Curiosity: Doppler effect for speakers?

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You might guess what I'm thinking.

I have been wondering, that if a low frequency is mixed with a higher frequency and put through a speaker, then will the low frequency making the cone vibrate back and forth, cause slight doppler effect on the higher frequency?

Even if this is true, however, then vibrating strings on say a guitar are no different. The string won't experience frequency fluctuations due to doppler effect, but when the energy is transferred back through the air, this is when I imagine that doppler shift would occur.

Even considering that a guitar string would work the same way, however, it does not mean that this property would not have any effect on sound quality. If the guitar were recorded using a microphone, then the doppler shifts would add when it comes through the speaker.

Also, if this is true, then it might explain one of the things that make acoustic and electrical instruments sound different. On an electric guitar, the pickup coil would not pickup any doppler effect, because it is just sensing the magnetic interference with the string, not pressure waves from the air. Thus, acoustic instruments would logically be slightly different sounding than electric ones.

I don't find any reasons why my theory could be false, if you do please tell. Or if it is a known fact, links would be nice.

My main reason in asking about this is because I am interested in what corrective circuitry might do to the sound if used to cancel out this effect. As far as I can theorize, the doppler effect gets worse the farther away you are from the speaker, so perhaps corrective circuitry might also be used to give a sense of distance, if used with an effect that simulates the way higher frequencies will arrive sooner over a distance.

Does this spark anyone's imagination?

- keantoken
What you're describing is IM distortion in audio terms and yes, it exists.
Saying that a guitar is distorted by the same mechanism kinda misses the point--that's the sound of a guitar. By the same token, brass instruments are horribly distorted if you compare what goes in one end to what comes out the other. But that's the sound of a brass instrument. With the exception of a synthesizer, there is no instrument that produces a pure sine wave. It's the "distortions" that make the instruments what they are.

I understand about harmonics and things and how they give instruments their unique sounds. I was mainly using the guitar as a example, I was not trying to say that it's sound was 'distorted' although it may have sounded that way :p.

However, I have always heard of intermodulation distortion referred to as an amplifier characteristic rather than a speaker characteristic. As far as I can theorize, even if the amplifier has zero distortion, you will still get IMD from the speakers because of the natural inertia of the air, which is essential for it to carry sound as well.

So, if one could build a device to cancel these effects, then would the difference be noticeable?

I'm sure I'm not committing argumentum ad nauseum, but if I'm missing something here, it's because I'm tired. (;

- keantoken
A few things:

The sound from an acoustic guitar radiates mostly from the guitar body. The force from the string shakes the body, which in turn radiates the sound. The sound from the strings only is very quiet.

Since the excursion of the guitar body is very small, the frequency modulation (=doppler effect) will also be very small.

However, if the sound from the guitar is played bac by a loudspeaker with a small cone, the cone will have to have a larger excursion than the guitar body. This will lead to a much larger frequency modulation.

...then again, the effects of this frequency modulation is still very small, it is so small that it completely drowns in other mechanisms that produce IM distortion.

In other words, the doppler effect is a non-problem in almost all loudspeaker systems.

...and yes, electric and acoustic guitars sound different, but not due to the doppler effect :D .
Whoa... I was looking at your avatar... And it blinked! *regains composure*

At any rate, thanks, I now have a more qualified sense of scale of this phenomenon. Makes my wonder if this thread wouldn't be beneficial to other readers like me.


- keantoken

So to my next question! I've been running my reality simulator (the brain) and I've been wondering what doppler distortion does to positive and negative peaks in relation to listener position.

If the phase distortion on a noisy car is equal while you're in front of it, to when you are behind it at the same distance, then as the graph portrays, both positive and negative halves of the wave would be distorted equally.

However, if pressure before and after the car are not simply 'mirror' forms of each other, then at the same distance behind the car the distortion would be different than in front of it (i.e. the phase difference would not be the same).

So, drawing again on my reality simulator, then if the above paragraph has truth to it, the results in the graph would be most closely achieved when listening to the speaker from its side. However, if you were listening from the front, then when the speaker cone pulled away from you, you would get different distortion then whjavascript:checklength(document.vbform);
[check message length]en it came back at you.

So if we had an ideal speaker (no effects like cone inertia and phase shift vs. frequency) that was not in a box, and that was of quite large diameter (say, 5-6 foot). We would theoretically be able to measure these effects.

So the validity of this essentially hinges on how similar the properties of higher-density air (compression) are to lower-density air (rarefaction).

I believe that higher-density air would have some difference in characteristics to lower-density air because of particle spacing (i.e. in higher-density air the particles bounce off each other more, whereas in lower-density air, they have more room to move around.

However, if this is true (that the doppler effect of a compression wave is different than that of a rarefaction wave) because of the points in the above paragraph, this likely would only be significant in high-power systems, where there is lots of difference in density between the compression and rarefaction.

Is more mathematical analysis needed to clarify this?

- keantoken
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