Volume 110, Issue 6, December 2001
Index of content:
- ARCHITECTURAL ACOUSTICS 
A power conservation approach to predict the spatial variation of the cross-sectionally averaged mean-square pressure in reverberant enclosures110(2001); http://dx.doi.org/10.1121/1.1409538View Description Hide Description
Although it is commonly assumed that broadband mean-square pressure levels are spatially uniform in reverberant enclosures, there is a gradual spatial variation, especially if the room is long in one direction, and/or the acoustic absorption is not applied uniformly to the enclosure boundaries. An equation for predicting the average cross-sectional sound pressure levels in a lightly damped enclosure with absorption is derived based on conservation of acoustic power. The derivation involves a one-dimensional boundary value problem, the solution of which is an estimate of the average sound pressure level at cross-sections in the interior. In its simplicity, the resulting formula is reminiscent of the classical Sabine formulation; however, this prediction contains a spatially varying function that depends upon the distribution of absorption (side-wall versus end-wall). The formula is demonstrated on a model problem consisting of a rectangular acoustic enclosure with a source on one end-wall, absorption on the opposing end-wall, and a combination of hard and absorbing side-walls. Comparisons with an exact numerical simulation show that the prediction works well for a wide range of absorption levels and provides an improvement over classical diffuse field theory, where the levels are assumed to be spatially uniform. A formula for the volume-averaged, broadband, mean-square pressure (a modified Sabine formula) is also derived and shown to give excellent agreement with the numerical simulations.
110(2001); http://dx.doi.org/10.1121/1.1412443View Description Hide Description
It is possible to obtain good absorption from Schroeder diffusers if suitable alterations to the design are made. Interestingly, previous work has shown that good absorption appears possible below the design frequency when the diffusers are poorly constructed. This has inspired the design of a profiled absorber using perforated plates in some wells; the absorber has extended bass response. The paper presents a theory for the enhanced absorption and the important design parameters are discussed. Good agreement is shown between the prediction model and impedance tube measurements. The design of this absorber was first carried out using a numerical optimization, although a simplified design procedure is also outlined which is almost as good. The results clearly show that this type of profiled absorber extends the absorption at low frequencies while maintaining the good absorption at mid frequencies as well.
An acoustic boundary element method based on energy and intensity variables for prediction of high-frequency broadband sound fields110(2001); http://dx.doi.org/10.1121/1.1416201View Description Hide Description
A boundary element method is formulated in terms of time-averaged energy and intensity variables. The approach is applicable to high modal density fields but is not restricted to the usual low-absorption, diffuse, and quasiuniform assumptions. A broadband acoustic energy/intensity source is the basic building block for the method. A directivity pattern for the source is derived to account for local spatial correlation effects and to model specular reflections approximately. A distribution of infinitesimal, uncorrelated, directional sources is used to model the boundaries of an enclosure. These sources are discretized in terms of boundary elements. A system of equations results from applying boundary conditions in terms of incident, reflected, and absorbed intensity. The unknown source power for each element is determined from this system of equations. A two-dimensional model problem is used to demonstrate and verify the method. Exact numerical solutions were also obtained for this model problem. The results show that spatially varying mean-square pressure levels are accurately predicted at very low computational cost.
The Monte Carlo method to determine the error in calculation of objective acoustic parameters within the ray-tracing technique110(2001); http://dx.doi.org/10.1121/1.1409373View Description Hide Description
This article is an extension of the procedure to estimate errors in ray-tracing calculations of room response proposed by Giner et al. [J. Acoust. Soc. Am. 106, 816–823 (1999)]. The previous article presented an expression to estimate the error in computing the energy reaching a receptor during a small time interval. This expression was obtained assuming that a pure ray-tracing program is used and a Monte Carlo Technique is applied. In the present work these ideas are extended in order to compute the objective acoustic parameters of a room. The corresponding equations are presented in closed form. Our results show that the number of rays involved in the analysis depends on the nature of the parameters to be evaluated. Some examples are shown in order to validate our conclusions.
110(2001); http://dx.doi.org/10.1121/1.1416200View Description Hide Description
This article reports the results of a series of measurements of the sound transmission loss of exterior wood stud walls. The measurements were made using standard laboratory procedures in which the walls were built between two reverberation chambers. The outdoor–indoor transmission class is used to rate the relative effectiveness of the various constructions. The measurement results are used to illustrate the influence of key parameters of the constructions on measuredsound transmission loss values and to give guidance for future designs. The overall sound insulation of these wood stud walls, to typical outdoor noises, is shown to be limited by two types of low-frequency resonances. An understanding of these low-frequency limitations can most effectively lead to superior sound insulation in similar wood stud walls.