Yuvi Kahana

A multiple microphone recording technique for the generation of virtual acoustic images

A new recording technique based on multichannel digital signal processing is suggested. The system uses a dummy-head that is modeled as a rigid sphere with two pairs of microphones mounted on opposite sides of the sphere in the horizontal plane. Reversals—front back confusion, is a well-known phenomenon when localizing virtual acoustic images produced by either headphones or loudspeakers. Reproduction with two loudspeakers to the front of the listener causes rear virtual acoustic images to be perceived primarily at “mirrored” angles in the frontal hemisphere. The problem is tackled here by using a multichannel signal processing technique rather than by mimicking accurately the acoustomechanical properties of a human head. The acoustic signals which are recorded at the microphones are filtered by a 4×4 matrix of digital filters before being transmitted via four loudspeakers. The performance of the system is investigated by means of computer simulations, objective measurements, and also by subjective experiments in an anechoic environment, where the listeners are asked to localize the perceived angle of the signals which were prerecorded with the sphere dummy-head. Successful discrimination of reversals is achieved primarily due to the dominant role of the interaural time delay (ITD) for localization at low frequencies, but the accuracy with which listeners can localize virtual acoustic images is reduced in comparison to a conventional two-ear dummy-head (e.g., KEMAR) with a two-loudspeaker arrangement. The system is robust with respect to head rotations—virtual acoustic images do not disappear and localization ability improves when listeners use small head rotations.

Factors in the performance of systems for the production of virtual acoustic environments

This paper will describe recent progress in the development of systems designed to present, at the ears of a listener, the signals necessary to produce the illusion in the listener of the existence of a ‘‘virtual’’ source of sound. Attention will be restricted to systems which use a relatively small number of loudspeakers whose input signals are determined by optimal processing of the signal to be associated with the virtual source. It will be demonstrated that the form of the sound field produced is crucial to the success of such systems. Emphasis will be given to a system that uses two very closely spaced loudspeakers to transmit a particular form of sound field which most easily produces the iteraural time delay associated with a given virtual source position. Results will be presented of subjective experiments and of numerical simulations of the sound field. Factors determining the system performance will be discussed, particularly with regard to front–back confusions and individual differences in hea...