[sdiy] ot: rotating speaker simulation or stupid approach

[jhaible at debitel.net]

> 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 [...]
> 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. 

Richard, you would almost convince me. But I have just recently
built something like that, not even using speakers, just
a tapped delay line and a scanner to interpolate between 
the different delay times (like the Hammond scanner vibrato does),
and the frequency modulation is clearly audible.
And it was clearly audible ever since Hammond invented the scanner
vibrato.

> If you don't believe me, try it. 

I tried it. The FM is there. (And some side effects from phase 
canellation - right.)

> 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.

I'm not familiar with the details of surround systems. But that's just 5
speakers, isn't it? Probably not enough to cause that effect. Probably
the phase canellation will be more prominent than the delay effects.

It may be similar to what we all know from phasers: If you listen to
the phase shifted signal alone, you clearly hear FM when the poles and
zeros are modulated. But when you make a 50:50 mix with the dry signal,
creating the notches of phase cancellation, the FM is much obscured
and will only be perceived with quite extreme modulation depth.

I guess it's similar with tapped delay lines or crossfaded speaker 
arrays: If you have enough taps (or speakers) to keep the delta T
between two adjacent channels small, the phase cancellation will
only occur at high frequencies, and in midrange you will preceive
the FM / simulated dopper effect.

JH.


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