Michael Vorländer

Simulation of the transient and steady‐state sound propagation in rooms using a new combined ray‐tracing/image‐source algorithm

A new method for the calculation of room acoustical impulse responses is described, which is based on two well‐known computer algorithms, the ray‐tracing and the image‐source models. With the new method, the procedure of sieving the ‘‘visible’’ image sources out of the enormous quantity of possible sources is carried out by examination of the histories of sound particles. From the obtained list of visible image sources, the impulse response of the enclosure is easily constructed. The new method combines the advantages of the ray‐tracing process, namely, the relatively slow increase of computation time with the length of the impulse response, with the accuracy inherent to the image‐source model, which is even sufficient to calculate the Fourier transform, i.e., the steady‐state transmission function of the room, or to convolve the impulse response with sound signals.

Virtual reality system with integrated sound field simulation and reproduction

A real-time audio rendering system is introduced which combines a full room-specific simulation, dynamic crosstalk cancellation, and multitrack binaural synthesis for virtual acoustical imaging. The system is applicable for any room shape (normal, long, flat, coupled), independent of the a priori assumption of a diffuse sound field. This provides the possibility of simulating indoor or outdoor spatially distributed, freely movable sources and a moving listener in virtual environments. In addition to that, near-to-head sources can be simulated by using measured near-field HRTFs. The reproduction component consists of a headphone-free reproduction by dynamic crosstalk cancellation. The focus of the project is mainly on the integration and interaction of all involved subsystems. It is demonstrated that the system is capable of real-time room simulation and reproduction and, thus, can be used as a reliable platform for further research on VR applications.

Switching in the cocktail party: exploring intentional control of auditory selective attention.

Using a novel variant of dichotic selective listening, we examined the control of auditory selective attention. In our task, subjects had to respond selectively to one of two simultaneously presented auditory stimuli (number words), always spoken by a female and a male speaker, by performing a numerical size categorization. The gender of the task-relevant speaker could change, as indicated by a visual cue prior to auditory stimulus onset. Three experiments show clear performance costs with instructed attention switches. Experiment 2 varied the cuing interval to examine advance preparation for an attention switch. Experiment 3 additionally isolated auditory switch costs from visual cue priming by using two cues for each gender, so that gender repetition could be indicated by a changed cue. Experiment 2 showed that switch costs decreased with prolonged cuing intervals, but Experiment 3 revealed that preparation did not affect auditory switch costs but only visual cue priming. Moreover, incongruent numerical categories in competing auditory stimuli produced interference and substantially increased error rates, suggesting continued processing of task-relevant information that often leads to responding to the incorrect auditory source. Together, the data show clear limitations in advance preparation of auditory attention switches and suggest a considerable degree of inertia in intentional control of auditory selection criteria.

The impact of background speech varying in intelligibility: Effects on cognitive performance and perceived disturbance.

Noise abatement in office environments often focuses on the reduction of background speech intelligibility and noise level, as attainable with frequency-specific insulation. However, only limited empirical evidence exists regarding the effects of reducing speech intelligibility on cognitive performance and subjectively perceived disturbance. Three experiments tested the impact of low background speech (35 dB(A)) of both good and poor intelligibility, in comparison to silence and highly intelligible speech not lowered in level (55 dB(A)). The disturbance impact of the latter speech condition on verbal short-term memory (n = 20) and mental arithmetic (n = 24) was significantly reduced during soft and poorly intelligible speech, but not during soft and highly intelligible speech. No effect of background speech on verbal-logical reasoning performance (n = 28) was found. Subjective disturbance ratings, however, were consistent over all three experiments with, for example, soft and poorly intelligible speech rated as the least disturbing speech condition but still disturbing in comparison to silence. It is concluded, therefore, that a combination of objective performance tests and subjective ratings is desirable for the comprehensive evaluation of acoustic office environments and their alterations.

Practical aspects of MLS measurements in building acoustics

Abstract The maximum length sequence (MLS) technique is established in various acoustic measurements for fast and accurate broadband determination of acoustic quantities. In this paper a general overview of the benefits and limitations of the MLS technique in building acoustics is presented. Further-more, an approach is described to investigate the influence of time variances on the use of synchronous averaging. It is shown that averaging with the presence of time variances causes an apparent level loss. Time variances are investigated specifically in the context of temperature drift in a room and of wind fluctuations under free field conditions. For both cases formulae are derived to enable error estimation for practical cases. The theory is validated by experimental results.

