[pianotech] re.:Bladeless fan

[Ron Nossaman]

Shawn Hansen wrote:
> The organ statement has me interested.
> I have experienced standing waves in this way.  Moving through a space 
> with a stagnant/constant sound source will give you the pleasure of 
> hearing certain partials as being louder in specific locations in the 
> room.  

Yes, I've experienced that, but this is different.


>If you move through the space you are essentially animating this 
> effect.  
>Sine waves, which have no partials, will do the same thing, but 
> that the sound will disappear in certain places in the room, so when 
> moving through the space it will sound like beating.  

I disagree. The beats change at a rate corresponding to your 
rate of speed, regardless of your position in the room.


> Doesn't doppler reference a change in frequency?  

Yes.


>Wether it is light or 
> sound or some other set of frequencies?  When I turn fans off when 
> tuning I certainly do it because I hear "beats".   

Because the echoes from the moving blades differ in frequency 
from the source tones. We hear both the source, and the 
Doppler shifted echo (to a much lesser degree) at the same 
time and perceive beats. If the sound source is stationary, as 
with the church organ, you hear a lower frequency from the 
organ as you walk away from it, and a higher frequency from 
the echo as you walk toward it. I know it's not a single point 
echo, and the interaction is much more complex than that, but 
that's the basic principle.


>But now I am curious 
> if the sound waves are actually being distorted in the way that would 
> compress and or expand the wave's structure resulting in a frequency 
> change.  

Of course. That's what a Doppler shift is. Listening to an 
ambulance go past, the sound lowers in pitch as it passes you. 
If the sound source was stationary and you drove past it, 
you'd hear the same effect. From recordings of a passing 
vehicle, it's possible to calculate it's rate of speed from 
the pitch difference coming and going.


>So what we hear are perhaps not only amplitude changes based on 
> the periodic reflection of sound, but ALSO a vibrato or pitch change 
> because the reflecting surface is moving.  

We hear the same thing as we hear when two strings aren't 
producing the same frequencies in a unison. We hear beats from 
mismatched frequencies.
Ron N