frequency modulation of the upper ranges by the bass!! - diyAudio
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Old 25th June 2013, 12:19 PM   #1
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Default frequency modulation of the upper ranges by the bass!!

i came across this at Will You Like My Speakers? page

"The nemesis of single-driver speakers is frequency modulation (FM) of the upper ranges by the bass. (Doepler distortion is a type of FM distortion, not the other way around.) A higher fequency becomes a warble tone, the frequency and depth of the warble depending on the frequency and driver displacement of the bass tone. The result is when there are many bass frequecies in the music, as the music becomes loud (more driver excusion) the upper ranges become harsh and tonally indistinct. The driver "falls apart""

does all fullrange drivers suffer this? or are there any exceptions?
any technique to tame this ?
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Old 25th June 2013, 01:36 PM   #2
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Not sure which ones actually suffer. IMO, it varies from driver to driver and listener to listener. You need to decide for yourself if there are problems with a specific driver for the music you prefer at the levels you like.
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Old 25th June 2013, 04:49 PM   #3
chrisb is offline chrisb  Canada
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As Bob says in the very first sentence: "Full range drivers are not for everyone", and he does a pretty good job of summarizing why.

I think the closing section answers your question:

Quote:
How bad is the FM distortion on a single-driver speaker? It depends! Music with simple bass lines never sounds better than on a single-driver speaker. Jazz trio, girl-and-a guitar, classical chamber music. If you listen to your music a moderate levels -- ~70dB and the listening chair 6-8feet from the speakers, you can play almost anything. Music to avoid: Hard rock, most anything mastered this century for iPods and ear-buds, romantic symphonies at concert levels.
So you have been warned. If you are into polite genre and moderate levels, my speakers are for you. If you are into heavy, loud music, you need to look elsewhere.
emphasis (and typo corrections) mine, and this will be true of any brand and size of FR driver and enclosure design


(keep the change)

The mitigation of IM distortion effects is one of the primary reasons why many of us have come out of the closet about the use of "helper woofers" - i.e. what has not become categorized as FAST systems. When implemented with HP filters on the FR drivers, these are really nothing less than a 2-way system - the salient difference being moving the XO point out of the middle of the critical "telephone" band, allowing the FR driver to do all the things that Bob outlines, and that we longtime FR geeks appreciate.
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Old 25th June 2013, 06:52 PM   #4
Bigun is offline Bigun  Canada
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Doppler doesn't happen in a speaker driver, it's an oft repeated misunderstanding, like many in audio. There are of course various opportunities for non-linear behaviour in a speaker that can cause IM products and are mistakenly attributed to Doppler. However, I have generally found full range drivers to have only one issue - cone break up modes, otherwise they sound superb. And not all drivers have cone break up problems to be worried about. Parasitic cone break up oscillations could be subject to Doppler shift.
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Last edited by Bigun; 25th June 2013 at 07:01 PM.
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Old 25th June 2013, 09:52 PM   #5
chrisb is offline chrisb  Canada
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Well, as Rod Elliot (and no doubt others) would explain, the incorrect use of a technical term (as short-hand) does not eliminate the occurrence of a phenomena that could be attributed as "similar to .."

Doppler Distortion in loudspeakers


IINM, Full Range loudspeakers work over as wide a range as they do in no small part due to the designer's understanding, and careful use of what I think you're describing as "break-up" modes. An absolutely rigid "pistonic" cone driven by the conventional moving coil method would have rather limited frequency response; for example those massive car sub woofers with thick metal cones strong enough to support more than their own weight.

