As the crow flies

I went this weekend to see the brilliant immersive soundscape installation The Murder of Crows by Janet Cardiff & George Bures Miller, currently showing at the Park Avenue Armory. If you will be in NYC between now and September 9, I highly recommend it!

Essentially a dreamscape audio movie realized by 98 speakers placed around the Armory’s immense Diff Hall, the experience reminded me of the huge emotional power that sound can convey. I could literally feel, in my bones, doors creaking open and closed, marching footsteps, howling winds and other powerful sonic “images”, with a visceral immediacy far beyond what could be conveyed by mere visual means.

And of course I immediately set about trying to figure out how I could make one. It could be done with a binaural headset, but I wouldn’t want to have to wear earphones all the time. With a stationary audience of just a few people, you can do it with four speakers, each positioned at the corners of a square around the listeners.

To capture the soundscape, you would use four microphones arranged into a square exactly the same size. If pairs of microphones are the wrong distances from each other, then phase information is destroyed, and the experience will lose realism and immediacy. And you probably want the square to be fairly large, so it captures the way things move between the speakers.

So a faithful audio “camera” partly comes down to placing each microphone the correct distance away from the other microphones as the crow flies (so to speak). If you are doing this in a secluded location (eg: a beach), then you can just use six strings of the proper lengths — one between every pair of your four mike stands — and separate the mike stands until the strings are taut.

But if you are doing it in a crowded place, such as a New York City street, then you need to use some other method of calibrating distance. You could use optical measuring devices, such as time-of-flight cameras, but to me that seems needlessly expensive.

Since we already have high quality microphones, perhaps we can generate a sound and measure when that sound arrives at each microphone (a simple form of sonar). As a calibration device, we can place two small speakers a small known distance apart from each other (say, on opposite ends of a separating rod). A computer sends two simultaneous pings, one to each of the two calibration speakers — with the two pings having different pitches. From the time each of the pings arrives at each of the four mikes, we can calculate the exact position of all four mikes, and how each mike needs to be moved to get into proper position.

I’m sure there are other ways to do this. The important thing is to be able to capture wondrous immersive soundscapes wherever they may be. If a library of such immersive soundscapes could be developed, then this would be an experience anyone could enjoy at home.

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