ARTICLES

A Simple Function for Modelling Three-Dimensional Scattering Strength from the Ocean Surface
Zhi Yong Zhang

A Variation to the Sound Level Conversion Measure of Hearing Protector Performance
Warwick Williams

Relationship Between Speech Recognition and Self-Report Measures
Andrea Simpson, Hugh J. McDermott & Adam A. Hersbach

Determination of Dynamic Properties of Rail Pads using an Instrumented Hammer Impact Technique
Alex Remennikov & Sakdirat Kaewunruen

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A simple function for modelling three-dimensional scattering strength from the ocean surface

Vol. 33, No. 2 pp 47 - 50 (2005)
ABSTRACT: Both the rough air-sea interface and entrapped air bubbles due to wave breaking scatter sound in all directions and contribute to so-called reverberation in active sonar. There are monostatic sonar systems where the source and receiver are at the same position, bistatic sonar systems where the source and receiver are separated, and multistatic sonar systems involving multiple sources and receivers at different positions. In monostatic situations, reverberation is mainly due to backscattering. In bistatic and multistatic situations, forward and out-of-plane scattering are significant contributors. The empirical Chapman-Harris formula is often used to predict surface backscattering strength in monostatic sonar. To better predict reverberation from the sea surface in bistatic or multistatic sonar, a three-dimensional scattering formula that includes a forward scattering lobe will be desirable. Following earlier work, in this paper the separable form of backscattering models are extended by including an expression of forward scattering lobe obtained under the Kirchhoff approximation, taking into account shadowing effects. Comparison with another more sophisticated model shows that shadowing corrections are important at low grazing angles. The formula obtained here is simple and includes scattering effects from both the roughness of the sea surfaces and the sub-surface bubbles. It may be useful for modelling multistatic surface reverberations.

A variation to the sound level conversion measure of hearing protector performance

Vol. 33, No. 2 pp 51 - 55 (2005)
ABSTRACT: This work looks at a variation in the method of calculating the single number rating of hearing protector attenuation performance, the SLC80. The resulting figure has a slight variation from the current method of calculation but comparison with 111 devices that had recently been tested shows that in practice this difference is minimal. The advantage of the variation in the method is that the uncertainty in attenuation performance is reduced to one standard deviation replacing the conventional seven standard deviations. This makes for easier error analysis and future statistical analysis of the relative performance of different devices.

Relationship between speech recognition and self-report measures

Vol. 33, No. 2 pp 57 - 62 (2005)
ABSTRACT: The performance of a prototype digital high-power hearing instrument was evaluated using tests of speech understanding in quiet and a questionnaire. The subjects were 26 adults with moderate-to-profound sensorineural hearing loss, most of whom were experienced hearing aid users. There was no significant difference between the group mean scores on monosyllabic word tests conducted in quiet for the prototype device versus the subjects own hearing aids. However, responses to the questionnaire showed that 85% of the subjects preferred the prototype device to their own hearing aid(s). Although there was a positive correlation between the questionnaire results and the speech recognition score differences, overall there was a stronger tendency for subjects to prefer the prototype device than could be explained by their speech test results alone.

Determination of dynamic properties of rail pads using an instrumented hammer impact technique

Vol. 33, No. 2 pp 63 - 67 (2005)
ABSTRACT: The repeated impact of train wheels over sleepers can reduce the lifetime of a sleeper and degrade ballast. In more extreme cases it can lead to the breakdown of the concrete sleeper. Concrete sleepers are rigid compared to steel and wooden sleepers and therefore it is necessary to provide impact attenuation to prevent premature breakdown of the concrete. One of the measures employed to attenuate the effect of the impact loads on concrete sleepers has been the use of the resilient rail seat pads. Numerous analytical and numerical models have been formulated to investigate the dynamic behaviour of railway track substructures. All models require careful selection of the track component properties to satisfactorily represent track vibration response. However, there is currently no standard method available that can be used to evaluate the dynamic properties of the rail pads. At the University of Wollongong, an instrumented hammer was used to excite an equivalent single degree-of-freedom system (SDOF), incorporating a rail pad as a resilient element, to determine the dynamic properties using methods of modal analysis. The analytical SDOF dynamic model was applied to best fit the experimental modal measurements that were performed in a frequency range of 0-500 Hz. The curve fitting gives such dynamic parameters as the effective mass, dynamic stiffness, and dynamic damping constant, all of which are required for numerical modelling of a railway track.