Why the devil?
For those who want to reintroduce voltage-drive into a current-drive amplifier for electromagnetic damping: Why re-invite the devil again?
When you understand F = Bl x i, you stop using voltage-drive…
When you understand Eemf = Bl x v, you stop using electromagnetic damping…
Electromagnetic damping introduces and amplifies unwanted cone movements, and is a way to loose control over the cone.
In every component in the audio chain, one wants to avoid microphonics. Why introduce it in your loudspeaker? It is like sticking a microphone in or to your loudspeaker to pick up all the inside and outside sounds to destroy your input signal. The first law (above) describes how the amplifier drives the loudspeaker driver, the second law describes how the loudspeaker driver drives your amplifier (this is the microphone effect, the law behind the workings of an (electrodynamic) microphone). Now you know where modulated noise comes from… (All the sounds inside the enclosure of drivers and ports and all the sounds outside the enclosure.)
And for those who think it is some form of negative feedback: the loudspeaker output (SPL), cone acceleration, force on the cone and current relate to each other, and cone speed and back emf is something completely different and 90º degrees too late! Try to imagine what happens to the amplifier when a low loud bass note suddenly stops… (It is known that some loudspeakers can kill amplifiers.)
And for those who think of electromagnetic damping as a brake. There is one essential component missing: the brake peddle, it is always braking! So it is more like drag than a brake. And for those who think that will be a small signal: at resonance it is about 90% of the original signal! (It brings the Qm down to Qts.)
The only challenge is to lower the Q at resonance. Of all the possible ways to lower the Q, electromagnetic damping is by far the most imprecise and unstable (no control) and introduces the most distortion (unwanted cone movements).
So, stick to current-drive, above, at and below resonance.
For those who want to reintroduce voltage-drive into a current-drive amplifier for electromagnetic damping: Why re-invite the devil again?
When you understand F = Bl x i, you stop using voltage-drive…
When you understand Eemf = Bl x v, you stop using electromagnetic damping…
Electromagnetic damping introduces and amplifies unwanted cone movements, and is a way to loose control over the cone.
In every component in the audio chain, one wants to avoid microphonics. Why introduce it in your loudspeaker? It is like sticking a microphone in or to your loudspeaker to pick up all the inside and outside sounds to destroy your input signal. The first law (above) describes how the amplifier drives the loudspeaker driver, the second law describes how the loudspeaker driver drives your amplifier (this is the microphone effect, the law behind the workings of an (electrodynamic) microphone). Now you know where modulated noise comes from… (All the sounds inside the enclosure of drivers and ports and all the sounds outside the enclosure.)
And for those who think it is some form of negative feedback: the loudspeaker output (SPL), cone acceleration, force on the cone and current relate to each other, and cone speed and back emf is something completely different and 90º degrees too late! Try to imagine what happens to the amplifier when a low loud bass note suddenly stops… (It is known that some loudspeakers can kill amplifiers.)
And for those who think of electromagnetic damping as a brake. There is one essential component missing: the brake peddle, it is always braking! So it is more like drag than a brake. And for those who think that will be a small signal: at resonance it is about 90% of the original signal! (It brings the Qm down to Qts.)
The only challenge is to lower the Q at resonance. Of all the possible ways to lower the Q, electromagnetic damping is by far the most imprecise and unstable (no control) and introduces the most distortion (unwanted cone movements).
So, stick to current-drive, above, at and below resonance.
Voltage drive for electrostatic loudspeakers.
There is certain symmetry hobbyists should be aware of.
There is certain symmetry hobbyists should be aware of.
- Voltage drive is good for electrostatic loudspeakers.
- Current drive is good for electrodynamic loudspeakers.
In “Computer-Aided Electroacoustic Design with SPICE” from 1991, especially in paragraph 6.3 Condenser Electrostatic-Mechanical Transducer, W. Marschall Leach JR showed the formulas and modeling of an electrostatic transducer. The output is dependent on the current and not on the voltage.
In an electrodynamic transducer force is created by moving charge (current), in an electrostatic transducer force is created by static charge (voltage is an indirect quantity via capacitance).
I think that electrostatic drivers also benefit from current-drive.
In an electrodynamic transducer force is created by moving charge (current), in an electrostatic transducer force is created by static charge (voltage is an indirect quantity via capacitance).
I think that electrostatic drivers also benefit from current-drive.
Since electrostatics are always (!) dipoles, current-drive is good for electrostatics. Falling drive voltage is desirable, and usually implemented in practical implementations.
