Rendell R. Torres

Computation of edge diffraction for more accurate room acoustics auralization

Inaccuracies in computation and auralization of room impulse responses are related in part to inadequate modeling of edge diffraction, i.e., the scattering from edges of finite surfaces. A validated time-domain model (based on analytical extensions to the Biot-Tolstoy-Medwin technique) is thus employed here to compute early room impulse responses with edge diffraction. Furthermore, the computations are extended to include combinations of specular and diffracted paths in the example problem of a stage-house. These combinations constitute a significant component of the total nonspecular scattering and also help to identify edge diffraction in measured impulse responses. The computed impulse responses are then convolved with anechoic signals with a variety of time-frequency characteristics. Initial listening tests with varying orders and combinations of diffraction suggest that (1) depending on the input signal, the diffraction contributions can be clearly audible even in nonshadow zones for this conservative open geometry and (2) second-order diffraction to nonshadowed receivers can often be neglected. Finally, a practical implementation for binaural simulation is proposed, based on the singular behavior of edge diffraction along the least-time path for a given source-edge-receiver orientation. This study thus provides a first major step toward computing edge diffraction for more accurate room acoustics auralization.

Audibility of edge diffraction in auralization of a stage house

A recent model for edge diffraction based on physical acoustics was applied to the auralization of a source in a stage house. In this paper the results of two methods of comparison (computation of frequency responses and evaluation of binaural listening tests) are used to judge the audibility of diffraction effects associated with such geometries. The computed impulse responses are also compared via binaural listening tests with measured impulse responses from a physical scale model. Current results indicate that, from an auralization standpoint, first‐order diffraction is significantly more influential than second‐order diffraction, which can lie more than 45 dB below the direct or specularly reflected arrivals. Thus, higher‐order (and computationally more prohibitive) edge diffraction calculations may possibly be simplified or even neglected in auralization programs. [Work supported by Axel & Margaret Axson Johnsons Stiftelse, Sweden.]

Room acoustics auralization studies of laterally incident boss‐model scattering

Computation of room impulse responses by commercial auralization programs typically employs simplified Lambert models to account for nonspecular scattering. However, Lambert scattering models, developed primarily for diffuse lighting computation, have limited validity in acoustics. Instead, in this study, surface scattering from hemispherical bosses on walls is computed with numerical models based on exact classical solutions. Scattering impulse responses for auralization are calculated with various configurations of bosses on infinite planes on both sides of a binaural receiver (approximately 7–10 m away). The physical sound field is analyzed along with the resulting subjective effects of varying boss size and scatterer density.

The color of early sound arrivals in an auditorium

In computational room acoustics, most present‐day methods calculate specular reflections complemented by various models for surface scattering. A recent method of edge diffraction calculation [U. P. Svensson et al., J. Acoust. Soc. Am. 106, 2331–2344 (1999)], builds on the BTM technique which has been proven to be accurate in such diverse situations as noise control by barriers and sound scatter from the rough ocean surface. Here, it is applied to a large concert hall. The hall chosen is one of those in an International Round Robin study. It is shown that calculations of edge diffraction give quantitative details of what has until now been approximated and loosely called ‘‘scattering.’’ Here, the early part of the impulse response is considered in order to study aspects such as the frequency content of individual early diffractions and reflections and the low‐frequency energy distibution in the room. Only rigid surfaces, and up to second‐order edge diffraction and/or specular reflections are taken into ac...

Influence of surface scattering characteristics on the sound quality of reverberation

The influence of the scattered sound from different sound diffusing surfaces on the subjective characteristics of the late reverberation of a room was studied using a simulation approach. The impulse response of various scale model scatterers was measured for a set of angles and used in combination with a simple room model for auralization. As shown by Kleiner one can quite easily hear the differences between the scattered sound from different scatterers [M. Kleiner et al., Proc. 93rd Audio Eng. Soc. Convention, San Fransisco. Vol. 43, ‘‘Auralization of QRD and Other Diffusing Surfaces using Scale Modelling’’ (1992)]. The results obtained here, for the late reverberation, are not as clear, indicating that the ‘‘individual’’ sound of scattering surfaces will influence primarily the sound quality of the early part of the reverberation. [Work supported by RPI.]

