[sdiy] ot: rotating speaker simulation or stupid approach

[Ian Fritz]

At 08:10 PM 7/3/2003, Richard Wentk wrote:

>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.

I would look at this differently.  :-)

Suppose you have two sticks to dip in the water and an observer in line 
with them. Now do two experiments:

1.)  Dip the first stick twice with a time separation dt.  The two pulses 
will reach the observer
spaced by a time interval dt.

2.) Dip the first stick and then the second, again with a time separation 
dt.  The two pulses will now reach the observer spaced by a time interval 
that is different from dt -- larger or smaller depending on which side side 
of the sticks the observer is located.

This is a simple and obvious Doppler shift.

>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.)

Well, in a phased array all the sources are continuously emitting (cw). 
That's why you get beam steering due to phase cancellations.  But if the 
row of speakers is excited one at a time then the phase cancellation 
effects do not exist.  The result is a Doppler shift, since the source is 
effectively moving.  Of course, it is a discretized effect, so wavelengths, 
speaker size and aliasing effects will have to be properly accounted for.

   Ian