Direct Sound Field Research Articles

Coupled auralization and virtual video for immersive multimedia displays

The implementation of maximally-immersive interactive multimedia in exhibit spaces requires not only the presentation of realistic visual imagery but also the creation of a perceptually accurate aural experience. While conventional implementations treat audio and video problems as essentially independent, this research seeks to couple the visual sensory information with dynamic auralization in order to enhance perceptual accuracy. An implemented system has been developed for integrating accurate auralizations with virtual video techniques for both interactive presentation and multi-way communication. The current system utilizes a multi-channel loudspeaker array and real-time signal processing techniques for synthesizing the direct sound, early reflections, and reverberant field excited by a moving sound source whose path may be interactively defined in real-time or derived from coupled video tracking data. In this implementation, any virtual acoustic environment may be synthesized and presented in a perceptually-accurate fashion to many participants over a large listening and viewing area. Subject tests support the hypothesis that the cross-modal coupling of aural and visual displays significantly affects perceptual localization accuracy.

ACTIVE CONTROL OF ENVIRONMENTAL NOISE, VI: PERFORMANCE OF A FUNDAMENTAL FREE-FIELD SOUND CANCELLING SYSTEM

Before the adaptive cancellation performance of multichannel free-field acoustic systems could be evaluated in detail, it was considered prudent to establish first the performance of a single channel free-field cancelling system. The adaptive theory for this basic system, including the stability process, is described. Measured adaptive performance is given confirming the theory.The concept of stability bands and their prediction is considered. The bands are a consequence of phase differences between the primary source and secondary cancelling field, generated by the transfer functions of the control system. At the edge of these stability bands satellite pole frequencies “beat” with the cancelling frequency, generated by the zeros, to produce side bands. The relation between the stability bandwidth and the adaptive speed, in terms of reference signal strength and adaptive step size, are investigated.The sound generation and cancellation performance of this basic canceller are then considered. The acoustic sound field directivity characteristics are similar to that of a dipole, tripole or quadrupole type source, depending on the primary–secondary source separation distance. The acoustic shadow characteristics are established in detail and compared with measurement. Good agreement is obtained.

Studying room acoustics using a monopole-dipole microphone array

The use of a sound field microphone for examining the directional nature of a room impulse response was reported recently [R. Essert, J. Acoust. Soc. Am. 100, 2837(A) (1996)]. By cross-correlating monopole and colocated dipole microphone signals aligned with left–right, up–down, and front–back axes, a sense of signal direction of arrival is revealed. The current study is concerned with the arrays ability to detect individual reflections and directions of arrival, as a function of the cross-correlation window duration. If this window is too long, weak reflections are overlooked; if too short, spurious detections result. Guidelines are presented for setting the window width according to perceptual criteria. Formulas are presented describing the accuracy with which direction of arrival can be estimated as a function of room specifics and measurement noise. The direction of arrival of early reflections is more accurately determined than that of later reflections which are quieter and more numerous. The transition from a fairly directional sound field at the beginning of the room impulse response to a unidirectional diffuse field is examined. Finally, it is shown that measurements from additional dipole orientations can significantly improve the ability to detect reflections and estimate their directions of arrival. [Work supported by NASA.]

An analytical model for bandlimited response of acoustic-structural coupled systems. I. Direct sound field excitation

An analytical model is proposed in this paper for predicting time-averaged energies of boundary structures and enclosed sound field. The sound field is directly driven by an acoustic source and the structures are excited through acoustic-structural coupling. The present model is based on the classical modal coupling method but is developed in such a way to improve the computational efficiency in estimating the bandlimited response of acoustic-structural coupled systems particularly in the medium frequency range where a large number of acoustic and/or structural modes is involved. In this frequency range, one often has to deal with manipulation of large complex matrices in order to obtain full mathematical solutions to the system response using the classical modal coupling method. However, this is avoided if the present bandlimited model is used. This paper describes the mathematical development of the model. Numerical examples for a panel-cavity system are presented and the proposed model is compared with the classical modal coupling method in terms of computational efficiency and accuracy in the prediction of the system energies.

