The classic Leslie tweeter high frequency horn driver also "stays still", coupled to the high frequency horn through a gasket.
Only one of the two rotating horns is driven, the other is used for dynamic balance.
If both horns were driven, the Doppler frequency modulation (FM) would be reduced, the speed would have to be doubled for the same FM. Spinning faster
The horns have diffraction/diffuser/deflector/reflectors which redirects the higher frequencies(5-10kHz) in a sort of a torus (donut) shaped pattern emanating at 90 degrees from the direction the horn points at.
Some Leslie HF horns have had the reflectors removed, without the reflector, the effect is more amplitude modulation, AM.
A woofer reproduces a multitude of frequencies, the lower frequencies modulate the higher frequencies, the low excursion higher frequencies being carried (modulated) forward and back by the much higher excursion low frequency cone movement.
Doppler distortion (FMD) exists and at extremely high excursions can be measured in small fractions of a cent, 1/100 of a semitone, occurring too fast to be perceived as FMD.
FMD is not the source of the objectionable types of loudspeaker distortion, dominated by intermodulation (IMD).
Electrostatic speakers have very large diaphragms which are capable of very little excursion before the diaphragm would arc against the stator grid, so are incapable of enough excursion to cause AM or FM effects.
The higher the amplitude of the low frequencies, and the wider the bandwidth separation, the more apparent the IMD becomes.
Small, wide bandwidth drivers are generally the worst offenders from an IMD standpoint, or AM and harmonic distortion as the voice coil is driven out of the magnetic gap.
Art
Doppler distortion is a type of IMD. Intermodulation products are present in the form of sidebands just the same as for AM modulation. In spectral analysis terms, AM and FM intermodulations can be indistinguishable as demonstrated by Lars Risbo at Purifi: https://purifi-audio.com/blog/tech-notes-1/doppler-distortion-vs-imd-7
I just give up on you. There have been enough responses now to disprove your statements.
The summation of the Rod Elliot article @Markw4 linked:
”The effect is very small (to the point of being virtually inaudible by itself), and is usually swamped (or masked if you prefer) by amplitude modulation and intermodulation distortion, so could be considered immaterial in any typical loudspeaker system. If people really want to describe the effect as a distortion - not an unreasonable assumption, since it does exist - then it should be re-named. The correct term is (in my opinion) Phase Modulation Distortion (PMD), and I suggest that the term 'Doppler distortion' be dropped from usage….
The methods that may be used to minimise PMD are exactly the same as those used to minimise intermodulation distortion, primarily, reduce the excursion of the mid-bass driver. This may be done by using a crossover and a subwoofer, or by choosing an alignment that reduces the low frequency excursion to the absolute minimum. Naturally, a 3-way system will outperform a 2-way in this respect, since the midrange driver's excursion will be minimal with no bass content.”
So, beyond the bombast of the initial statement(s) and lengthy reading we find that it’s:
Oh..
So just to be clear IMD IS IMD and Doppler IS Doppler.
”The effect is very small (to the point of being virtually inaudible by itself), and is usually swamped (or masked if you prefer) by amplitude modulation and intermodulation distortion, so could be considered immaterial in any typical loudspeaker system. If people really want to describe the effect as a distortion - not an unreasonable assumption, since it does exist - then it should be re-named. The correct term is (in my opinion) Phase Modulation Distortion (PMD), and I suggest that the term 'Doppler distortion' be dropped from usage….
The methods that may be used to minimise PMD are exactly the same as those used to minimise intermodulation distortion, primarily, reduce the excursion of the mid-bass driver. This may be done by using a crossover and a subwoofer, or by choosing an alignment that reduces the low frequency excursion to the absolute minimum. Naturally, a 3-way system will outperform a 2-way in this respect, since the midrange driver's excursion will be minimal with no bass content.”
So, beyond the bombast of the initial statement(s) and lengthy reading we find that it’s:
- not really Doppler
- ’almost’ inaudible (?)
- and please just use a sub or dedicated bass driver.
Oh..
So just to be clear IMD IS IMD and Doppler IS Doppler.
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So why doesn't this happen to vinyl records, which are cut by a lathe that has a motor that works just like a speaker cone in principle?
Or for that matter to our ears that have a membrane driving the coupling bones?
(quoting Rod Elliot). This is conceptually the most obvious of the "Doppler distortions" (plural) -- HF formed at the LF extreme cone excursions are distance-offset 2*Xmax which can be a large phase shift. That it is "inaudible" even for a fullrange driver, is (I hypothesize) due to averaging: the HF phase is perceived as that of the center position. When I "LX" two drivers (could even be the same model) to align time or phase, easily within 1mm repeatable precision, the method relies on the bottom driver phase being perceived as coming from its physical center i.e. averaging over the cone near and far.
