The significance of high Qms..?

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#161
For those, who don't want to simplify things too much 😉
Antoine Falaize and Thomas Helie have developed interesting approach using port-Hamiltonian formalism to model loudspeakers, viscoelastic creep in particular.
There is a very interesting graph of displacment response to the step force for loudspeaker with viscoelastic suspention having different creep time constants.

Modeling and simulation of the electrodynamic loudspeaker | Antoine Falaize
 
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#162
If we move to that direction, let's not forget Speakerbench. Although it might not be that relevant to creep and viscoelasticity.

And, giving it a thought, while Falaize and Helie sure do bring us more insight and we should be thankful for that, the effects should show in normal loudspeaker measurements. Among others that have been discussed before.
 
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#163
There is nothing more practical than a good theory © 🙂. There are not many papers investigating experimentally the loudspeaker suspention creep in the time domain (of course, the frequency domain is mathematically equivalent, but not so representative), but a couple of experimental studies you can find (Djurek et al., Vanderkooy et al.).
 
#164
kstrain said:
Q vs loss

I'm hesitant to interject where there may be strongly-held opinion, but I believe Qms is unlikely to be a reliable guide to high-frequency behavior of a loudspeaker.

People like speakers that happen to have high (or low) Qms, and that's fine, of course: I'd not argue with preference. It might even be that within a given range of speakers, the preference scales with Qms. That does not mean that Qms can be used to predict the sound of another speaker more generally.

The following overview of some of the different types of damping may be helpful. It represents a modern understanding developed over the last several decades in engineering and material physics.

Damping in Structural Dynamics: Theory and Sources | COMSOL Blog

This was the first hit in a search, I have no connection to the author or companies involved.

The problem comes from the definition of the quality factor Q of a resonance:

Q is defined to be 2 pi times the energy stored in the resonance divided by the energy lost per cycle of the resonant frequency. As a consequence, it has no well-defined meaning other than at the resonant frequency. It used to be thought that all damping was viscous, and Q at one frequency could be used to predict loss at another, but I do not believe that's a good guide given the kinds and range of materials used in loudspeakers.

A more informative quantity which is defined at all frequencies is the damping loss, aka the loss angle, or often tan delta in electronics - often given the symbol phi(f) - a function of frequency (f).

For real materials phi is often a nontrivial function of frequency; at some frequency like 10 times fs, it can differ from phi(fs) by easily a factor three; or in other cases, it could take almost the same value. That's a big spread, as Qms values are not very spread out in the first place.

Thinking of cone-edge resonances (often at 10 to 50 times fs). You could find two speakers with the same Qms (pick 5), and would know that the loss phi(fs) =0.2 for both. If one of the speakers has most of the loss in the surround, but the other has more of its loss in the spider, you'd have to figure this into the argument along with Qms. So in this case you might expect the speakers to sound very different even though Qms is the same.

I couldn't find two speakers that make a tight comparison, with Qms =5, but the following come close enough to make the point:

B&C 12NDL76 Qms 4.2, double roll surround, single spider
B&C 12NDL88 Qms 5.0, triple roll surround, double spider

Double spiders tend to have considerably more loss than single spiders, yet the Q of the 12NDL88 is higher, but then so is the cone mass.

If someone wants damped or undamped cone resonances, isn't it better to look for direct measurements of the break-up resonances, rather than a proxy that is not necessarily reliable?

Ken
Click to expand...

Someone mentioned driving the speaker with a current amp, and I'm also inclined to think that way.

If Qms is tuned to some 'reasonable' value for a speaker's bass resonance and marketable T-S parameters, then a few octaves further up in frequency the plastics and rubber used to achieve it may behave very differently. Yield (creep?) and hysteresis suggest that any attempt at mechanical damping could contribute to IM distortion.
Stored energy bouncing back and forth between the spider edge and voice coil also come to mind.

If we combine high sensitivity with high Qms, the worse a high damping factor voltage amplifier would seem to perform, because of the speaker's tendency to convert stored energy back into voltage at a later time. Not only do many amps distort when that happens because there's no control of the phase angle of the (electro-mechanical) feedback, so global feedback circuits become dysfunctional, but high efficiency speakers would exacerbate those effects because they produce higher voltages.

