequency and a specific modal composition of the incident field, there is infinitely large variety of possible source configurations leading to acoustic invisibility. Some issues related to acoustic invisibility in viscous fluids and free space are discussed.
Centre for Marine Science & Technology, Curtin University of Technology, Perth WA 6845, Australia;
ABSTRACT
Calibration of bearing accuracy was conducted for the hydroacoustic station (HA01) deployed in the Indian Ocean off Cape Leeuwin, Western Australia, as part of the International Monitoring System of the Comprehensive Nuclear-Test-Ban Treaty. Both the random and systematic components of the bearing error were investigated using the azimuth measurement of various underwater events detected at the Cape Leeuwin station. The RMS value of the random component of azimuth estimation was examined using long-lasting low-frequency underwater events, such as harmonic tremor signals from drifting iceberg and seismic events including Sumatra earthquakes (main shock and after-shock). The random bearing errors were associated with horizontal deviation of hydrophones' moorings from the position based on a model of mooring motion. The systematic component was estimated through inversion of the signal travel time difference to the HA01 hydrophones from a number of underwater explosions made in the Indian Ocean at known locations. It is shown that the standard deviation of bearing estimates due to the random component is around 0.5 degree. The systematic error, which is about 0.8 degree clockwise, can be compensated by small correction of moorings' coordinates. Potential effects on azimuth estimation of horizontal refraction along cross-ocean acoustic propagation paths are also considered through numerical modelling.
(1) Dynamics Group, Sinclair Knight Merz, St Leonards, Sydney 2065 (2) University of Technology of Compiegne, Compiegne, France
ABSTRACT
As the blades of a propeller pass through the water they produce characteristic amplitude modulated random noise signals which can be detected using sonar. A popular empirical technique for passive acoustic detection of surface ships from submarines using these sonar signals is DEMON (Detection of Envelope Modulation on Noise) process-ing. As the name suggests, DEMON processing seeks to detect the frequencies of modulation, i.e. the shaft and blade pass frequencies. It works by isolating the frequency band in which the modulation is most distinct to the operator, taking the envelope of this filtered noise band and producing a waterfall spectrogram. Harmonics associated with the rotating components of the propeller will be manifest in the waterfall, allowing the vessel to be identified. DEMON processing has several drawbacks however, most importantly the requirement for operator skill in the selection of the noise band. This paper presents the preliminary findings of work underway to provide a mathematical formalisation of the empirical DEMON processing technique. This formalisation is based on the observation that DEMON processing unknowingly exploits the cyclostationary properties of the propeller signals. Cyclostationary signal processing, a technique which has recently found application in mechanical systems involving rotating machinery, offers great in-sight into the detection problem, and has the potential to overcome the weaknesses of DEMON processing as well as expanding its capability to include frequency as well as amplitude modulation.
(1) Maritime Operations Division, Defence Science and Technology Organisation, Edinburgh, Australia
ABSTRACT
Inputs to mathematical models can be very uncertain because of incomplete knowledge. These input uncertainties propagate to output uncertainties, and an estimation of these output uncertainties, and of the importance of the individual inputs to these output uncertainties, would be desirable. These estimates can be calculated via Uncertainty Analysis (UA) and Sensitivity Analysis (SA) respectively. A software library, Algorithms for Stochastic Sensitivity Analysis (ASSA) has recently been written (Jansen 2005) to carry out these analyses. ASSA estimates the total variance (VTOT) for uncertainty, and for each input parameter, the "top marginal variance" (TMV) for its sensitivity - the reduction in variance that would occur if the input were perfectly known. We adapt the ASSA methodology to underwater acoustic propagation. Our tool uses efficient uncertainty models for sound speed profile (SSP) and bottom loss (BL) to compute an ensemble of SSPs and BL curves. A propagation model is then used to compute an ensemble of transmission losses (TLs) from which VTOT and the TMVs are used to estimate TL uncertainty and sensitivity respectively. Example results for a real ocean environment are presented.
Centre for Marine Science and Technology, Curtin University of Technology, Bentley, Western Australia
ABSTRACT
Offshore pipelines are integral to the oil and gas industry and vital for the transport of resources from subsea wells. As fields progress toward deeper water and more rugged terrain, an effective method of real-time monitoring of both environmental and pipeline conditions is essential for maintaining environmental and safety standards. This study seeks to assist in the development of real-time methods of data retrieval by characterising the horizontal performance of various underwater acoustic modems. Preliminary trials involve investigating performance of two commercially available modems in shallow water. In the first of these the system is found to have worked effectively, with reliable communications obtained at ranges up to 500m. Whilst this was less than the expected range for the modems, the characterisation of their performance gives an initial insight to the functionality of horizontal underwater communications in the varying oceanographic conditions.
(1) MOD, Defence Science and Technology Organisation, HMAS Stirling, Western Australia (2) CMST, Curtin University of Technology, Bentley, Western Australia
ABSTRACT
During September 2006, an exploratory underwater acoustic recording was made in the Nornalup-Walpole Estuary located 350 km south of Perth, Western Australia. Immediately obvious in the recording was a sustained crackling sound, characteristic of snapping shrimp. The presence of snapping shrimp in the estuary was unexpected, since the salinity is generally quite low, and snapping shrimp were not mentioned in biological literature surveyed prior to the recording. Individual impulses showed precursor and bubble characteristics, and spectral and statistical properties of the impulsive noise were consistent with the source being the snapping shrimp.
9 Moya Crescent, Kingsgrove NSW, Australia
ABSTRACT
To compute Transmission Loss (TL) in shallow water, the required environmental parameters include a geoacoustic model (GAM) of the seabed, the sound-speed profile of the water layer, and roughness of the sea-surface and sea-floor. The GAM is estimated with regression equations from geological properties, which are held in a geographic database. TL can be affected by the GAM down to a few wavelengths below the seafloor, but such data is sparse. In practice, sonar prediction systems assume that the seabed is uniform from the seafloor down. A question that arises is the degree to which computed low-frequency TL can consequently differ from actual TL. This issue is addressed by examining the results of a TL experiment that was accompanied by measurements of seafloor and seawater proper-ties. At 100 Hz the computed TL compared well with the data, whereas at 50 Hz the computed TL was too high (by 18 dB at 20-km range). This error can be attributed to the existence of a reflecting basement within the seabed that was not accounted for. When such acoustic effects are found, seabed databases should be modified to include an appropriate basement.
School of Mechanical and Manufacturing Engineering, University of New South Wales, Sydney, NSW 2052, Australia
ABSTRACT
A model to describe the low frequency dynamic and acoustic responses of a submarine hull subject to an eccentric harmonic propeller shaft excitation is presented. The submarine is modelled as a fluid-loaded, ring stiffened cylindrical shell with internal bulkheads and conical end caps. The stiffeners are introduced using a smeared approach. A harmonic axial force is introduced by the propeller and is transmitted to the hull through the shaft. It results in excitation of the accordion modes only if the force is symmetrically distributed to the structure. Otherwise the excitation can be modelled as the sum of a distributed load and a moment applied to the edge of the hull. This leads to excitation of the higher order circumferential modes that can result in high noise signature. Structural and acoustic responses are presented in terms of deformation shapes and directivity patterns for the radiated sound pressure. Results for the case of purely axisymmetric excitation and the case in which an eccentricity is introduced are compared.