The theory of recording and reproduction of three-dimensional sound fields based on spherical harmonics is reviewed and extended. Free-field, sphere, and general recording arrays are reviewed, and the mode-matching and simple source approaches to sound reproduction in anechoic environments are discussed. Both methods avoid the need for both monopole and dipole loudspeakers—as required by the Kirchhoff–Helmholtz integral. An error analysis is presented and simulation examples are given. It is also shown that the theory can be extended to sound reproduction in reverberant environments.

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Three-Dimensional Surround Sound Systems Based on Spherical Harmonics

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A High-Accuracy, Low-Latency Technique for Talker Localization in Reverberant Environments Using Microphone Arrays

As teleconferencing systems evolve to an ever more lifelike and transparent audio/video medium, it will be necessary to incorporate multichannel audio, which at a minimum involves two channels, i.e., stereophonic sound. However, before full-duplex stereophonic teleconferencing can be deployed, the acoustic echo cancellation (AEC) problem must be solved in this regime. This paper draws attention to a fundamental problem of multichannel AEC that concerns the nonunique nature of the estimated receiving room impulse responses. Various potential solutions are discussed but are shown to be not entirely satisfactory. >

Stereophonic acoustic echo cancellation-an overview of the fundamental problem

This paper presents a new method of designing a beamformer having a desired nearfield broadband beampattern. The methodology uses the spherical harmonic solution to the wave equation to transform the desired nearfield beampattern to an equivalent farfield beampattern. A farfield beamformer is then designed for a transformed farfield beampattern that, if achieved, gives the desired nearfield pattern exactly. Salient features of the new method are as follows. (i) The nearfield patterns can be achieved for all angles, not just the primary look direction. (ii) There is no theoretical restriction on the bandwidth. (iii) General array geometries may be used. As an illustration, we apply the method to the problem of producing a practical array design that achieves a nearfield beampattern that is frequency invariant over an octave bandwidth, where at the lowest frequency, the array-source separation is three wavelengths.

Broadband nearfield beamforming using a radial beampattern transformation

In this thesis, we develop computational tools for analyzing conversations based on nonverbal auditory cues. We develop a notion of conversations as being made up of a variety of scenes: in each scene, either one speaker is holding the floor or both are speaking at equal levels. Our goal is to find conversations, find the scenes within them, determine what is happening inside the scenes, and then use the scene structure to characterize entire conversations. 
We begin by developing a series of mid-level feature detectors, including a joint voicing and speech detection method that is extremely robust to noise and microphone distance. Leveraging the results of this powerful mechanism, we develop a probabilistic pitch tracking mechanism, methods for estimating speaking rate and energy, and means to segment the stream into multiple speakers, all in significant noise conditions. These features gives us the ability to sense the interactions and characterize the style of each speaker's behavior. 
We then turn to the domain of conversations. We first show how we can very accurately detect conversations from independent or dependent auditory streams with measures derived from our mid-level features. We then move to developing methods to accurately classify and segment a conversation into scenes. We also show preliminary results on characterizing the varying nature of the speakers' behavior during these regions. Finally, we design features to describe entire conversations from the scene structure, and show how we can describe and browse through conversation types in this way. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253-1690.)

Conversational scene analysis

The theory and the design of a logarithmically spaced microphone array with a spatial filtering capability are presented. The spatial filtering reduces the effects of reverberation and interfering noise sources that degrade the quality of sound pickup in teleconference systems. This feature is a result of the array's ability to discriminate against sound arrivals from all directions except that of the desired source

Fadi Khalil

Papers

Microphone array for sound pickup in teleconference systems

Annulation d'écho en téléconférence stéréophonique