Sorry, but your explanations sound like mix of data from under-digested articles. I never said nothing about acoustic centers, nor I said that they are at coils. What I said, that Doppler effect is not a property of a speaker driver. If your cone is infinitely rugged and infinitely light you have no Doppler effect at all. Or, at least if it does not resonate on the frequency that "rides" on cone movement.
If you haven't considered the acoustic centre of the driver you haven't understood the problem. Modulation of the acoustic centre position is the crux of the whole issue. The acoustic centre is the zero reference point for the phase of the high frequency signal around which the phase is modulated.
You can say it all you like, but that doesn't make it true.
Doppler distortion / phase modulation still occurs on a perfectly terminated cone (where no standing wave resonances occur because the bending wave is fully terminated at the edge) and it also still occurs on a driver working as a perfect piston. It's nothing to do with standing wave resonances on the cone.
Honestly, do some reading on the subject (there are some references in Rod's article as a starting point) and take some measurements of your own if you're still not convinced. Seriously.
Honestly, please do some reading on elementary physics to understand the difference between phase-related distortions caused by modulation of properties, and Doppler effect that is totally linear phenomenon. I often take measurements, in order to improve things. When people take measurements and assume wrong causes of effects they observed, they often make things worse trying to improve things.
I don't get it. If I vibrate a cone sinusoidally at 10 kHz my microphone will pick up a pure tone at 10kHz, hopefully. If I start to move the cone (still vibrating at 10kHz) away from the microphone at a constant 1m/s my microphone will pick up a tone of 330/331 x 10kHz = 9969 Hz. And proportionately the same the other way. Isn't this what happens to my cone if I start vibrating it with loud bass notes while reproducing a 10kHz tone? What has this got to do with the cone's mass, ruggedness, and resonances?
I'll be thinking about this all night now! We all know "doppler" effects happen in air, when the source and target are in relative motion. But if I introduce a really loud really low bassoon note in the orchestra, does that modulate the pitch of the triangle? Can I tell? Why? I guess the 'displacement' of the air due to the bassoon, compared to the wavelength of the treble from the triangle, is again the critical issue? In the air, in the mic dipahragm, in the speaker, the problem arises due to the amplitude? So, what about in my chair back at my ear, where the bass amplitude is again very small. The amplitude diperses but the frequency doesn't...the treble source is moving with respect to the target. OK I guess I buy it but now I have to quantify it.
And is the effect the same in the air as in the driver? If we separate the signal into woofer and tweeter mounted close to each other, won't the bass again modulate the frequency of the treble when the amplitude gets sufficient?
Why is it that we don't notice this? I assume it takes a LOT of bass excursion to be significant, and I'm not sure my pitch discrimination is very good at the extreme treble.
So is this one reason huge planars sound "good" (not my assessment)? Sure makes an arguement for square inches versus excursion.
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Honestly, please do some reading on elementary physics to understand the difference between phase-related distortions caused by modulation of properties, and Doppler effect that is totally linear phenomenon. I often take measurements, in order to improve things. When people take measurements and assume wrong causes of effects they observed, they often make things worse trying to improve things.
Making sure I understand you: Areas of the speaker cone are excited by the motor's input and radiate sound according to area, shape of speaker, surround terminus qualities, mass and stiffness of cone, frequency and energy of inputted energy, etc. This radiated sound from the surface of the cone is then "doppler modified" by the pistonic movement of the whole cone. Is that it?
No. Electric signal does not create movement. It creates forces that move the membrane and the air. The membrane is moved by vector of already summed forces, with a curve that is already the result of sum of 10 kHz tone and bass tone. No Doppler effect. But when you move with bass frequency already excited membrane that oscillates by itself on 10 KHz frequency, you have Doppler effect.
My head hurts! If the bass tone was DC and the cone could keep moving in one direction, would we not hear Doppler on the 10kHz then?
When the whole thing does pistonic movement it renders already "modified" curve. No separate bass and highs that ride on bass. Just a curve that is the sum of two signals.
However, it is like a theoretical "Round Horse in Vacuum", so when you go to real thing from this ideal model you should add other effects that are non-linear. Areas are excited differently by different shapes of the signal, that creates distortions. And that creates also Doppler effect that is caused by resonating areas of cones. Once excited, they literally ride on cone.
No. We would be forced to move by the same DC, if the membrane is infinitely large. Or, we would hear nothing if it is infinitely small.
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I'm with Wavebourn on this one.
The speaker cone moves because of the force between its voice coil and the magnet. If the voice coil is fed a signal at a particular frequency, the cone vibrates relative to the magnet, then the listener will hear that sound at that frequency.
If the cone is fed two frequencies, say a low amplitude at 10kHz and a larger amplitude at 10Hz, then there will be a slow moving cone with a high frequency vibration.