AM/FM/IM/horn/coax-WG distortions are more subtle.
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Doppler shift is defined as a change in pitch or frequency from the wavelength shortening as the source approaches the listener. This can be described mathematically.
Tacking ’Doppler’ to other phenomena is simply misleading and wrong.
Tacking ’Doppler’ to other phenomena is simply misleading and wrong.
Because "The Dopler Effect" is a known & true physical phenomenon, the subject does have validity.
However, from reading, I understand that it is a very contentious subject when it comes to speakers.
A fundamental point regarding the subject is [ obviously ] cone excursion/movement. It is only upon large excursion that Dopler is relevant.
If we look at coaxial/dual concentric drivers, it is only small ones [ KEF ] that will exhibit a degree of D.D. when driven hard.
Classic Tannoy 12's & 15's have relatively lower cone excursion for the same SPL, and this is why they do sound very good.
The way Bose 901's got around excessive D.D. was to use many drivers, thereby reducing cone excursion. [ but still forcing sound with EQ ]
The 'long & short' reality of the subject IS > single full-range drivers, driven reasonably hard, exhibit the greatest amount of Dopler Distortion.
PS.
As has been noted in a lot of posts recently, the amount of bass distortion in so many speakers is in the order of 3 to 5 % to start with >
but Dopler Distortion is especially relevant to higher frequencies.
However, from reading, I understand that it is a very contentious subject when it comes to speakers.
A fundamental point regarding the subject is [ obviously ] cone excursion/movement. It is only upon large excursion that Dopler is relevant.
If we look at coaxial/dual concentric drivers, it is only small ones [ KEF ] that will exhibit a degree of D.D. when driven hard.
Classic Tannoy 12's & 15's have relatively lower cone excursion for the same SPL, and this is why they do sound very good.
The way Bose 901's got around excessive D.D. was to use many drivers, thereby reducing cone excursion. [ but still forcing sound with EQ ]
The 'long & short' reality of the subject IS > single full-range drivers, driven reasonably hard, exhibit the greatest amount of Dopler Distortion.
PS.
As has been noted in a lot of posts recently, the amount of bass distortion in so many speakers is in the order of 3 to 5 % to start with >
but Dopler Distortion is especially relevant to higher frequencies.
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The effect is directly related to ratio between cone speed and speed of sound. As the size is (much) smaller than a regular speaker the doppler effect will be there but much more reduced.
You could say 'the Dopler Effect is omni present' > but it is really just a matter of what is perceivable 🙂
PS.
Dopler Distortion is directly related to INTERMODULATION.
PS.
Dopler Distortion is directly related to INTERMODULATION.
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I've seen this, it's called a whizzer cone on a full-range driver.
Guys; let’s understand/agree on first principles before getting even more hermetical.
Doppler shift is the compression of the wavelength in the medium (air) due to the relative movement between the source and the observer.
How do you suggest this applies to a needle playing a groove?
Doppler shift is the compression of the wavelength in the medium (air) due to the relative movement between the source and the observer.
How do you suggest this applies to a needle playing a groove?
So full range drivers are the worst in respect of this xy-distortions. The smaller they are, the earlyer the cone will show extreme movement.
A simple 2-way should prevent most of this or am I wrong?
A simple 2-way should prevent most of this or am I wrong?
I think we are debating this for philosophical not practical reasons. Why would anyone expect or wish a wideband driver (fullrange 20-20khz loudspeaker driver does not exist, so something like my JBL 2226H Fs40hz-1.5khz) to generate high SPL deep bass directly by the cone, instead of vented BR/QW resonance with the cone hardly moving? Problem solved right?
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The above is correct.
Part of the confusion here is that doppler effect is usually first taught in school in the case where the moving sound source has a constant velocity. However in the case of doppler distortion in a speaker, the moving source is in an oscillatory state of motion. If we write the equations properly including for the source motion then we should get the right answer whether we think of the problem in terms of doppler effect (frequency modulation) or in terms of phase modulation. After all, frequency is just the rate of change of phase.
That's why Elliott says the problem can be thought of in terms of doppler effect or in terms of phase modulation. Its just that he feels the way we usually model phase modulation as opposed to the way we are usually taught about doppler effect can make the latter a little more confusing than the former.
Also, if we think of the problem in "piece-wise linear" terms, such as the source velocity as it passes through its zero (rest) position, then maybe doppler would the more intuitive model.
BTW, in a more general way, similar confusion can arise from using continuous FT, versus typical audio FFT, versus short-time FFT. In many cases (with non-PSS signals) they all give somewhat different answers yet we may tend to think of whatever results we obtain as showing the one and only true physical reality, rather than showing a particular view of the physical reality.
All the above being my two cents only.
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At least that defines what the distortions is, and it's not Doppler. Too bad it didn't go a step further and demonstrate the effect on a real woofer.