Documentation for Project #2 - Ambisonic Soundscapes - More Documentation and a Sample File

Ambisonic microphones 101:
It is easy to gloss over the terms used in ambisonics, so I offer some further deciphering that I started in Project 1. The B-format recording channels consist of 4 microphone equivalents:
w - an omnidirectional microphone;
x - a figure-of-eight microphone oriented front/back;
y - a figure-of-eight microphone oriented left/right;
z - a figure-of-eight microphone oriented up/down.
(A figure-of-eight microphone is equally sensitive in the front-to-back directions and least sensitive to the side.)

The construction of an ambisonic array using 4 regular microphones is quite possible, but results in at least 2 problems: the space required to mount them limits their closeness, and an omnidirectional and figure-of-eight microphone are not of the same construction. Ideally, all of the capsules should be identical and matched, and the distance between the capsules when using regular microphones (even if the z-axis is omitted) introduces at least frequency response and phase issues.

Michael Gerzon's answer was to use 4 cardiod capsules in a tetrahedral array, thus, addressing the above issues. My experimental tetrahedral microphone allows the capsules to be in an array less than 2.5 cm apart.

Software information:
The software I am using to convert the 4-channel B-format recordings to 5.1 (also known as G-format) is VVMic by David McGriffy. This is freeware available on the Internet (PC only) and is also available as a VST plugin for both PC and Mac OS X. There are other standalone programs as well as tools for Max/MSP, PureData, etc. Some of these allow playback directly from the B-format (including VVMic) depending upon the computer interface capabilities, of course.