Edge diffraction and scattering in the early room impulse response

Although reverberation is a common and important measure of quality in architectural acoustics, the early part of a room’s impulse response is equally critical, as its spectrum and transient structure predominantly determine the initial filtering (i.e., the natural coloration) that a room endows on an (otherwise anechoic) source signal. Since this initial room filtering could make the difference between an acoustical impression that is, for example, either ‘‘clear and defined’’ or ‘‘murky and unfocused,’’ various studies have been performed to better understand and model the effects of edge diffraction and surface scattering on the early reflections in room impulse responses. These studies include auralizations with varying ‘‘diffusion’’ coefficients, computations and measurements of edge diffraction in early room impulse reponses, and application of ‘‘boss models’’ to compute hemispherical scattering in room acoustics auralization. Various results show that these effects can be clearly audible and that t...

Scale‐model MLS measurements of edge diffraction

To better understand edge diffraction in room acoustics, scattering is measured from 1:10 scale models of reflector arrays and a stage house. The source is an 11‐cm dodecahedron with tweeter elements having an effective frequency range of 600–6000 Hz. Maximum‐length sequences (MLS) are used, which have various advantages over spark sources for scale models. The main three reflector arrays have approximately the same equivalent area: an array of five long rectangular panels, a 7‐by‐5 array of smaller square panels, and a single solid panel. Measurements are taken along a line under the reflector arrays and repeated for different array sizes and coverage. The stage‐house measurements include a stepped reflector array and receiver positions shadowed from the receiver. Results compare well with previous work [R. W. Leonard et al., J. Acoust. Soc. Am. 36, 2328–2333 (1964)], and some practical rules of thumb are discussed. [Work performed as guest of the Inst. fuer Techn. Akustik, RWTH‐Aachen, Germany; support ...

Considerations for including surface scattering in room‐acoustics auralization

Recent research in auralization has centered on including wave‐theoretical aspects that are unaccounted for by well‐developed but frequency‐limited geometrical models [Dalenback et al., J. Audio Eng. Soc. 42(10) (1994)]. For example, inclusion of edge diffraction (or edge scattering) has been shown (via scale‐model measurements and listening tests) to contribute audibly to the early part of the room impulse response [Torres and Kleiner, Proc. 16th Intl. Congress on Acoust. (1998)]. Modeling room surface scattering requires consideration of at least two main concerns: (1) characteristics of typical room surfaces and (2) types of numerical models most suitable for reasonable speed and accuracy. Typical room surfaces (e.g., in concert halls) can have a periodic nature at one wave number‐scale and a statistical nature at another. This suggests use of a multiscale, multiple‐scattering description possibly based on a combination of perturbation models, ‘‘boss’’ models, variational models, or higher‐order Kirchh...

Modification of Skudrzyk’s mean-value theory parameters to predict fluid-loaded plate vibration

The geometric-mean drive-point admittance (or “mobility”) of a complex structure is given by the admittance of the corresponding infinite structure (i.e., the “characteristic admittance,” Yc). The frequency response of an infinite plate, for example, coincides with the geometric-mean response of a finite one. Eugen Skudrzyk’s “mean-value theorem” was derived and experimentally verified without consideration of fluid loading. This paper shows that Skudrzyk’s method can be applied to fluid-loaded plates well below the coincidence frequency. Skudrzyk’s general mathematical expression allows simplified modifications that account for fluid loading and result in an approximate fluid-loaded characteristic admittance that differs only by a small multiplicative factor (<2u2009dB) from a correct analytical expression derived by Crighton.

Computation of impulse response for stage‐house geometries with use of edge‐diffraction models

A mathematical model following diffraction studies by Medwin [J. Acoust. Soc. Am. 72, 1005–1013 (1982)] is developed to calculate the impulse response of a three‐dimensional ‘‘stage house’’ geometry in a baffle. The model includes multiple diffraction components in addition to specular reflection, and it forms a more complete approach than current room impulse‐response predictors based on geometrical acoustics. Thus it may also be suitable for more accurate auralization, particularly at low frequencies. A comparison is made with results from a ‘‘cone‐tracing’’ computer model [B. Dalenback, J. Acoust. Soc. Am. 100, 899–909 (1996)] and results from physical scale‐model measurements. [Work supported by Axel & Margaret Axson Johnson’s Foundation, Sweden.]