"Doppler Shift" reflected from fixed objects?

You can get a chirp from a flexible pipe with circumferential ridges, but I would expect that to be caused by the sharp multiple reflections off the ridges.

Corrugated surfaces do to sound what optical gratings do to light. Put in an impulse and you'll get a chirp as echo if you happen to be in the right spot.

Jeroen Belleman

A "howl round" stabilizer would do this. They shift the frequency up or down by about 5 Hz.

Amplitude modulation implies some form of non-linearity. This isn't the case here.

There should be no new 'frequencies'.

AM is a non-linear operator. And you need to get a better handle on standing waves.

If you're ever walking alongside an iron bar fence, try generating a sharp sound. Slam your foot down, throw a rock against the ground, something like that. You'll hear a chirp from the fence. (Making a click with your mouth doesn't seem to have enough contrast -- probably because your head is making the sound in the first place. Clapping doesn't seem to be loud enough, or sharp enough, to work very well.)

Tim

Doesn't have to be ridged -- a smooth pipe will do. A good example is if you see a pile of pipes unattended on a construction site. Have an accomplice clap at the far end, and you'll hear a Star Wars blaster style "pew!"

The underlying mechanism is that acoustic waveguides are dispersive, just like EM waveguides are; they just happen to work down to DC, because acoustic waves include longitudinal modes.

Tim

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You don't seem to be actually understanding the Doppler effect. Saying it has to do with "changes in the path length" is not the same thing as unders tanding it.

The cause of Doppler shift is the change in wavelength of the reflected (or transmitted) signal due to the continuous movement of the reflector (or so urce). Notice the word "continuous". If the reflector pops up in one loca tion it will not have a Doppler effect. If it then pops up in another loca tion, that reflector also has no Doppler effect. So how would having both of them present product a Doppler effect??? It would only produce two echo es. Make it N reflectors and you still have only N echoes, all at the orig inal frequency.

Rick C.

The ear has a nonlinear response. Why don?t we only hear mixing pro ducts?

CheaterW

How is that implemented? My understanding is a pitch shifter is very hard to implement.

Rick C.

OK for a totally wacky idea, driving home I was thinking all 'your' closed spaced columns could make an array of scatterers (that might not be a real word) things that scatter. like x-rays and a crystal.... some frequencies would might bounce off the columns and interfere constructively.

You need to go in and ping it, measure the pulse response. I tried to do that with my smart phone and culvert under my road. but mostly failed... I need better audio kit.

George H.

Which says " Amplitude modulation results when the carrier c(t) is multiplied by the positive quantity (1+m(t))".

This is a passive environment, which should not result in a multiplication.

There is no mention of standing waves in that article.

At low pressure levels it is largely linear. At higher levels the effects become more apparent to some. I'm certainly aware of distortion in my hearing at very high levels.

Right, I'm not sure how important the ridges/corrugations are... seems like they would be close to the right size for HF. (v=f*lamda, 300m/s and 1 kHz is 0.3m)

George H.

Oh I should add, smooth pipes give a chirp too. it's not just the corrugations. GH

frequency shift and pitch shift is not the same thing

Is this the abbey in question?

Well, if that's true, you might be able to measure the effect. Drag a function generator, audio amp, and speaker into the abbey. Also an oscilloscope and electret microphone. Have the function generator produce a "chirp" or burst of tone and listen for the echo return. If there is a frequency or phase change, you might be able to see something in the rotation of the Lissajous pattern on the scope formed by the original tone and the reflected return.

There has been quite a bit written on Gothic cathedral acoustics. Since the layouts are similar, following a floor plan in the shape of a cross, and an elevation plan following numbers and ratios extracted from the Bible, the acoustics tend to be similar: The composers and choir masters of the day tweaked their compositions to take advantage of these characteristics to create a feeling of awe as well as avoid notes that might tend to destructively cancel or otherwise sound strange. For example: "WHY DOES THE ACOUSTIC SPACE OF CHURCHES EXALT GREGORIAN CHANT?"

Drivel: There have been non-liturgical music played in Gothic cathedrals, such as Jethro Tull at Canterbury cathedral. I couldn't find a references, but I'm told that the concert didn't sound very good. You decide:

Makes no difference how exactly it occurs, the important end point is that an interfering wave or group of waves results in an amplitude modulated wave, and this wave has the frequency offset sidebands.

It makes every difference. Unless there is a non linearity there is no multiplication and no amplitude modulation.

Standing waves are a form of cancellation and enhancement as a function of position. That is not Amplitude Modulation.