The Ambi-Alice Is an Ambisonic Microphone for Directional Recording and Listening
By using four separate microphones in a single housing, audio can be encoded spatially and selectively played back.
What is ambisonic audio?
In a typical audio stream, waveforms are encoded into discrete channels that correspond to different speakers. Most people are probably familiar with mono, stereo, and 5.1/7.1 speaker setups, but these normally lack true depth and cannot be easily mapped between each other. Ambisonic audio, on the other hand, encodes waveforms as a speaker-independent sound format called B-format which gets dynamically mapped onto the listener's speakers after. Microphones used with ambisonics are quite expensive, so a maker on Instructables who goes by the name DJJules wanted to construct a DIY version for just $100.
An initial prototype
When starting his design, Jules selected four TSB2590 microphone capsules due to their 1" diaphragms, built-in EMI/RFI suppression capacitors, and a durable metal housing. A set of three wires were then attached to both the microphone's leads and to the XLR connector's pins, along with a single capacitor and resistor in series. Once some color coding was added, he assembled the four modules into a simple array. However, this initial design was a bit fragile and made it hard to identify which microphone corresponded with which direction.
Upgrading the microphone hardware
The next and final attempt at the hardware brought about a couple of substantial changes that greatly improved how easy the device was to use. Perhaps the most obvious enhancement was the inclusion of a custom 3D-printed spherical housing that both protected the microphone array and provided labels next to each module for easier identification.
The four XLR connectors for the microphones got plugged into a ZoomF6 multichannel field recorder. The resulting audio was then passed to the host PC running the encoding software.
Encoding audio
As previously stated, sound files are ordinarily recorded in channels that correspond to either the individual microphone or speaker. Taking this data from a set of four microphone channels and converting it into the B-format is handled by an encoder. Jules took a piece of software called Reaper and configured it with four tracks, along with some calibration data, to calculate the wave pressure and spatial information as outlined here.
Getting some output
B-formatted sound files in hand, Jules was then able to run it back through Reaper to extract speaker channels, no matter which configuration they might be in. At the end of his build log video was a short demonstration of how the Ambi-Alice microphone array could be used, including on a tripod within a forest and mounted on a truck.
There was also a short video that showcased how clear and precise the sound could get when placed next to a moving object, such as a train.
