Audio Engineering Society

Chicago Section

Meeting Review, November 16, 2004

other meeting reports

11/16/04 Meeting Highlights
by Jeff Segota

The November 16th meeting of the Chicago Section featured a presentation on surround sound systems and associated recording techniques by distinguished guest John Eargle. Well-known as an author of several audio engineering books, recording engineer and/or producer of over 280 compact disks, and for his decades of product development experience, Mr. Eargle drew a large and enthusiastic crowd of 68 attendees. This was our first dinner/presentation meeting of the season.


Recent developments in microphone technologies and practice for surround sound fall broadly into four categories. In a stereo-derived approach such as 5.1, the loudspeakers create both real and phantom images as well as de-correlated ambience. Phantom images are created in the same method as in stereo, and it is important to avoid positioning an

image both as phantom between front left and right and directly in the center. Regarding ambience reproduction, it was shown via inter-aural correlation measurements that with proper recording technique, four speakers in a living room can replicate a very live, large space to a fairly close degree. Several near-coincidence microphones have been used in practice, as well as the Trinnov Azimuthal Array and the Gerzon 2-3 Martrix.



A different approach is the Mapping Technique. An array of directional microphones is used to pickup sound from all three directions, with each mic covering an equal sector. For playback, a loudspeaker for each microphone/channel is placed in the location of each mic's maximum pickup axis. The minimum number of microphones required is four of the first-order type, and the Soundfield Microphone is an example. More channels requires higher order microphones, which can be created virtually from an array of omnidectional mics as in the Eigenmike system. The mapping technique is probably most suited for special venues and in conjunction with visual presentations.


In the transaural technique, binaural reproduction is achieved for a carefully placed listener with just two loudspeakers. This method requires cross-talk signals to be canceled, which is complex but the technique can be expanded to enable panning in post processing.


The final subject, systems with parallax, generated the most audience discussion. A large array of microphones and a "wall of speakers" are used to reproduce the original three-dimensional acoustic field of a source, and thus the image location is independent of listener location. This is the cutting edge of directional sound reproduction, with Delft Technical University and the Fraunhofer Institute making major contributions.