Recording Ambience In Surround

Michael Williams has dramatically influenced my recording techniques for stereo and multi-channel. His seminal Stereo Zoom paper [PDF] helped reconcile the coincident vs. spaced pair perspectives into a unified theory of spacial recording. So I jumped at the chance to hear him present along with Helmut Wittek from Shoeps microphones.

My notes from the Oct 20, 2013 presentation at AES 135 Convention in New York:
W28: Practical Techniques for Recording Ambience in Surround

Helmut Wittek

What is ambience? Understanding sound in three layers:
(1) Reverb, Diffuse Noise
Diffuse, location-independent, not localized, room information.
Must be de-correlated, balanced energy distribution.
(2) Early Reflections, Discrete Sounds (spread)
Discrete, location-independent, localized, but the location is arbitrary, info on position of the source in the room.
Must be correlated, balanced directional distribution.
(3) Discrete Sounds
Discrete, location-dependent, localized, source information.
Must be correlated.

Factors that help de-correlate:
Larger distance between the two mikes
More directional mic patterns
Greater angle between mikes

DFC: Diffuse Field Correlation

Arrays can be built using the Image Assistant Java applet and/or the Michael Williams curves.

After the session I spoke with Helmut about using boundary layer techniques to record ambience. We agreed: this could work very well to achieve de-correlation, but the necessary size of the movable boundaries would render such an approach far less practical than any of the arrays demonstrated.

Helmut’s full presentation plus all of the surround sound demonstration files are available here.

Michael Williams

5 channel umbrella, 12 channel umbrella, 16 channel array including height information

Unified Perspective:
All sound is recorded at the same time; there are no layers. The layer concept provides additional control for later in the process whereas array placement is absolutely critical for the Williams arrays. But layers are to sound what hamburgers are to food, whereas the Williams arrays are “like fine French cuisine.”

The 5, 12, and 16 mic arrays are scalable: turn on as many microphones as there are speaker channels.

2nd volume of his book is now available. I bought it.

A collection of papers about recording by Michael Williams, plus his Multichannel Microphone Array Design pages here.

For more convention presentations, photos, audio gear, etc. see: 135th AES NY Roundup