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L. Oddo, INSA de Lyon (France)
B. Laulagnet, INSA de Lyon (France)
Jean-Louis Guyader, INSA de Lyon (France)
A machinery is often composed of a hood radiating sound to the exterior and internal substructures. Ignoring this substructures when calculating the sound radiated by the hood can lead to bad results, when compared to experiment. Howevery taking into account the substructure in a deterministic sense, is generally unrealistic due to their complexity. The aim of this paper is to use the S.E.A. equation in order to derive an equivalent damping due to the internal substructures, and then use it to compute the sound radiated from the hood having an increased damping. This allows us to use the standard computer program of thin structures radiation by, just modifying their damping loss factors. In particular conditions, that will be described, the internal substructures can increase considerably the damping of the hood, and then reduce the radiated sound. The paper present the theoretical derivation of the method, on the case of a cylinder coupled to internal system described by point mobilities. The prediction with exact calculation will be compared to this approach. To show the phenomenon of increasing damping of a master structure coupled to internal vibrating systems, an experiment was made with beam like structures. It will be presented and experimental results compared to theoretical predictions.
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P.J.G. van der Linden, LMS International (Belgium)
J. Schnur, Mercedes-Benz (Germany)
T. Schomburg, Mercedes-Benz (Germany)
Oilsumps, valve covers and distribution covers belong to the main noise radiating parts on modern engines. These parts have in common that their main excitation is a velocity excitation at their fixation to the engine structure. A method was developed to quantify the noise emission of the individual part under artificial excitation. This measurement technique allows the assessment of the parts and their mounting to the engine for efficient variant comparisons. The difficulty lies in the simulation of near to rigid boundary conditions and in the effective velocity excitation. A combination of reciprocal acoustic excitation and inverse force identification is used. The objects are excited by volume velocity sound sources. On the basis of a reciprocity relationship and inverse force identification the effective noise emission is quantified for spatially uncorrelated velocity excitation. A comparison of aluminium, plastic and rubber decoupled oilsumps is shown as an example.
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Ahmad Bahrami, Australian Defence Force Academy (Australia)
Hugh M. Williamson, Australian Defence Force Academy (Australia)
Joseph C.S. Lai, Australian Defence Force Academy (Australia)
It has been shown theoretically by Evensen [1] that the noise radiated from impact forming operations is related to the time derivative of the force induced in the machine. In this paper, the validity of this theory is explored for a high speed sheet metal shearing process. Noise measurements were made for various shear blade tooling parameters, such as blade profile, blade clearance and speed of cutting, while dynamic cutting forces were measured simultaneously using a pressure transducer. Experiments include the cutting of profiled sheet metal products as well as flat sheets. The profiled specimens used for this investigation were roll formed sheet steel products with an approximately sinusoidal profile produced by BHP Building Products, Australia. For this product, eight different shear blade profiles of varying blade angles and rake were designed and produced. Comparison of experimental results with theory is discussed along with practical applications to the design of high speed sheet metal shearing operations with reduced radiated noise.
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N J Agrawal, Kirloskar Copeland Limited (India)
R. Dubey, Kirloskar Copeland Limited (India)
S.R. Joshi, Kirloskar Copeland Limited (India)
The noise generated by a hermetically sealed reciprocating compressor reaches the outside environment by traveling through compressor shell. The intensity of the noise reaching outside is known to be sensitive to the geometric configuration of compressor shell. As part of a noise reduction program, compressor shell has been modeled on ANSYS Finite Element Analysis (FEA) Software and modal analysis was performed on the same. The results from this analysis were then cross checked with the near field sound intensity analysis of the compressor on dual channel FFT sound analyzer, B&K 2144. It was found that there were compressor shell natural frequencies of vibration in 1000 Hz to 3000 Hz frequency range and these were very close to the high intensity peaks in the sound intensity spectrum of compressor obtained by using dual channel FFT analyzer. Shell shape alternatives were analyzed for their forced response using ANSYS FEA software; shell of modified shape has shown significant improvements in forced response characteristics in the 1000 - 3000 Hz range. A modified shell has been prototyped. An overall noise power reduction of 1.8 dBA has been achieved, without any associated cost penalty.
sv970382.pdf (Scanned) TOPNelik Dreiman, Tecumseh Product Company (U.S.A.)
The scheme of hermetic rolling piston compressor vibration and sound generation mechanism has been developed to understand complex inter-relationship between suction and discharge gas pulsations, mechanical forces, such as vane reciprocation roller revolution drive forces of the rotor rotation and electro-magnetic forces of the electric motor. One of the sound sources within the compressor is the mechanical friction between the crankshaft thrust surface and facing surface of the outboard bearing. The sound radiated by the compressor has been reduced by useof the special designed thrust bearing (USA Pat. 5,554,015 dated Sept. 10, 1996) which has been pressfit (interference fit) within countersunk recess in the rotor. By press fitting the thrust bearing within the counterbore formed in the rotor, rotation of the thrust bearing relative to the rotor is prevented This results in forming a single frictional pair, the lower surface of the thrust bearing against the upper end faceof the compressor main bearing hub, thereby reducing the amount of sliding fiction within the compressor. The polyamide material used to form the thrust bearing is characterized by a very low coefficient of static and kinetic friction. It helps to diminish the resultant friction thereby increasing the efficiency, reducing overall sound radiated by the compressor as during operation, so at the start, and improve reliability of the rotary compressor.
sv970365.pdf (Scanned) TOPIlja E. Tsukernikov, AO "NIIpolygrafmash" (Russia)
The problem of technical standardization of noise emission values of machinery and equipment is considered as a problem of how to determine limit admissible values of sound power levels of machines by satisfactoring of which it is ensured the fulfillment of noise sanitary norms at operators stations when the scheme of machinery disposition in the room is specified. The system of linear algebraic equations for the limit admissible A-weighted sound power values of machines is derived. Some equations for the mean sound absorption coefficient of a room where machines operate are proposed, which can be used when dealing with A-weighted quantities. The comparison of the values calculated is done using the recommendations of ISO 3744 too, and the most suitable equation is found. As an example the limit admissible A-weighted sound power levels of curds production workshop equipment are calculated.
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Zbigniew Engel, University of Mining and Metallurgy (Poland)
Dariusz Pleban, Central Institute for Labour Protection (Poland)
The authors worked out two indices of the acoustic assessment of machines: a power index and an emission index, as well as indicatory methods of acoustic assessment of machines. The worked out indices are functions of several parameters such as e.g., variants of the operational conditions of the machine, the acoustic properties of the room. The results of the simulation tests, illustrating the effects of variations of different parameters on the values of the indices of the acoustic assessment, are presented in the paper.
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Wan-Ho Jeon, Korea Advanced Institute of Science and Technology (Korea)
Duck Joo Lee, Korea Advanced Institute of Science and Technology (Korea)
Centrifugal fans are widely used, so the noise generated by these machines causes one of serious problems. Especially, in vacuum cleaner fan, not only the discrete tones but also broadband noise are dominant. Past researchers have been focused on the generation and reduction of the BPF tones. In order to understand the generation mechanism of tone/broadband noise, a DVM (Discrete Vortex Method) is used to calculates the flow field and the Lowson's equation is used to predict the acoustic pressure. The broadband noise of a circular casing centrifugal fan is due to the unsteady force fluctuation around the impeller blades related to the vortex shedding.
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