D. de Vries

Acoustic control by wave field synthesis

The acoustics in auditoria are determined by the properties of both the direct sound and the later arriving reflections. If electroacoustic means are used to repair disturbing deficiencies in the acoustics, one has to cope with unfavorable side effects such as localization problems and artificial impressions of the reverberant field (electronic flavor). To avoid those side effects, the concept of electroacoustic wave front synthesis is introduced. The underlying theory is based on the Kirchhoff–Helmholtz integral. In this new concept the wave fields of the sound sources on stage are measured by directive microphones; next they are electronically extrapolated away from the stage, and finally they are re‐emitted in the hall by one or more loudspeaker arrays. The proposed system aims at emitting wave fronts that are as close as possible to the real wave fields. Theoretically, there need not be any differences between the electronically generated wave fields and the real wave fields. By using the image source...

A new method to acquire impulse responses in concert halls

A new method is proposed to acquire impulse responses in concert halls with large signal‐to‐noise ratios and with a high resolution. In our proposal an omnidirectional loudspeaker is used which is driven by an amplified sweep: a signal containing all frequencies of interest smeared out in time. By using a deconvolution technique, an almost perfect pulse is obtained with a high peak pressure and a short effective duration. Measurements were made in two different concert halls to illustrate the practical implications of the new technique.

Array technology for acoustic wave field analysis in enclosures

A method is proposed to calculate and measure impulse responses in an enclosure along closely spaced receiver arrays. Hence, instead of using a sparse distribution of receiver positions with single microphones, as is common practice now, arrays of microphones are applied to register the complex sound fields within enclosures. This way, there is a strong spatial correlation between adjacent responses, enabling one to analyze individual reflected wavefronts. It is shown that visualization of the recorded data in a two-dimensional domain, defined by detector position and travel time, gives a significantly improved insight in the structure of complex sound fields. This insight is further increased by applying the linear Radon transform (plane wave decomposition), yielding a representation of the data in the so-called ray parameter versus intercept time domain.

Velocity analysis based on minimum entropy

Seismic resolution is determined by the sparsity of reflection events together with the dispersion of the wavelets representing those events. In this paper, minimum entropy (ME) norms are introduced as a measure of spatial resolving power. It is shown that the lateral dispersion of inverted diffractor responses (inverted spatial wavelets) increases with increasing velocity error. Using this property, minimum entropy velocity analysis (MEVA) is proposed to extract velocity information from diffraction energy. MEVA can be successfully applied to zero-offset (including poststack) data and common-offset data with a sufficient amount of diffraction energy. In addition, MEVA can be used as an alternative to existing CMP velocity estimation techniques.

A wave field extrapolation approach to acoustical modeling in enclosed spaces

In practice, impulse responses in enclosed spaces are calculated with algorithms that are based on the ray tracing model, the mirror image source model, or a mixture of both. Using those algorithms, however, the wave character of sound propagation is not properly taken into account and complex boundaries cannot be included. This paper presents an alternative approach adopted from the seismic imaging field. The proposed model is based on the concept of wave field extrapolation. Absorption properties of the boundaries can be specified in detail by means of reflection matrices. Propagation between boundaries is formulated by propagation matrices. Impulse responses along an array of receivers are numerically simulated by executing a sequence of matrix multiplications that can be interpreted as generalized spatial convolutions. Diffraction phenomena due to the finiteness and irregularity of the boundaries are correctly taken into account. The proposed algorithm is illustrated on a two-dimensional configuration...

Acoustic imaging in enclosed spaces: Analysis of room geometry modifications on the impulse response

Sound propagation in enclosed spaces is characterized by reflections at the boundaries of the enclosure. Reflections can be wanted in the case when they support the direct sound or give a feeling of envelopment or they can be unwanted when they lead to echoes and colouration. When measuring multiple impulse responses in an enclosed space along an array the reflections can be mapped to the reflecting objects. Similar to seismic exploration, medical diagnostics, and underwater acoustics, an image of the reflecting objects is obtained in terms of reflected energy. The imaging process is based on inverse wave field extrapolation with the Kirchhoff–Helmholtz and Rayleigh integrals. The inverse of the imaging process recreates the measured impulse responses from the image and it allows one to remove or alter reflecting objects in the image and investigate their influence on the wave field in the enclosed space in a physically correct way. This can be verified by reimaging the altered wave field. Preliminary res...

Quantification of Uncertainty in Transfer function Estimation: A Mixed Deterministic-Probabilistic Approach

Abstract In this paper a procedure is presented to obtain an estimate of the transfer function of a linear system together with a confidence interval, using only limited a priori information. By applying Bartletts procedure of periodogram averaging to the non-parametric empirical transfer function estimate, and by employing a periodic input signal, the statistics of the resulting estimate asymptotically can be obtained from the data. The model error consists of two parts: a probabilistic part, due to the stochastic noise disturbance on the data, and a deterministic part, due to the bias in the estimate. The latter is explicitly bounded with a deterministic error bound, while the former asymptotically results from a F distribution. For this analysis only minor assumptions are made on the distribution of the noise.

An Approach to Tissue Characterization Based on Wave Theory Using a New Velocity Analysis Technique

In this paper we will discuss an approach to tissue characterization which was developed within our group. First our approach will be outlined.One of the basic steps we use here is a high resolution Synthetic Focusing procedure (Berkhout et al.,1982). For this procedure, and also for tissue characterization itself, correct knowledge of the sound propagation velocity is essential. In the second part of this paper a method will be put forward to determine values of the sound propagation velocity with little prior knowledge of the medium.This method is based on the minimization of the entropy of the synthetically focused image. Now an optimally focused image can be produced,which may be analysed in detail.In the final part of this paper we will discuss a method of analysis,based on two dimensional Fourier transformation of the r.f.data of part of the image.

Deriving content-specific measures of room acoustic perception using a binaural, nonlinear auditory model

Acousticians generally assess the acoustic qualities of a concert hall or any other room using impulse response-based measures such as the reverberation time, clarity index, and others. These parameters are used to predict perceptual attributes related to the acoustic qualities of the room. Various studies show that these physical measures are not able to predict the related perceptual attributes sufficiently well under all circumstances. In particular, it has been shown that physical measures are dependent on the state of occupation, are prone to exaggerated spatial fluctuation, and suffer from lacking discrimination regarding the kind of acoustic stimulus being presented. Accordingly, this paper proposes a method for the derivation of signal-based measures aiming at predicting aspects of room acoustic perception from content specific signal representations produced by a binaural, nonlinear model of the human auditory system. Listening tests were performed to test the proposed auditory parameters for both speech and music. The results look promising; the parameters correlate with their corresponding perceptual attributes in most cases.

Identification and Control of a Compact Disc Mechanism using Fractional Representations

Abstract This paper discusses the approximate and feedback relevant parametric identification of the radial servo system present in a Compact Disc player. In this application the problem of approximate identification based on data from closed loop experiments will be analyzed to find a finite dimensional linear time invariant discrete time model, suitable for model-based control design. The feedback relevant identification in this paper is based on the algebraic theory of fractional representations, which has lead to a framework for equivalent open loop identification of (normalized) coprime plant factors and a manageable approximate transfer function estimation of the feedback controlled plant. A mixed worst-case/probabilistic approach to model uncertainty quantification is used to construct an upper bound on the uncertainty of the normalized coprime factors being estimated. Both the nominal model and the uncertainty description are used to design an enhanced robust controller.