Computer simulations in room acoustics: Concepts and uncertaintiesa)

Geometrical acoustics are used as a standard model for room acoustic design and consulting. Research on room acoustic simulation focuses on a more accurate modeling of propagation effects such as diffraction and other wave effects in rooms, and on scattering. Much progress was made in this field so that wave models also (for example, the boundary element method and the finite differences in time domain) can now be used for higher frequencies. The concepts and implementations of room simulation methods are briefly reviewed. After all, simulations in architectural acoustics are indeed powerful tools, but their reliability depends on the skills of the operator who has to create an adequate polygon model and has to choose the correct input data of boundary conditions such as absorption and scattering. Very little is known about the uncertainty of this input data. With the theory of error propagation of uncertainties it can be shown that prediction of reverberation times with accuracy better than the just noticeable difference requires input data in a quality which is not available from reverberation room measurements.

Acoustic centering of sources measured by surrounding spherical microphone arrays

The radiation patterns of acoustic sources have great significance in a wide range of applications, such as measuring the directivity of loudspeakers and investigating the radiation of musical instruments for auralization. Recently, surrounding spherical microphone arrays have been studied for sound field analysis, facilitating measurement of the pressure around a sphere and the computation of the spherical harmonics spectrum of the sound source. However, the sound radiation pattern may be affected by the location of the source inside the microphone array, which is an undesirable property when aiming to characterize source radiation in a unique manner. This paper presents a theoretical analysis of the spherical harmonics spectrum of spatially translated sources and defines four measures for the misalignment of the acoustic center of a radiating source. Optimization is used to promote optimal alignment based on the proposed measures and the errors caused by numerical and array-order limitations are investigated. This methodology is examined using both simulated and experimental data in order to investigate the performance and limitations of the different alignment methods.

Virtual reality for architectural acoustics

Over the last decades, powerful prediction models have been developed in architectural acoustics, which are used for the calculation of sound propagation in indoor and/or outdoor scenarios. Sound insulation is predicted rather precisely by using direct and flanking transmission models of sound and vibration propagation. These prediction tools are already in use in architectural design and consulting. For the extension towards virtual reality (VR) systems, it is required to accelerate the prediction and simulation tools significantly and to allow an adaptive and interactive data processing during the simulation and 3D audio stimulus presentation. This article gives an overview on the current state-of-the-art of acoustic VR and discusses all relevant components in terms of accuracy, implementation and computational effort. With the progress in processing power, it is already possible to apply such VR concepts for architectural acoustics and to start perceptual studies in integrated architectural design processes.

Relationship Between the Scattering Coefficients Determined with Coherent Averaging and with Directivity Correlation

There exist three kinds of methods to determine the scattering coefficients of architectural surfaces: I) the reverberation room method, using coherent averaging of room impulse responses during sample rotation, 2) the free-field method, using coherent averaging of reflected responses during sample rotation, 3) the free-field method, using directivity correlation between reflected fields with a sample and with a flat reference surface. This paper discusses the relationship among the scattering coefficients determined by the three methods. Numerical simulation demonstrates that the two free-field methods give little different values at mid and high frequencies for a 1D sinusoidal surface, but not to the same extent for a 2D sinusoidal surface. Subsequently, this disagreement is examined in comparison with experimental results obtained by the reverberation room method and with theoretical ones. Theoretical consideration reveals that the dominant reason for the disagreement is due to different averaging processes in the three methods, which take the arithmetic mean or the quadratic mean of specular reflection factors with respect to azimuthal incidence angle, respectively.

Acoustic centering of sources with high-order radiation patterns

Surrounding spherical microphone arrays have recently been used in order to model the radiation pattern of acoustic sources that are assumed to be at the center of the array. Source centering algorithms are applied to the measurements in order to reduce the negative effect of acoustic source misalignment with regard to the physical center of the microphone array. Recent works aim to minimize the energy that is contained in the high-order coefficients of the radiation pattern in the spherical harmonics domain, in order to directly address the problem of increased order and spatial aliasing resulted by this misalignment. However, objective functions which directly minimize the norm of these coefficients were shown to be convex only when employed on sources with low-order radiation patterns. This work presents a source centering algorithm that operates on plane sections and aims to achieve a convex objective function on every plane section. The results of the proposed algorithm are shown to be more convex than the previous algorithms for sources with higher-order radiation pattern, usually at higher frequencies.