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Toronto AES Bulletin

Mar 2002

Meeting Review

A Better Understanding of Stereo Microphone Techniques

Dr. Geoff Martin started his presentation by talking about different types of microphones for stereo micing. He talked in depth about omni directional, sub cardioid, cardioid, hyper-cardioid and bi-directional, illustrating the pressure component, pressure-gradient component and sensitivity equation for each type. He demonstrated a software program called 'Matlab' that created a 3 dimensional graph of each pickup pattern.

Dr. Martin likes to use omni-directional microphones for stereo micing. John Eargle also prefers omnis for micing which I used to find surprising, thinking that a directional pickup pattern would be preferred for critical micing applications. However, the advantage of an omni mic is that it is equally sensitive to sound sources from all directions. He introduced a concept I hadn't considered before, where will the sound(s) appear for the person listening to the recording. A sound coming from one speaker is a real image. When a sound comes from two speakers then a phantom image is created, however where does that phantom image appear and can that image spot be predicted? For the purposes of this presentation, a reference playback system is a pair of stereo speakers equidistant from and at a 30 degree angle to the listener.

Dr. Martin illustrated a table for location of phantom image location based on interchannel amplitude differences measured in milliseconds and interchannel time differences measured in dB. This table is based on work done by Michael Williams and Martin warned that these results will not be the same for all people. Different listeners and different acoustical spaces will find differences, however the results will be consistent for each person. By using amplitude and time differences one can place a sound anywhere in the 60 degree field between the two speakers.

Dr. Geoff Martin has a Bachelor of Music in pipe organ, is a graduate of McGill Masters program in Sound Recording and in 2001, completed his doctoral studies. A faculty lecturer at McGill for the Music technology area, he is a member of the development team for McGill's new Centre for Interdisciplinary Research in Music, Media and Technology (CIRMMT). Dr. Martin is the papers chairman for the upcoming 2003 International Conference of the AES on multichannel audio at Banff, Alberta.

The Toronto AES chapter would like to thank Dr. Geoff Martin for his presentation and Peter Cook for organizing the evening

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Originally posted: 26 March 2002
Last update: 26 March 2002