Current drive for moving-coil speakers takes out Le and Re (and therefore variations in these). But is more susceptible to artefacts which affect back-emf such as resonances and variations in B, where voltage drive would result in compensatory changes in drive current.
I've mentioned this before in these forums, but for midrange drivers operating above resonance, best practice may be for driving resistance which matches the mechanical impedance of the speaker cone itself - to damp any bell-like resonances.
Current-drive takes out all impedance effects: Re, Le and back-emf.
Elimination of back-emf is a good thing, because
* It eliminates the microphone effect, thus no pickup of vibrations, sounds in the enclosure of back wave and ports, sounds of adjacent drivers, external sounds (other loudspeakers) and thus avoids a lot of modulated noise (which effects imaging)
* It lowers (!) the IMD by nonlinearities in B, as Klippel shows in their non-linearities poster
* It doesn’t help at all in countering vibrations, because it “measures” the wrong signal (speed in stead of acceleration), it “measures” only low frequency signals and it is modulated and distorted
* It is also heavily modulated by temperature and is thus a very unreliable “signal”
* It has no benefits at all
* There are better ways of damping
Elimination of back-emf is a good thing, because
* It eliminates the microphone effect, thus no pickup of vibrations, sounds in the enclosure of back wave and ports, sounds of adjacent drivers, external sounds (other loudspeakers) and thus avoids a lot of modulated noise (which effects imaging)
* It lowers (!) the IMD by nonlinearities in B, as Klippel shows in their non-linearities poster
* It doesn’t help at all in countering vibrations, because it “measures” the wrong signal (speed in stead of acceleration), it “measures” only low frequency signals and it is modulated and distorted
* It is also heavily modulated by temperature and is thus a very unreliable “signal”
* It has no benefits at all
* There are better ways of damping
There is a very simple way to test whether this theory is correct or not.
Take an unconnected speaker driver. Tap it and note how the sound dies out.
Now short the connections, and tap it again. You will notice that the sound dies out more quickly and sounds more 'dead' by want of a better term.
Conclusion: a shorted voice coil driver is better damped.
A shorted connection is equivalent with voltage drive because a voltage source acts as a short for external excitation.
Even better if you have an iron core/coil meter laying around: jiggle the unconnected meter and the pointer will move around wildly. Now short the connections and wiggle it again: the meter doesn't move nearly as much because it is heavily damped by the short.
In fact, manufacturers always recommend to short such meters when transporting them, to damp the movement and protect the mechanism.
Jan
Take an unconnected speaker driver. Tap it and note how the sound dies out.
Now short the connections, and tap it again. You will notice that the sound dies out more quickly and sounds more 'dead' by want of a better term.
Conclusion: a shorted voice coil driver is better damped.
A shorted connection is equivalent with voltage drive because a voltage source acts as a short for external excitation.
Even better if you have an iron core/coil meter laying around: jiggle the unconnected meter and the pointer will move around wildly. Now short the connections and wiggle it again: the meter doesn't move nearly as much because it is heavily damped by the short.
In fact, manufacturers always recommend to short such meters when transporting them, to damp the movement and protect the mechanism.
Jan
The point is that unwanted cone movements are resisted. With current drive, external influences have a free rein to flap the cone around.
And in the case of variations in B, those are countered also - if B diminishes, back-emf falls and I increases (and vice-versa).
Better ways of damping might be mechanical, where the voice-coil joins with the cone*. But only with voltage drive, and low Re. And only for midrange drivers operating above primary resonance.
(* Damping between voice-coil and cone - as in back-matching a transmission line.)
Last edited:
You probably drive your car with your handbrake allways on?
If you care about audio, you don’t want electric damping because it’s very problematic: modulation, distortion, compression, instability in frequency response and crossovers, etc, etc and you don’t need it, because there are better ways of damping.
Another interesting viewpoint: if the temperature of your voice coil is raised with 256 °C you get an extra resistance of 100%, thus the damping factor of your driver alone (impedances of amplifier, cables and crossover = 0 Ohm) is 1 (probably your voice coil is melted).
This also means:
128 °C gives a DF 2
64 °C gives a DF 4
32 °C gives a DF 8
16 °C gives a DF 16
8 °C gives a DF 32
4 °C gives a DF 64
2 °C gives a DF 128
1 °C gives a DF 256
This means that if you put some music on, or it’s a sunny day, or you light a candle, your DF and your electric damping is out of the window.
