[sdiy] ot: rotating speaker simulation or stupid approach

[Richard Wentk]

At 10:50 03/07/2003 +0200, Czech Martin wrote:
>Why would the doppler effect be lost?
>
>Imagine the following: 100 speakers in a single line, every cm a speaker.
>A "sequencer" with interpolation and VCA will make the active of those
>speakers move forth and back. What is the difference of that arraw
>to a single speaker on a model railway track waggon, moving
>foth and back?

The difference is that you're not getting the wavefront compression that's 
created if you move a single speaker.

Look at it this way. If you move a boat through water, it leaves a wake. 
The waves bunch up at the front and spread out behind.

If you set up a system with tens of dippers in a line, you *won't* get 
anything like the same wave pattern. The wave fronts from each dipper will 
make a discrete contribution to the wavepattern you see at the lakeside. 
But if you fire the dippers in turn you won't get the same simple bunching 
effect followed by a stretching out that you'd get if a single boat passed by.

Technically you're creating what's called a phased array, which has very 
different properties to a moving point source. A moving sound point source 
physically, and literally, compresses the spacing of the wavefronts ahead 
of it. A phased array can be used to steer a beam of wavefronts by using 
phase cancellation, but it doesn't produce any Doppler effects. (Some 
phased array radars do use Doppler effect measurements to estimate the 
velocity and vector of a target. But that's something different.)

If you don't believe me, try it. When was the last time you heard any 
dopplering from surround panning on a 5.1 system? I'd bet anything that you 
can spin a source sound round a 5.1 system as fast as you want, and you 
won't be able to hear any dopplering taking place.

Richard