cube

This is a simple loudspeaker array for 3D "full-sphere" sound. Eight loudspeakers form the corners of a cube. They're driven from a PC with a cheap multichannel audio card. It's great for editing and producing my location recordings, listening to others such as the soundofspace collection, playing immersive games, and occasionally bemusing the neighborhood.

Why?

I've been recording lots of local material in Ambisonic surround, using a TetraMic. This has time and four (relative) dimensions in space: velocity front-back (X), velocity left-right (Y), velocity up-down (Z), and pressure (W). In microphone terms that's equivalent to three figure-eight mics and an omni, exactly coincident.

It's fun to record like this because you can mix down later to any combination of standard first-order polar patterns. Want a forward-facing cardioid? That's W plus X. (Rear-facing cardioid, similarly, is W minus X). Blumlein stereo is simply X+Y and X-Y. Crossed hypercardioids at 100 degrees, pointing slightly upward? Just as easy. (Listen to some examples here). Ambisonic surround is like a 3D generalization of mid-side recordings techniques; you get incredible flexibility in post.

But monitoring and mixing only by listening to the stereo mix-down has a couple problems. One, it's hit and miss: rotation is easy but making sure you have the best possible perspective is hard. Two, it's really only a single slice through the recording. Like a black-and-white photo of a sculpture, you can tell there's depth and color, without exactly experiencing it. But the raw recording isn't two-dimensional; it's four-dimensional, and includes front, back, left, right, up, and down. So I want a way to listen to these Ambisonic recordings in their full natural state. And, in particular, I want the equivalent of a little mixing desk. Nearfield monitoring, so I can sit with keyboard and maybe a little control surface, playing with the sound. Rotation, zoom and pan, stretch, reverb, mixing: all the usual tools, but for enveloping surround. Then, after constructing a good thing in full surround, I can choose a slice across the mix for stereo.

I also wanted a standing-up version, so I made the height easily adjustable. The standing-height position is intended for getting this thing out into the world, as a little experiential curiosity. It sounds, well... interesting.

How it sounds

With a good location recording, standing inside the cube really does sound like you are there. Positions are fairly stable and precise if you move around and face in different directions. Above really is overhead. Distance is tangible, and the acoustics of a hall or bar or street are very distinct. This is immersive surround-sound.

From the outside, it sounds as though they are here. There's a solidity and space that extends to maybe twice the cube's dimensions; air with music in it. Quite curious.

With Ambisonics or with suitable drivers for positional audio systems such as OpenAL, discrete sounds can appear from and move in any direction. 5.1 surround is more immersive than stereo, but this makes horizontal surround sound flat.

Construction

The frame is 4 feet (1.2m) on a side and about 7'5" (2.25m) tall. That gives plenty of room for one person to stand inside (two can, too), and it's also easy to walk in and out of. Mountings are vertically adjustable, to suit standing and seated listening positions. When the cube is configured at standing height, the speakers are about 7'2" (2.16m) and 3'2" (0.96m) from the floor. As with stereo monitoring, nearfield reproduction ensures a minimum of unwanted reflections from walls and other surfaces.

Frame: anodized aluminum tubing with some extra internal bracing. The pillars are detachable for moving.
Mounts: adjustable swivel (detail photo)
Loudspeakers: eight Gallo A'Diva Ti. There's no need for a sub, these little speakers go plenty deep enough.
Amplification: Russound DPA-6.12, using 8 of 12 channels.
Audio interface: ESI Gigaport HD, using all eight channels.

For software, I'm mostly using AudioMulch, and some of my own code for rotation/push/stretch and other effects. B-Format recordings are decoded to the cube's 8 speakers using Bruce Wiggins' VST. OpenAL decodes via Rapture3D.

this is another of hugh's audio projects.