What I have done that's worked well is to start with a driver that has a lower Qms, then tightly pack fiberglass insulation in the box (no port) leaving just a few inches of space around the back of the driver. It's a bit of trial and error to get the density right.
Dave
Ahh yes, passive damping, that should help to control it.
But I was also interested what Jerry had in mind when he said 'because there are better ways of damping'. So more than one?
Although, since he resorted to personal/snide remarks that mostly mean he has no factual/technical reply. But he may surprise us!
Jan
But I was also interested what Jerry had in mind when he said 'because there are better ways of damping'. So more than one?
Although, since he resorted to personal/snide remarks that mostly mean he has no factual/technical reply. But he may surprise us!
Jan
Last edited:
Do you need to?
I mean, EQ has to be used anyway so unless the Q is so high that it makes EQ difficult I don't see why it could not be addressed at the source.
Everyone is discussing current, voltage and speaker cones, but not air, which is the most important. A speaker cone interacts, first and foremost with air, causing a thin layer of air to undergo rapid changes in pressure. This imparted energy, moves in the form of pressure waves in air.
Unless a speaker cone is completely isolated from its back and is situated in an infinite medium, reflections, diffraction and refraction take place. The resultant wavefronts then superpose and interact, and since air is not a very good acaustic medium, possibly, some distortion arises. As far as I can remember, the speed of sound is dependent on the air's temperature, it is also dependent on its pressure and it also a function of amplitude!
What do I want to say with all this?
I think, the answer should be readily imaginable. If the medium through which sound travels to reach our ears is so imperfect, why should anyone worry about any unconventional way/s of driving a speaker?
Unless a speaker cone is completely isolated from its back and is situated in an infinite medium, reflections, diffraction and refraction take place. The resultant wavefronts then superpose and interact, and since air is not a very good acaustic medium, possibly, some distortion arises. As far as I can remember, the speed of sound is dependent on the air's temperature, it is also dependent on its pressure and it also a function of amplitude!
What do I want to say with all this?
I think, the answer should be readily imaginable. If the medium through which sound travels to reach our ears is so imperfect, why should anyone worry about any unconventional way/s of driving a speaker?
Typical electrodynamic speakers have back-EMF as a feedback mechanism to control cone movement so they are best driven by a low impedance aka buffering factor and the "parallel" resonance is well controlled. But there will be mechanical resonances that behave like a series resonance, at higher frequencies, and hence are best controlled with a high impedance drive. So an series inductor without a shunt capacitor soothes out the woofer sound. Electrostatic and piezo drivers produce a current that follows diaphragm movement so a high impedance better controls them.
But most drivers will have a series of parallel and series resonances at different frequencies, so the best solution may be a nominal impedance like 8 Ohms that dampers both series and parallel resonances.
Even when electronic crossovers are used, inductors on woofers and caps on horn or tweeters is recommended.
But most drivers will have a series of parallel and series resonances at different frequencies, so the best solution may be a nominal impedance like 8 Ohms that dampers both series and parallel resonances.
Even when electronic crossovers are used, inductors on woofers and caps on horn or tweeters is recommended.
The usual method in the 1930's was a thick cloth around the back of the loudspeaker. That and the acoustical short due to the ventilation holes in the back of the valve radio was apparently enough to get a reasonably flat response from a loudspeaker driven from an open-loop pentode stage (reasonably flat to 1930's standards, that is).
Another method is motional feedback using a sensor mounted on the moving part of the loudspeaker.
That's correct in the far field, and Peter Walker already proved it in 1980 in an AES article about the ESL 63.
Air is much more linear than the mechanical speaker construct. Only at extremely high sound pressure levels, air non-linearity starts to appear.
In home use it is not a factor and the air can be considered perfectly linear.
As far as the air load on a speaker is concerned, that is much less than, again, the mechanical frictions and losses. This means that the air load on the speaker is very small, which is the reason why the speaker efficiency is so poor. Its a classical case of bad load matching!
In electrostatic and similar speakers that have a large area with respect to the mechanical losses, the air load becomes significant and then you see a large increase in efficiency, because the load matching is much better.
Jan
When recording, we record voltage of the microphone.
When playback, we should use voltage drive. At least, make more sense to me.
There are some subwoofer drivers using position feedback, physically. I am not sure whether the relation between position and signal voltage is 1 to 1 relation. Probably, also a function of frequency.
When playback, we should use voltage drive. At least, make more sense to me.
There are some subwoofer drivers using position feedback, physically. I am not sure whether the relation between position and signal voltage is 1 to 1 relation. Probably, also a function of frequency.