-September 30, 2013

First there was mono. Then came stereo, three-channel "stereo", quad, and now, the cinema-inspired five-channel "surround". Trouble is, none of these systems really address how to recreate a real audio event in a listener's ear-brain system. Except for one. Or two.

The most realistic way to recreate a real acoustic event for a listener is to record it binaurally using a dummy-head microphone system, and listen back over headphones. This can produce an eerily realistic, you-are-there experience – incredibly effective for a production like, say, Stephen King's The Mist. Two ears. Two channels. But what if we want to clearly and unambiguously sense the spatial character of sounds at one listening location, for reproduction over speakers say?

Let me introduce you to a system called Ambisonics. Even though it's been around for over 40 years, it's not well known. What if I told you there was a system that needed only four channels to fully encode a 3d sound field? What if I said recordings could be played back over four or forty speakers? Maybe the ease of sound manipulation would impress you – being able to rotate, tilt, and zoom a soundscape using simple audio mixing? And what if there were 3d acoustic sensors (a.k.a., microphones) that could capture such full-surround audio? That's Ambisonics.

How Ambisonics represents a 3d sound field using fewer channels than the so-called surround used for movies is as ingenious as it is simple. The four signals are labelled W, X, Y, and Z. Speaking from a microphone/sensor point of view, W is the omnidirectional pickup, receiving sounds from all directions equally well. The X, Y, and Z pickups all have what are known as "figure-8" sensitivity patterns, one for each orthogonal axis. The W sensor produces a positive signal when impinged upon by a positive pressure part of an acoustic wave from any direction. The X, Y, & Z pickups, however, respond positively for pressure from the front, left, and upwards, and negatively to pressure from the rear, right, and downwards.

Simple adding and subtracting of various combinations of these four signals can produce a peak response pointing in any direction – not quite phased-array, but in the family.

And that's why the number of speakers doesn't really matter. My own system uses six speakers arranged in a hexagon. I could just as easily have five, or eight – it's trivial to create the appropriate WXYZ mix for any speaker position. True, it's only a 2d array, where the Z signal is ignored. One day, I'll set up a crazy 3d system.

The surround-sound processor I designed using Ambisonic principles

How are Ambisonic microphone systems constructed? There are a number of options. Check your collection of recordings for ones on the Nimbus label. They use a homemade array of one omni and two figure-8 mics that directly generate WXY, but unfortunately not Z – not that there'd be any current way to distribute 3d recordings. Yes, there is a way to create a stereo-compatible signal that can also be Ambisonically decoded with the proper equipment, but that's another story.

Most Ambisonic microphones take a different approach (keep in mind that we want our individual mic capsules to be as close as possible to each other – we're trying to approximate a single point). Here, we rig up four mics into a tetrahedron. All mics/capsules have the same spatial pickup response – in the cardioid family – but again, using just simple mixing, the WXYZ group can be created from these mics' signals.

A tetrahedral mic capsule array (source: SoundField)

We've focused on the pickup sensor aspect of Ambisonics here, but that's just one option. It's pretty simple to create an Ambisonic mix from either a bunch of regular pickups, or even purely synthesized sounds.

The TetraMic, another tetrahedral Ambisonic pickup (source: Core Sound)

So, to summarize, here's a technology that supports 3d audio representation using only four channels (or three for 2d), that's easy to manipulate, that isn't tied to a specific speaker arrangement, and that has an established track record.

My own Soundfield mic with control box

What cool uses can you think of for Ambisonic technology? Immersive computer gaming? Simulators? 3d hydrophones? Your imagination is the only limit.

Speaking of sensing and sensors, don't forget to enter the TI Sensing Design Challenge 2013. The $3,000 grand prize would be enough to build a homebrew Ambisonic mic – just sayin'.

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