The acoustic feedback problem has intrigued researchers over the past five decades, and a multitude of solutions has been proposed. In this survey paper, we aim to provide an overview of the state of the art in acoustic feedback control, to report results of a comparative evaluation with a selection of existing methods, and to cast a glance at the challenges for future research.

Fifty Years of Acoustic Feedback Control: State of the Art and Future Challenges

The sensitivity of listeners to small changes in the early sound field of auditoria has been measured. This was done using a realistic artificial simulation system of a concert ball sound field. The simulation system was designed so that standard objective parameter values were typical of those found in real halls, and within ranges known to be subjectively preferred. The following difference limens were measured: early lateral energy fraction; inter-aural cross correlation coefficient; clarity index and centre time. From these results it was shown that when changes are made to the early sound field, changes in perceived spatial impression will usually be larger than those for clarity. Furthermore it was found that acousticians can gain most by paying attention to lateral sound. Other measurements showed that: (i) the initial time delay gap is not significant to listener preference, and (ii) diffuse reflections in the early sound field are not perceived differently from specular reflections.

The sensitivity of listeners to early sound field changes in auditoriums

The directional variation of sound at a point has been studied in three rooms, using a measurement system described previously [J. Acoust. Soc. Am. 112, 1980–1991 (2002)]. The system uses a pair of 32-element spherical microphone arrays to obtain directional impulse responses in each of 60 steering directions, with an angular resolution of 28°, covering all directions in the whole solid angle. Together, the array measurements span the frequency range from 300 to 3300 Hz. The angular distribution of incident sound energy is visualized on a three-dimensional plot, and quantified by computing the directional diffusion and the directional peak-to-average level difference (“anisotropy index”) of the sound field. The small-to-medium-sized rooms had reverberation times of 360, 400, and 600 ms. Measurements were made for several source and receiver locations in each, and were analyzed over several time ranges (full decay time of room, late time decay, 2-ms windows throughout the decay). All measured sound fields ...

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Measurements of directional properties of reverberant sound fields in rooms using a spherical microphone array

Principles of binaural room simulation

When we estimate a sound field in a room, it is important to grasp the spatial informa-tion, especially of the early reflection periods. In this paper we'll discuss a way to grasp the spatial information of sound fields from impulse responses measured at closely located four points, the origin and three points of the same distance (3∼5cm) from the origin on the rectangular coordinate axes. From these four impulse responses the coordinates and powers of virtual image sources are calculated by correlation technique or intensity technique. Concert halls, opera theaters and many other sound fields are measured by this technique. The distributions of virtual image sources and directivity patterns of some concert halls are shown.

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Measurement of spatial information in sound fields by closely located four point microphone method.

In this paper, we describe some new sound field measurement methods by using a laser Doppler vibrometer (LDV). By irradiating the reflection wall with a laser, we can observe the light velocity change that is caused by the refractive index change from the change in air density. It means that it is possible to observe the change of the sound pressure. We measured a sound field projection on a 2D plane using a scanning laser Doppler vibrometer (SVM) which can visualize a sound field. And we made a 3D sound field reconstruction from some 2D laser projections based on computed tomography (CT) techniques. We made the reconstructed image for the sound field near the loudspeaker or in the room.

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Sound field measurements based on reconstruction from laser projections

Parametric loudspeakers are known for a very sharp directivity due to their ultrasonic carrier wave. A desired audio signal modulated onto this carrier wave is reproduced along the beam by intrinsic non-linearity of the air. Because of this directivity, parametric speakers provide for distinguished audio applications. In this paper we present our parametric array consisting of 576 ultrasonic transducers controlled individually by using high speed 1-bit signal processing. We are able to control the angle of an audio beam by creating a synthesized surface wavefront. Furthermore, we present an experiment where multiple independent audio beams are emitted from the single parametric loudspeaker.

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Acoustic Projector Using Directivity Controllable Parametric Loudspeaker Array

An investigation was made of cross‐correlation functions with transient signals between two points, which correspond to both ears of a listener in a room. The interaural cross correlation is closely related to the subjective impressions of sound fields. The study attempted to account in a comprehensive way for the combined effects that initial reflected and reverberation sounds from music or other transient signals have on such impressions. To this end, cross‐correlation functions of the rise and fall of the sound field from transient signals were derived from the impulse responses at two points in the hall. These results were combined with image‐sources distribution patterns derived by the closely located four‐point‐microphone method; then, a comparative explanation was made of the changes with transient time duration of the cross‐correlation functions. Good agreement was found between changes with time in experimentally derived maximal cross‐correlation function values and the changes with time in image‐sources distribution of the sound field.

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Effect of transient signal length on cross‐correlation functions in a room

Visualizations help us to understand the sound field behavior. A well-known method of sound field visualization is Kunt’s experiment which visualizes standing waves using light particles. Comprehending of both accurate and transient information on sound fields requires measurement of information at multiple points, and also their visualization. Microphones are commonly used to implement such measurements, which means that numerous microphones are needed. On the other hand, Laser Doppler Vibrometer (LDV) can be used to measure an average sound pressure over a laser path. We have conducted fundamental research on sound field measurement and visualization using LDV and Computed Tomography (CT) without ordinary microphones. The measured value contains integrated information on the laser path. If we have data on an area measured from all directions using LDV, we can estimate the sound field in the area without having to measure at many points using microphones. The new technology of observing sound field and vibration is proposed and being put to practical use in this way. The sound field is generated by sound source and it is important to know the relationship between sound field and sound source. It is very useful to observe both sound field and sound source vibration simultaneously. In this paper, we describe the integrated visualization of sound field and sound source vibration using 3D laser measurement method. We used Processing programming language in order to realize the interactive visualization of sound field and sound source vibration. In addition, we conducted an experimental measurement of impulse responses with laser CT and Time Stretched Pulse (TSP) signal.

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Yoshio Yamasaki

Papers

Measurement of spatial information in sound fields by closely located four point microphone method.

Sound field measurements based on reconstruction from laser projections

Acoustic Projector Using Directivity Controllable Parametric Loudspeaker Array

Effect of transient signal length on cross‐correlation functions in a room

Visualization of sound field and sound source vibration using laser measurement method