Mechanical echoes always undergo a bit of distortion when the motor converts them into current and voltage. A voltage amplifier then echoes that already-distorted signal with current, so as to clamp the output voltage to the 'correct' value, thus attempting to mechanically lock the voice coil whenever the spider (or cone or box or air pressure or finger) applies a force to it.

The more I think about it, the more solidified I become as a current amplifier convert, with a view that given a speaker's mechanical limitations, at least the amplifier should let the vc float freely. The option of adding electrical damping is always there, but it's really subtle and seems to be reserved mainly for the deep end of active speaker design.

Lech
 
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#165
FYI/FWIW, some of the widest range, highest resolution 12", 15" woofers around are vintage Altec with Qms ranging from ~1.4-4.5 with most of them in the ~2.2-2.8 range. Note that all are (relatively) low Xmax (~4-6 mm), (super) high Vas by today's standards (~16 - 32 ft^3 measured).
 
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#166
Isnt that something....Qms is a measure of mechanical Q... yet Vas is sort of a a measure of compliance too as well with air volume as the unit, referring to compliancy of air....higher volume of air has more give... Vas is focused on just Suspension as in the surround? And Qms is the driver as whole, mechanically.
 
#167
GM said:
FYI/FWIW, some of the widest range, highest resolution 12", 15" woofers around are vintage Altec with Qms ranging from ~1.4-4.5 with most of them in the ~2.2-2.8 range. Note that all are (relatively) low Xmax (~4-6 mm), (super) high Vas by today's standards (~16 - 32 ft^3 measured).
Click to expand...
Large diaphragm low moving mass woofers tend to be to be additionally damped by resistive air load which limits Qms.
Only when measured in vacuum we'll see their true mechanical Qms (unless it is limited by eddie currents in a conductive VC former).
 
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#168
Fascinating thread. Many good discussions.

My thought is that, Qms is at best a hint of the driver's damping behaviour. It's certainly not a reliable prediction of sound quality. Similar to "I found high efficiency drivers sounds better."

  • Problem 1:
As mention before, by definition, Qms describes damping at resonance frequency, it does not make much sense to predict SQ at other frequencies. RMS is a much better parameter.

  • Problem 2:
Combine all mech damping into a single value is very problematic. RMS(Qms) comes from many different sources. Ventilation, surround, spider, and voice coil former eddy current. Each of them has very different non-linear behaviours.

The most non-linear damping come from insufficient ventilation, as air traval is extremely non-linear between low speed to high speed, and should be avoided at all cost. Best example of my driver collection is the removal of foam plug on the back of Morel SCM634.

I believe the second most important damping source is surround. In fact, in the earlier Joachim Gerhard interview, when he's talking about "low loss drivers", he mostly specifically talking about low loss surrounds and uses Seas CA17RCY as an good example. That "low loss driver" has a QMS of 1.7, which is not high at all by today's standard. Joachim Gerhard used many aluminum voice coil former drivers in his design in the past, for example Canalis Anima.

Eddy current deserves another section..

  • Problem 3:
RMS(Qms) and its effect on mid-high frequency. Actually Qms makes no sense here so I just focus on RMS. RMS is defined as kg/s as unit. In my understanding, RMS effect is proportionally less with frequency rises, the higher the frequency, the smaller cone speed, but damping is proportional to speed(hence the "/s" part in its unit), not acceleration(which is what makes sound). That's why Qms only affects LF efficiency around resonance, A good example is to compare Seas 27TDFC and 27TDC. Any claim that high-Qms drivers result in good mid-range detail because of less damping is physically problematic(sorry for my English), especially for a low FS driver.

  • Problem 4:
Voice coil wire is also conductive, which generates electro-magnetic damping exactly the same mechanism as an conductive voice coil former. But it's represents in QES. Let's assume we have a pure voltage amplifier, after an impulse audio signal there's quiet passage, but the cone is still moving. At this moment, QES and QMS(Voice coil former part) works exactly the same way by using shorted eddy current to stop the cone motion. Now, why low Qms is considered bad but low Qes is considered good?