and doesn't your last sentence contradict your first?
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Old 25th June 2013, 10:52 PM   #6
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AMD/FMD exists. Doppler is essentially just a convenient term, a little like 'TL', accurate or otherwise. Problematic? They can (as in 'can') be, but depends on the listener, the material, the enclosure, the room, average & peak SPLs, listening distance... the list goes on. Same goes for cone breakup. As Chris points out, there's breakup & there's breakup. Most of the BW covered by fullrange drive units is created by resonant, not oscillatory, action (i.e. 'breakup'). Uncontrolled breakup is another matter entirely, and not exactly something that is desireable.
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Old 25th June 2013, 10:55 PM   #7
jcx is offline jcx  United States
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I can't see how there is any misuse of "Doppler" in connection with IMD from a physically moving speaker diaphragm - calling it FM or PM, the zero mean cyclic motion of the diaphragm doesn't change the physics

the linear model of the driver's motion in response to signal works just fine - it is the step where you convert the extended physical motion into sound radiation that most people make the mistake
the cheap assumption is that the driver motion causes an ideal air velocity modulation in its nominal, immobile position
but the diaphragm really does move, does modify the radiation equation from the idealized "linear" assumption


I have seen people who occasionally try clubbing you with “argument from authority” citing their physics backgrounds double think themselves into bad corners on Doppler from speaker cones though

despite the ability to find papers measuring the effect, correlating with the equations...
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Old 27th June 2013, 08:09 PM   #8
Bigun is offline Bigun  Canada
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Quote:
Originally Posted by chrisb View Post
and doesn't your last sentence contradict your first?
I see it like this.

An idealized speaker driver has a perfectly rigid cone that moves in response to the music signal. The signal drives a current through voice-coil which interacts with the fixed magnetic field of the permanent magnet. So the cone moves relative to the fixed magnet. Both the high and low frequency signals are translated into motion of the cone relative to the fixed magnet. The time-average position of the cone is fixed. The cone + magnet form the radiating source and it's position is stationary. There is no Doppler shift in the traditional sense because the sound source is not moving. The classic example is a Fire truck siren that changes pitch as it comes towards you and then away from you. If this siren were a speaker driver, the whole thing magnet + cone is in motion toward and then away from you. For a stationary speaker there is no motion of the source.

If there were a sound source on the cone surface, say a small tweeter, then a low frequency vibration of the main cone would cause the tweeter to move back and forth and it would exhibit Doppler distortion. Cone break up modes are parasitic vibrations of the cone surface, they are stimulated by the music but they exist in the fabric of the cone and are not movement referenced to the fixed magnet but referenced to the cone surface. In this way, sound from cone break up is like the small tweeter glued to the cone, the source will move with the cone and Doppler distortion is possible.

It's not a case of fancy physics, its about separating out the movement of the source from the movement of the parts that constitute the source.

There are several sources of non-linear behaviour in a real speaker system and so it is conceivable that measurements of those sources might have similarities to the signature expected from Doppler distortion.
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Last edited by Bigun; 27th June 2013 at 08:12 PM.
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Old 27th June 2013, 08:33 PM   #9
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Quote:
Originally Posted by Bigun View Post
An idealized speaker driver has a perfectly rigid cone that moves in response to the music signal.
Idealized to keep the math used in analysis in the 5% of the problems that can be done with "regular" prblem solving techniques. This does not make it an ideal loudspeaker.

dave
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Old 27th June 2013, 08:41 PM   #10
chrisb is offline chrisb  Canada
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Gee, I can remember when one of the models for an "idealized" speaker was a massless pulsating sphere - which to my thinking at the time (yes, that was late mid last century, so some "perception enhancement modalities" may have been involved, and perhaps are still flashing back ) suggested elasticity of the surface, if not imaginary materials and energy sources, and certainly not perfect rigidity

I can see your point about parasitic vibrations/resonances in cone (or other diaphragm type) being unrelated to the signal induced movement of the voice coil (or alternative motive method), but does not the type of wide frequency bandwidth that FR drivers such as under discussion here are targeted to achieve rely on a certain amount of controlled decoupling of emitting surfaces? as well as their profiles, variations in material density, perimeter termination / damping , and no doubt other factors?

Perhaps not (see first parenthesis above)
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