The mistake some of you are making is to visualize a moving surface - the cone - and a high frequency vibration of the surface of that cone about it's slowly moving average position. You are treating it as if the surface of the cone is the reference point against which the high frequency vibrations occur. If this were correct, there would be Doppler effect for the listener.
But this is not what happens. The high frequency vibration is not relative to the average position of the cone surface. The electrical signals driving the voice coil produce movement relative to the magnet. The magnet is stationary relative to the listener. This means that the high frequency sound is not Doppler shifted.
The speaker cone moves because of the force between its voice coil and the magnet. If the voice coil is fed a signal at a particular frequency, the cone vibrates relative to the magnet, then the listener will hear that sound at that frequency.
If the cone is fed two frequencies, say a low amplitude at 10kHz and a larger amplitude at 10Hz, then there will be a slow moving cone with a high frequency vibration.
The mistake some of you are making is to visualize a moving surface - the cone - and a high frequency vibration of the surface of that cone about it's slowly moving average position. You are treating it as if the surface of the cone is the reference point against which the high frequency vibrations occur. If this were correct, there would be Doppler effect for the listener.
But this is not what happens. The high frequency vibration is not relative to the average position of the cone surface. The electrical signals driving the voice coil produce movement relative to the magnet. The magnet is stationary relative to the listener. This means that the high frequency sound is not Doppler shifted.
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Sigh,
I followed a similar thread on the Parts Express forum same result. Some people have heard about Doppler Distortion, but don't understand what it really is and then they apply it to speakers where it doesn't.
Listen up! Doppler distortion is a steady state effect. If a sound source is moving, the frequency will be shift. But the sound source KEEPS moving in the same direction.
What happens in speakers is FREQUENCY MODULATION. It is exactly like an FM radio signal, except that a high frequency is riding on a low frequency rather than vis versa. It is exactly what an oscilloscope says it it.
Bob
I followed a similar thread on the Parts Express forum same result. Some people have heard about Doppler Distortion, but don't understand what it really is and then they apply it to speakers where it doesn't.
Listen up! Doppler distortion is a steady state effect. If a sound source is moving, the frequency will be shift. But the sound source KEEPS moving in the same direction.
What happens in speakers is FREQUENCY MODULATION. It is exactly like an FM radio signal, except that a high frequency is riding on a low frequency rather than vis versa. It is exactly what an oscilloscope says it it.
Bob
This is exactly what occurs though. Let me pose a simple question to you.
Place two identical speakers near each other and equidistant from the listener, feed one speaker the low tone and one the high tone.
If we ignore any mutual coupling between the drivers the driver producing the high tone will will not have its "rest" position altered by the low frequency tone and will be reproducing only the high frequency tone.
The two tones will sum together at the microphone without any intermodulation products.
Now have one speaker produce both tones - when the woofer moves closer or further from the listener the reference point for the high frequency tone is moved further away and closer to the microphone. This results in phase modulation.
It's really that simple. Anyone who can't believe what's happening needs to do a little reading up on frequency and phase modulation, how they are generated, and what spectral components they produce.
I've tried repeatedly to explain why this concept of the voice coil/magnet as the apparent source of the sound is wrong, but I seem to be talking to myself.
Let me ask you this, why stop at the voice coil ? Surely the amplifier is the source of the sound so why not include the distance of the speaker cable in your acoustic centre calculation ?
The amplifier is not physically moving so there must be no doppler distortion, right ?We don't include the length of the speaker cable in calculating apparent acoustic centre because the signal propagation speed in the cable is the speed of light, and is ridiculously faster than the propagation speed of sound in air, thus removing itself from the equation.
Likewise the propagation speed of sound through a solid medium, especially longitudinal compression waves which you have in a voice coil former, is vastly (many times) faster than the speed of sound in air. This has the effect of largely but not entirely eliminating the length of the voice coil former as a factor.
Because the signal travels so quickly from voice coil to cone the apparent acoustic centre moves forward and becomes tied close to the position of the cone/air junction.
Please people, if you believe the acoustic centre is stationary at the voice coil/magnet, at least try to take some measurements to back up your belief, or find some respectable references which agree with that crazy notion. (Good luck finding them) I have measured this stuff, I'm not just pulling it out of thin air or talking it into existence through thought experiments.
Nobody has addressed my point regarding the difference between an overhung and underhung voice coil either, probably because its an inconvenient truth which doesn't fit into the belief system of the voice coil/magnet gap as the sound source.
Finally if you don't like Rod's work, here is a (brief) article from Linkwitz on the matter, although I'm sure this won't convince any true doubters either...
Frontiers
and
http://www.linkwitzlab.com/images/graphics/doppler1.gif
and
http://www.linkwitzlab.com/images/graphics/doppler2.gif
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Bob, I think you're quibbling over terminology, not the existence of what we're discussing. Yes of course it is classic frequency/phase modulation, that's the whole point. I generally refer to it as frequency modulation distortion, doppler distortion is not a term I would normally use.
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