  • But voice coil former eddy current does affect distortion
I believe that's how most people relates good midrange sound with high QMS. Aluminum voice coil former have lower QMS, at the same time they has higher distortion in the midrange if used without a shorting ring. But the midrange distortion has nothing to do with QMS/Damping!

as described in Problem 3, damping naturally has less effect at higher frequencies, and motion-induced eddy-current is linear in nature(proportional to speed. proportional = linear unless you reinvent physics).

But eddy-current distortion effect is real, at midrange it's not caused by coil movement, but the result of magnetic system's hysteresis induced distortion component. That's exactly why current-drive has less midrange distortion, but current drive can't stop eddy-current flow in voice coil former. However, shorting devices such as copper cap will greatly couple voice coil former and reduce it's eddy current at mid-high frequencies(and distortions). However, copper cap will not couple the VC former at low frequencies, so it will not change the driver's QMS. In such driver with aluminum VC former and copper cap, QMS is low, but midrange distortion is also Low. So these two effects(Low QMS, higher midrange distortion) are no longer connected because of shorting rings.

Please note that again, voice coil wire itself is also conductive as aluminum voice coil former. When using with a voltage amplifier, they generate the same rising midrange distortion regardless low or high QMS, if the driver has no shorting rings.

I've heard some very good FR drivers with very low QMS but also very low midrange distortion, thanks to it's copper cap and their midrange clarity is great. For example, many MarkAudios, Audience A3s.

Thanks for reading. Peace..🙂
 
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#169
Besides of Seas tweeter has nearly identical constructions of high and low loss(27TDFC and 27TDC)，there's also examples such as Scanspeak 18W/8545 vs 18W/8545K. But I found these 2 examples are pretty informative with SPL/distortion:

8-202 kapton former:
http://nedlab.com/wp/wp-content/uploads/2014/06/Eton-8-202-C8-37HEX-woofer.pdf8-202 aluminum former
http://nedlab.com/wp/wp-content/uploads/2014/06/Eton-8-200-A8-37HEX-woofer.pdf
Here kapton former version actually has higher midrange distortion probably due to less damping/higher breakup peak. Aluminum version seems has a bit higher LF distortion. But no huge differences.
 
#170
racingpht said:
Besides of Seas tweeter has nearly identical constructions of high and low loss(27TDFC and 27TDC)，there's also examples such as Scanspeak 18W/8545 vs 18W/8545K. But I found these 2 examples are pretty informative with SPL/distortion:

8-202 kapton former:
http://nedlab.com/wp/wp-content/uploads/2014/06/Eton-8-202-C8-37HEX-woofer.pdf8-202 aluminum former
http://nedlab.com/wp/wp-content/uploads/2014/06/Eton-8-200-A8-37HEX-woofer.pdf
Here kapton former version actually has higher midrange distortion probably due to less damping/higher breakup peak. Aluminum version seems has a bit higher LF distortion. But no huge differences.
Click to expand...
Thank you for all the great points - it was some much needed house keeping. It is all a bit vague as many of the parameter values float up or down relative to each other depending on tuning, with no clear benchmark.
Qms could probably be forced up in value by increasing mass while keeping resistive losses the same, and/or tightening Vas, so it seems like a game of looking at all of the parameters together.
 
#171
The damping caused by conductive formers is actually electromagnetic damping although it shows up in the Qms value.
The tales of high Qms giving better sonic performance stem from the assumption that less mechanical friction results in less influence of any nonlinear properties of said friction I assume.

Regards

Charles
 
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#172
This is what I wrote very long time ago about the topic in a german forum (translated by deepl):

Question:
Hello,

have read that you once wrote something about the mechanical losses (Rms). Have now googled all the time but nothing useful found. Could you explain it to me again? I'm about: do I see that correctly the higher the rms value is the more its losses are ie efficiency or? but if a bass with 95 db 1w/1m is angegebn then it does not matter how high the rms values are or? or the rms value indicates something else. Because if a loudspeaker makes 100db 1w/1m with 1,5 rms or 100db with 6 rms it wouldn't matter or does it shorten its lifetime?

Many greetings
Click to expand...

