Index of content:
Volume 128, Issue 2, August 2010
- NOISE: ITS EFFECTS AND CONTROL 
A re-expansion method for determining the acoustical impedance and the scattering matrix for the waveguide discontinuity problem128(2010); http://dx.doi.org/10.1121/1.3455836View Description Hide Description
The paper gives a new method for analyzing planar discontinuities in rectangular waveguides. The method consists of a re-expansion of the normal modes in the two ducts at the junction plane into a system of functions accounting for the velocity singularities at the corner points. As the new expansion has an exponential convergence, only a few terms have to be considered for obtaining the solution of most practical problems. To see how the method works some closed form solutions, obtained by the conformal mapping method, are used to discuss the convergence of the re-expanded series when the number of retained terms increases. The equivalent impedance accounting for nonplanar waves into a plane-wave analysis is determined. Finally, the paper yields the scattering matrix which describes the coupling of arbitrary modes at each side of the discontinuity valid in the case of many propagating modes in both parts of the duct.
128(2010); http://dx.doi.org/10.1121/1.3459838View Description Hide Description
The sound absorbing properties of a thin micro-perforated plate (MPP) coated with piezoelectric material with shunt damping technology is investigated. First a theoretical model is presented to predict the sound absorption coefficients of a thin plate attached with a piezoelectric patch and electrical circuits. Then the model is extended to analyze the sound absorption for a thin plate with micro perforations and piezoelectric material. Measurements are also carried out in an impedance tube and found to be in good agreements with the theoretical model. The sound absorption of the constructions can be much improved by tuning the electrical circuits.
128(2010); http://dx.doi.org/10.1121/1.3458853View Description Hide Description
This study evaluated the potential risk to hearing associated with the use of portable digital audio players. Twenty-eight university students (12 males, 16 females; aged 17–23) completed a 49-item questionnaire assessing user listening habits and subjective measures of hearing health. Sound level measurements of participants’ self-identified typical and ‘worst case’ volume levels were taken in different classrooms with background sound levels between 43 and 52 dBA. The median frequency and duration of use was 2 h per day, 6.5 days a week. The median sound levels and interquartile ranges (IQR) at typical and ‘worst case’ volume settings were 71 dBA and 79 dBA, respectively. When typical sound levels were considered with self-reported duration of daily use, none of the participants surpassed Leq(8) 85 dBA. On the questionnaire, 19 students reported experiencing at least one symptom of possible noise-induced hearing loss. Significant differences in MP3 user listening patterns were found between respondents who had experienced tinnitus and those who had not. The findings add to a growing body of literature that collectively supports a need for further research investigating MP3 player user listening habits in order to assess their potential risk to hearing health.