Volume 104, Issue 5, November 1998
Index of content:
- NOISE: ITS EFFECTS AND CONTROL 
104(1998); http://dx.doi.org/10.1121/1.423869View Description Hide Description
Boundary integralequations are formulated for the shape sensitivity analysis of the acoustic problems. The concept of the material derivative is employed in deriving the sensitivity equations. Since the equation is derived by the direct differentiation of the boundary integrals containing the field values, it is expected that the sensitivity would be computed more effectively and accurately than the conventional finite difference method. In addition, the equation has the potential to be applied to many complex acoustic problems, because the derived equation is regularized by using the integral identity that incorporates the one-dimensional propagating wave and its material derivative. The validity of the formulations is demonstrated through examples having regular shapes such as the three-dimensional pulsating sphere and the one-dimensional duct, for which the analytical solutions are available. As an example for an irregular domain, the two-dimensional model of an automotive interior cavity is dealt with in the view point of the noise level at the passenger’s ear position. The results show that the present method can be an effective tool for the shape optimization in designing the desired sound field. It is noted that the present method permits accurate sensitivities of the acoustic pressure on the boundary as well as at the field points. The present method is thought to be an alternative to the previous finite difference techniques for computing the shape sensitivity using the boundary element method and the formal derivative method using the finite element method.
104(1998); http://dx.doi.org/10.1121/1.423870View Description Hide Description
Many applications have been found for the microperforated panel (MPP) absorber, on which the perforations are reduced to submillimeter size so that they themselves will provide enough acoustic resistance and also sufficiently low acoustic mass reactance necessary for a wide-band sound absorber. The most important parameter of the MPP is found to be the perforate constant k which is proportional to the ratio of the perforation radius to the viscous boundary layer thickness inside the holes. This, together with the relative (to the characteristicacoustic impedance in air) acoustic resistance r and the frequency of maximum absorption of the MPP absorber, decides the entire structure of the MPP absorber and its frequency characteristics. In other words, the MPP absorber may be designed according to the required absorbing characteristics in terms of the parameters k, r, and Formulas and curves are presented toward this end. It is shown that the MPP absorber has tremendous potential for wide-band absorption up to 3 or 4 octaves and for low-frequency absorption with a cavity of depth small compared to the wavelength. Techniques of making minute holes (of 0.1–0.3 mm, say) have to be developed, though.
104(1998); http://dx.doi.org/10.1121/1.423871View Description Hide Description
A three-dimensional analytical approach is developed to determine the transmission loss of circular flow-reversing chambers in the absence of bulk flow. The study couples the continuity conditions of the acoustic pressure and particle velocity at the inlet and outlet with the orthogonality relations of Fourier–Bessel functions. The present analytical results are compared with the boundary element predictions to assess the approach, as well as with previous works to examine the effect of nonplanar wave decay in end ducts. The acoustic attenuation due to flow-reversing chamber is then investigated in detail as a function of the length-to-diameter ratio of the chamber and the relative locations of the inlet/outlet.
104(1998); http://dx.doi.org/10.1121/1.423872View Description Hide Description
The active minimization of harmonic sound transmission into an arbitrarily shaped enclosure using error signals derived from structural vibrationsensors is investigated numerically. It is shown that by considering the dynamics of the coupled system, it is possible to derive a set of “structural radiation” modes which are orthogonal with respect to the global potential energy of the coupled acoustic space and which can be sensed by structural vibrationsensors. Minimization of the amplitudes of the “radiation modes” is thus guaranteed to minimize the interior acoustic potential energy. The coupled vibro-acoustic system under investigation is modelled using finite element analysis which allows systems with complex geometries to be investigated rather than limiting the analysis to simple analytically tractable systems. Issues regarding the practical implementation of sensing the orthonormal sets of structural radiation modes are discussed. Specific examples relating to the minimization of the total acoustic potential energy within a longitudinally stiffened cylinder are given, comparing the performance offered using error sensing of the radiation modes on the structure against the more traditional error criteria; namely, the discrete sensing of the structural kinetic energy on the boundary and the acoustic potential energy in the enclosed space.
104(1998); http://dx.doi.org/10.1121/1.423911View Description Hide Description
Subjective reactions to artillery sounds were determined for over 400 respondents divided among 17 different residential areas. Also, for the same respondents, the subjective effects of road-traffic sounds were determined enabling a comprehensive comparison of the dose-response relations. For the sake of comparison with other field surveys, the noise dose for the shooting sounds was, among other things, expressed as the yearly average C-weighted day–night level (CDNL) and that for the road-traffic sounds was expressed as the A-weighted day–night level (ADNL). Similarly, for both sound types the community response was expressed as the percentage of respondents being “highly annoyed.” From the comparison of the two dose-response relationships it could be concluded that for numerically equal day–night levels, the artillery sounds were more annoying than the road-traffic sounds. Overall, the difference was equivalent to the change in annoyance produced by a 5-dB shift in the yearly average day–night levels of the sounds. With equal day–night levels for “downwind” conditions, the artillery and road-traffic sounds were equally annoying. Results from the present highly controlled field survey provided a new opportunity to optimize the parameter values in Schomer’s rating procedure in which the noise exposure for impulsive sounds (y) is expressed as the A-weighted SEL of equally annoying vehicle sounds. PNSE represents the point at which the impulsive and vehicle sounds with numerically equal levels are also equally annoying. With PNSE fixed at 103 dB, an optimal solution was found with slope β set to 1.3. With the previously recommended slope the rating sound level for artillery sounds would be underestimated by almost 12 dB.