Sound system with source material and surround timbre response correction, specified front and surround loudspeaker directionality, and multi-loudspeaker surround

Spectral imbalance (alteration in timbre) when playing home video versions of motion pictures is overcome by re-equalization according to a unique correction response curve which compensates for the equalization for playback in large theater-sized auditoriums inherent in motion picture soundtracks. Surround-sound home playback of motion pictures is enhanced by employing main channel loudspeakers that produce generally direct sound fields and surround channel loudspeakers that produce gnerally diffuse sound fields. In addition, the reproduced surround-sound channel is further enhanced by decreasing the interaural cross-correlation of the surround-sound channel sound field and by reducing comb filtering effects in the surround-sound channel at listening positions within the room, preferably by introducing slight pitch shifting in the signals applied to multiple surround loudspeakers. Preferably, further equalization is applied to the reproduced surround channel to compensate for the differences in listener perceived timbre between the surround-sound channel and the main channels.

Reflecting sound imaging speaker enclosure

The direct and reverberant sound fields heard at a live musical performance are reproduced by a speaker enclosure consisting of two sub-enclosures mounted on a base with each subenclosure housing at least one speaker, thereby realistically reproducing the music.

Noise radiation by sources near to plane reflecting surfaces

The object of this investigation was to gain information and understanding sufficient to enable the sound fields of noise sources close to room boundaries to be described. The effects of nearby reflecting surfaces on the sound power output and directional radiation of noise sources have been studied, and an existing theory for the far field case has been developed to cover near field directional radiation patterns. For a number of simple sources, practical measurements have been in good agreement with theory for the sound power output of the source, the variation of sound pressure level with distance from the source, and the qualitative determination of near field directional radiation patterns. The information obtained from this work is applicable to noise sources in practical environments such as domestic living rooms, where the direct sound field produced by a gas fire installed in a hearth with hard reflecting surfaces is analogous to that produced by an experimental source close to two perpendicular reflecting planes in an otherwise free field.

Experimental study of high velocity air discharge noise from some ventilating ducts and elbows

The air discharged by a 12-in ‘Aphonic’ (quiet) radial flow fan, further quietened by long oversize felt-lined ducts and package attenuators, was passed into a large acoustically-deadened test chamber through straight round (4–10 in diam.) ducts and rectangular ducts of equivalent cross-sectional areas. The ducts were open-ended and, in the case of the round ducts, also fitted with 90° short and long elbows with and without terminating ducts. Duct velocities up to 4000 ft/min were attained in some cases by controlling fan speed. The A-weighted sound levels were measured in each case at 2 ft from the center of the exit section at an angle of 45°, and in some cases also at 3 ft for comparison. The object was to make measurements in the direct sound field of the exit plane. Relations between duct velocities and measured sound levels are evaluated over the ranges tested.

Probable Deviation Range in an Anechoic Room from the Values Given by the Inverse-Distance Law in Free Space

The sound field in an anechoic room is not exactly the same as that in free space, for it can be divided into two distinct parts, the direct sound field of the source and the reverberant sound field. The probable deviations of the sound pressure in the room from the values given by the inverse-distance law in free space are considered by means of image statistics. The deviations at any point are dependent on the ratio of the energy density of the reverberant sound to the energy density of the direct sound. For all points in the room having the same ratio of energy densities, the magnitude of the probable deviations is the same. In case of a point source, the probable deviation range A is given by the approximate formula: A ≐ 40rRS−12 db, where r=distance from the source in m, R=sound-reflection factor of the boundaries of the room in percent, S=surface of the room in m2. The approximation holds only for the small deviation range A⩽±1 db; for a bigger deviation range, a diagram is given. For directional loudspeakers and microphones, the probable deviation range depends furthermore on its directivity factor.