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Electroacoustic Systems for 3-D Audio – A report from the Pittsburgh meeting

Our binaural hearing systems are immediately exposed to spatial sound fields at birth (and arguably before birth). As we begin to process acoustic signals, we learn to use our binaural hearing to analyze the sound field to which we are exposed. Human hearing responds to sound pressure, which is a scalar field having three spatial dimensions. Thus, as we strive for better recording and reproduction of sound, it is reasonable that we move towards audio systems that address our natural binaural ability and build systems that recreate real three dimensional sound fields. Stereo audio has been in existence for more than 70 years and is still the most common technique used for audio recording and playback. In the mid 1970’s, with the advent of quadraphonic audio systems, there was an attempt to increase the number of playback channels to enhance the spatial aspects of the sound field. However, quadraphonic systems were not widely accepted and quickly disappeared. The explosive growth of DVD players for home theater systems that allow for 5.1 and 6.1 channel (and more) playback using Dolby TM and DTSTM multichannel systems has led to a wider acceptance and appreciation for surround-sound audio. This short article describes some of the current research in this area with emphasis on the papers presented at a special session on 3-D audio at the ASA Pittsburgh meeting in June 2002. One advance in the recording of sound-fields has its origins in work done in the early 1970’s by Gerzon. A basic innovation in this work was to expand on the stereo microphone techniques invented by Blumlein. Gerzon proposed a tetrahedral arrangement of microphones, where the sound field is separated by appropriately combining 4 first-order differential microphones. The term “first-order differential” relates to the beamforming process where the beam pattern can be described by the combination of an omnidirectional term and a first-order difference between closely-spaced pressure microphones. The first-order term can be associated with the acoustic particle velocity due the Euler’s equation that relates the acoustic particle velocity to the gradient of the pressure field. By combining the omnidirectional term along with the pressure gradient term, a general steerable first-order microphone can be formed. This microphone has been commercially produced as the Soundfield microphone. Although first-order sound field microphones are useful for recording of the spatial sound field, they are probably still too broad in their spatial response for more accurate playback systems. At the Pittsburgh meeting, Meyer and Elko presented some new work on a spherical microphone array that further extends the previous work by allowing any desired steerable beamformer to be realized. This paper showed how to realize an efficient modal beamformer based on the orthonormal decomposition of the incident pressure sound field into spherical harmonics. Since this decomposition is similar in spirit to standard eigen-analysis techniques, the system was named: “eigenbeamformer” or EigenmikeTM. By choosing to place pressure microphones on a rigid sphere, it was shown that the decomposition allows the recording of all desired spherical harmonic components in the desired passband, contrary to results that would be obtained had one used an open sphere (where the microphones are placed in free space). A spherical Electroacoustic Systems for 3-D Audio – A report from the Pittsburgh meeting