My answer:

Hello,

I'll try to write a little essay about the mechanical loss factor RMS. Unfortunately, you have to understand how such a chassis works, especially how the "efficiency" comes about:

A loudspeaker chassis is an energy converter, similar to a motor. It converts electrical power - via the detour of mechanical power - into sound power. As with a motor, this does not happen without losses.
To put it clearly: a loudspeaker is a very bad energy converter, about 99.5% is converted into heat, only half a percent comes out at the back as real "sound power", often even less!

To illustrate: if my amplifier pumps 100W electrically into the back, 0.5 watts of sound power comes out the back. By the way, a whole symphony orchestra in tutti produces sound power of less than 10 watts.

Now, of course, the question arises where the remaining energy is;

The main problem lies in the conversion of the mechanical energy into sound energy. Such a loudspeaker diaphragm with a "usual" surface can hardly excite the air to vibrate. Analogous to electrical engineering, a lot of energy is converted into "reactive power", i.e. a lot of air is moved "back and forth" without producing sound. One says to it "the too small diaphragm couples too badly to the radiation resistance of the air".

That's why one tries - when high levels are required - to make the coupling area to the air as large as possible by means of horns (another effect of the horns is that they "direct" the sound energy better, but that's not relevant now).

Specific: Most of the sound is lost, because the diaphragm makes a large part of its stroke work "useless", the "production" of sound is very inefficient.

In order to understand the further behavior of a driver, I differentiate into three "operating states", whose distinguishing feature is the frequency range that is to be reproduced: 1) below the resonance frequency, 2) above the resonance frequency, 3) at the resonance frequency.

1) Here the drive must work against the spring forces. This includes the influence of the suspension (spider, surround, but also the air spring through the enclosure!) -> The system is "spring loaded". In this area the efficiency is composed by diaphragm area (coupling to the air), strength of the drive (Bxl product) and of course the total spring stiffness

2) Here, the diaphragm mass to be accelerated dominates, which must be decelerated and accelerated by the electrical signal at each oscillation -> a "mass-inhibited" system. In this area, the efficiency is composed of the membrane area (coupling to the air), the strength of the drive (Bxl product) and of course the membrane mass to be moved.

3) this is where it gets interesting: at resonance, a mathematically ideal mechanical spring oscillator would theoretically oscillate infinitely far. In practice, however, this does not happen because there is always damping - i.e. losses - which "eat up" the oscillation energy. On the one hand, the chassis is electrically damped, in the case of TSPs expressed by the electrical Qes. The electrical drive not only converts the electrical energy into movement (motor principle); if the diaphragm "overshoots the mark", a current is induced in the voice coil by the movement and short-circuited via the amplifier output -> generator principle (amplifier = ideal voltage source). This can also be tested in practice: Simply press the diaphragm in with the mandrel when the amplifier is switched on and off. When the amp is switched on, the counterforce is significantly greater. By the way, the electric quality usually dominates the total damping! (compare Qes and Qts of a driver....)

Now it gets significant: also mechanically a driver is damped. The losses are mainly caused by the deformation work of the surround and the centering spider, which are "rolled through" during excursion. This is expressed by the mechanical quality factor Qms, as well as by the mechanical loss factor Rms (both are linearly related, Rms=2*Pi*Fs*Mms/Qms, so the mechanical quality is normalized to diaphragm mass and resonant frequency). But not only the mechanical deformation work flows into Rms or Qts, but also an electrical effect; if the carrier of the voice coil is made of a conductive material (often aluminum), an eddy current is induced in it when current flows. Now a little bit of energy is used to reverse this eddy current with every movement.

Let's have a look at the "quality" of the main influences a) electrical damping by generator principle (Qes), b) electrical losses by eddy currents (included in Qms) and c) purely mechanical losses by deformation of the suspension (Qms); Harmful is always when effects behave non-linearly, i.e. when a damping does not act proportionally to the excursion, current flow, frequency, but e.g. enters into it quadratically or exponentially. Then distortions are generated, which should be avoided if possible.

