[sdiy] ot: rotating speaker simulation or stupid approach

[Richard Wentk]

At 14:29 04/07/2003 -0600, Ian Fritz wrote:
>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.

No it's not. You're confusing an impulse with a continous sequence of 
wavefronts. You can't Doppler shift an impulse because it has an infinite 
frequency response to start with.

A more accurate representation would be to replace each single dip with a 
vibrator (no, not that sort... - actually thinking about it, they'd work as 
well as anything :-) ) producing a continuous stream of wavefronts by 
dipping up and down. Or indeed vibrating.

What you get now with two continuous sources is an interference pattern. If 
you have a line of sources which produce wavelet-like shaped pulses in 
sequence, scanning through them in turn still won't produce the simple 
bunch-up spread-out effect of straightforward Doppler shifting, because by 
the time you get the fastest impulse from the nearest speaker, the 
wavefronts from the others will still be playing catch-up.

The technical term for the resulting pattern is 'a mess.' ;-)

The *only* way to get this to work would be to get each speaker producing 
just a single wavefront. That would work. But for obvious reasons that's 
not going to be practical, or useful.

Richa