Let's have a look at a) according to this criterion; the linearity is essentially dependent on constant magnetic field strength over the stroke, whereby a decrease of the field strength with deflection is partly compensated by the fact that, after all, the counterforce also decreases. Incidentally, there seems to be a tendency for distortion to also increase as the drive (Bxl) becomes stronger, since the field strength gradient becomes increasingly nonlinear. So the distortions that can be caused by electrical damping rise and fall with the quality of the magnetic field

b) The eddy current losses are very linear to the current strength if aluminum is taken as the voice coil carrier, since this is non-magnetic and has no hysteresis losses. Distortions due to this effect are negligible. If non-conductive carrier material is used, this effect does not occur at all (Kapton, Nomex etc.).

c) The "real" mechanical losses due to material deformation are VERY non-linear and generate a large part of the distortions!

So: Rms can be a quality criterion for a chassis. But one should pay attention to the voice coil's carrier material; if a substantial part of Rms (also Qms) comes together by eddy current losses, this is not bad: It costs a little bit of efficiency, but does not produce any distortions. The purely "mechanical" part of Rms is bad and responsible for about 1/3 to 2/3 of the distortion in an "average" chassis.
The linearity of the magnetic field in the motor (linearity of Qes) for the other part.

What has been stated refers to the quality of reproduction of "low" frequencies around the resonance of a driver, at "high" frequencies one must still take into account bunching, partial oscillation and breakup behavior.
Click to expand...

Smart guy "Cpt._Baseballbatboy" added / corrected some information:

good text, a few comments:

"If the carrier of the voice coil is made of a conductive material (often aluminum), an eddy current is induced in it when current flows. Now a bit of energy is used to reverse the polarity of this eddy current at each movement."


The losses are not caused by the reversal of polarity (which takes place because there is still some capacitance involved somewhere, but this can be neglected) but by the properties of a current generated by induction. This current creates a magnetic field which counteracts the cause of the current, i.e. it slows down the movement of the voice coil (because the cause is the movement).


"By the way, there seems to be a tendency for the distortions to also increase as the drive (Bxl) becomes stronger, since the field strength gradient becomes increasingly nonlinear. So the distortions that can be caused by electrical damping rise and fall with the quality of the magnetic field."


It depends on how the high Bxl is achieved: either by a very strong magnetic field or by many turns of the voice coil in the air gap. The latter then has to be split again into the possibility of using a deep air gap, or increasing the number of turns of the coil. The former is expensive (a lot of metal) and reduces the linear excursion (or you take an underhung coil, but it will be even more expensive because the air gap must be even deeper), so the second solution is almost always used.

Many coil turns mean at the same time a high inductance, and this leads to a modulation of the magnetic flux. If you simplify the calculation, you get a lot of K2, depending on the ratio of coil inductance to magnetic flux (<- please don't nail me down on this, I threw away the piece of paper where I calculated this). The higher the inductance the higher the distortions, the higher the flux the lower.

The nasty thing about these distortions is that they are also effective in the midrange, because they don't need any excursion, but only current flow through the coil.

In an aluminum carrier (or also short-circuit rings) this temporal change of the magnetic field generates a current flow, which is equivalent to an attenuation of the change (see above -> eddy currents).
Click to expand...

But as mentioned other factors than Rms or Qms are also relevant, my rule of thumb is to choose woofers with a difference between Qts and Qes of <=0,02 to ensure that Qes dominates the resonance behavior.

Best regards
Peter
 
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#173
I've written before about this, but it helps greatly to view the speaker cone as a transmission medium, with properties of mechanical impedance and velocity of transmission (though it's complicated by the fact that there will be to some extent 2 modes - transverse and longitudinal).

For ringing to be avoided, at least one end of the medium needs to be perfectly terminated resistively. But preferably both, because there will always be discontinuities, which cause reflections (which is why you get ringing in the first place).

The resistive termination can be electrical at the driving end, in which case amplifier output resistance needs to be known (and controlled) - and may need to be negative, to reduce the effect of voice-coil resistance.

If there is mechanical damping at the driving end, this would need to be factored in too. This damping could be in series with the cone, or parallel. In the former case, electrical driving resistance would need to be low (maybe negative); shunt mechanical termination would imply current drive as a requirement.

The above would be true for a midrange driver. The requirement for bass drivers is at odds with this, because the aim is for large excursions and an assumption of a relatively rigid cone...
 
#174
phase_accurate said:
The damping caused by conductive formers is actually electromagnetic damping although it shows up in the Qms value.
The tales of high Qms giving better sonic performance stem from the assumption that less mechanical friction results in less influence of any nonlinear properties of said friction I assume.

Regards

Charles
Click to expand...
The same goes for damping from copper rings, which is why I have to concentrate and frown in order to figure out why exactly one increases distortion while the other decreases it.

The way I understand it, the coupling between primary and secondary windings (VC and former) is modulated by at least 2 independent (but related) factors:
1) the magnet is imperfectly saturated and provides a parasitic transformer core that increases the coupling between primary (VC) and secondary (former) "air coils", and the resulting "mechanical" friction is modulated by dynamic changes in geometry as the VC and former system moves around.
2) the amount of current that generates its own magnetic field. As above, an imperfectly-saturated magnet becomes a transformer core, but the coupling itself is also non-linear because the saturation varies non-linearly with current.

The above would also be a good argument for light-weight cones and high sensitivity drivers in general.
 
#175
abstract said:
The same goes for damping from copper rings, which is why I have to concentrate and frown in order to figure out why exactly one increases distortion while the other decreases it.
Click to expand...

I found it very easy to understand (thanks to some Purifi articles on AudioXpress) by this way:

The magnetic strength is non-linearly modulated by the voice coil current. At low frequency, it's force factor modulation, mostly 2nd order. At high frequency, it's hysteresis loop and is mostly 3rd order.

Now both copper ring and aluminum voice coil former acts as a "shield" to these non-linear effect. Neither couple the low frequency part very well(2nd order), but mostly the hysteresis part. However, copper ring is not allowed to move, so it's a pure shield to reduce it from magnet system. But voice coil former moves as the cone, so by shielding these non-linear effects, it actually "translates" these non-linear magnetic modulation into sound waves.

I think it's worth mentioning aluminum voice coil former is many orders of magnitude less conductive than copper cap(mostly due to a slit). so the effect is mostly quite small. I think the effect is proportional to QES/QMS(eddy current part only).
 
#176
Laws of physics:
That which is at rest shall remain at rest unless acted on my an outside force.
That which is in motion shall remain in motion unless acted on by an outside force.

In other words we have mass to deal with. Bad enough, now we add mechanical and electrical "springs" into the equation so we deal with resonance too.

The basic problem is the signal we send is representative of the sound wave we want and does not include the energy needed to overcome inertia. Jerk, acceleration, speed. Dt Dx you know, freshman calculus. Low Q drivers have a higher mechanical force impeding them so the relative effect of inertia making the jerk less if we have enough drive.

Pro drivers are all about efficiency. Home drivers are all about low distortion. Two use cases, two sets of tradeoffs.

A lot of the motor issued have been discussed. There are some really good CAD programs to model this.

None of that deals with bell modes, teeter-totter, cone resonances, cone standing waves, reflections from the surround, breakup etc. Glad I just use drivers, not design them.
 
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#178
... a conductive voice coil former does since it moves in the magnetic field.
Click to expand...

Hej Lars

Speaking of conductive formers: Is your opinion also - like some mentioned in this thread - that a conductive former increases nonlinear distortion, especially at low levels ? If so, what is the reason for this ? If there really are low level nonlinearities caused by a low Qms/Qes ratio then the purely menchanical part of Rms would be responsible for this IMO.

Regards

Charles
 
#179
Sure glad I don't design drivers, just use them. It would seem they are a horrible mix of catch-22 problems. Reading all the old ASE papers, we knew what we wanted 70 years ago, just can't build it.

Looking at a new build and gee it is tough, Purifyi vs CSS. Only used SEAS and SB in the past Step up time.
 
#180
Wow great to have Lars here!

@phase_accurate This paper seems to measure this effect and speculated about voice coil former and distortion effects in sector 11, by the auther of 《Current-Driving of Loudspeakers》:
https://acoustics.ippt.pan.pl/index.php/aa/article/view/1780/pdf_255
Personally I think hysteresis distortion in speakers seems pretty constant % to SPL at low level. However I'm not sure it's an effect rising % as level goes lower. Like transformer distortion. Hope Lars has an answer!

How Transformers Distort​

https://www.soundonsound.com/techniques/analogue-warmth​

transformer_distortion-jfO5To4HkDpvPJMlTwKqHD.8m7Z.PSQo.jpg
 
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