Increase (electrical) Damping of a Driver

[Tom79]

Hi,

I'm new in Audio but I have some technical background, altough my master in physics was 18 years ago - so I've forgotten a lot.
I'm interested in increasing the damping of a Midwoofer speaker-driver by modifying the driver itself and the crossover. So that the crossover destroys some of the energy (counter electro magentic force = cemf) that comes from the speaker coil movement. Why damping: because I want a very precise speaker-driver. I know there's a lot of potential because I know a company, who does exactly this and makes incredible good speakers. This company has no patents, so I think, it's a technique from the past.
As I understood: the perfect electric damping would be: if a signal comes from an amp, the coildeflection of the driver follows linearly the voltage of the Amp. If the signal from the amp "stops" (sharp transient) best would be, to have a short circuit over the driver-coil, so no voltage is there anymore, and the coil movement immediately stops. This would happen if the amp would have an output resistance=0, but we all know that's not the case. So any idea how it is possible to increase the electrical damping?

I've found this patent, but I don't understand how it works. Perhaps someone of you can explain me:

US5373563A - Self damping speaker matching device - Google Patents

A damping circuit for speaker systems of the type containing at least one speaker having a speaker coil with an input and an output connection and having a matching coil adapted to be connected in series with the input connection of the speaker coil and a damping coil adapted to be connected in...

patents.google.com

Please post also other links and ideas if you have.

would be great!

Tom

 

[Speedskater]

A power amplifier can have a negative output impedance (by messing with the feedback loop) (I don't remember the details). But speaker designers voice a loudspeaker with a normal amplifier, so response might change.

 

[Tom79]

Havent't read all of this, but neg. Output Impedance Amps seem useless (unstable...): https://sound-au.com/project56.htm

To increase transient response of a speaker you should increase it's (electric) dampening. that's the secret.

I've found here a little explanation how the circuit from above should work, but it's not really clear for me:

https://patentimages.storage.googleapis.com/6c/34/b8/789e456c4bfa3c/EP0761074B1.pdf

 

[DonH56]

Electrical damping usually happens in amplifiers; speakers include mechanical damping. I suppose you could try increasing wire gauge in the voice coil and working on the magnetic field strength, but damping in speakers usually involves cone, spider, and cone material trades. Electrical damping generally means lower amplifier output impedance and/or some sort of servo circuit to add feedback around the speaker itself.

The patent is confusing to me and I wasn't curious enough to try to figure it out. It includes a lot of unproven assumptions that seem a good example of why so many have lost faith in the patent system. The general idea, best I can follow what I did read, is to add a compensatory coil (inductor) to improve the transient response. That sort of approach is pretty common in amplifiers and various passive circuits so I'd be a little surprised it isn't already included in crossovers, or has been rejected as not beneficial. Trying to optimize transient response is tricky in a purely electrical system let alone an electromechanical system where driver characteristics are more difficult to define and control (e.g. change in compliance over time, effects of heating on voice coil impedance, etc.) It's possible "curing" the electrical response results in worse mechanical response (been there, done that). Sticking another pole/zero into the transfer function could improve the response, sure, but could also reduce stability and I wonder how the optimization holds over time and environmental changes. Think doublets in an amplifier (electrical) compensation circuit, where it looks more stable and has improved transient response, at the cost of (very) long settling tails without perfect pole-zero cancellation.

Let us know how it works, be an interesting experiment - Don

 

[Tom79]

thanks a lot for all your inputs. I know, that electric damping is usually made in the amp. Mechanical damping in mid/woofers is very low compared to electrical damping, so my main approach was electrical damping in the crossover. This also, because I know, that the company (Strauss elektroakustik), who produces these incredible speakers, uses such crossovers. It was proven by the ETH Lausanne, that a part of the energy of the coil, that normally goes back to the amp, is "burned up" already in the crossover. Unfortunately these Strauss speakers are that f.... expensive, that I can't just buy one and open it.

 

[NTK]

Dr Toole did his study and wrote this article almost 50 years ago. In his test the tweeter had a resonance at 12 kHz. Increasing the amplifier damping ratio from 0.5 to 200 made no difference.

https://www.diyaudioprojects.com/Technical/Papers/Damping-Damping-Factor-and-Damn-Nonsense-Floyd-Toole.pdf

 

[DonH56]

thanks a lot for all your inputs. I know, that electric damping is usually made in the amp. Mechanical damping in mid/woofers is very low compared to electrical damping, so my main approach was electrical damping in the crossover. This also, because I know, that the company (Strauss elektroakustik), who produces these incredible speakers, uses such crossovers. It was proven by the ETH Lausanne, that a part of the energy of the coil, that normally goes back to the amp, is "burned up" already in the crossover. Unfortunately these Strauss speakers are that f.... expensive, that I can't just buy one and open it.

Mechanical resonances in the drivers themselves are rarely amenable to electrical solutions.

 

[Tom79]

Dr Toole did his study and wrote this article almost 50 years ago. In his test the tweeter had a resonance at 12 kHz. Increasing the amplifier damping ratio from 0.5 to 200 made no difference.

https://www.diyaudioprojects.com/Technical/Papers/Damping-Damping-Factor-and-Damn-Nonsense-Floyd-Toole.pdf

View attachment 297071

What Is Loudspeaker Damping & Damping Factor (DF)? - ProSoundWeb

What damping does, types of damping, calculations, handy charts and more

www.prosoundweb.com

"Driver Variations:
The larger the driver the more import electrical damping is. Low frequency drivers and subwoofers have the most problems regarding damping. Their moving mass is quite high and their suspensions are comparatively weak compared to this mass.

Because of this they have relatively poor mechanical damping and therefore electrical damping is important. High frequency drivers have much lighter mass and stiff suspensions compared to that mass. As a result, electrical damping is relatively unimportant."

We all know : for a "thight" bass we need speakercables with very low resistance. So amp impedance is very important.

 

[Tom79]

To increase the mechanical damping of a tweeter, they drill holes into the "magnet" so some air behind the membrane can flow in and out and produces some friction (=damping). In this case, also the moving mass becomes higher and the spring constant smaller, the result is also a deeper resonance frequency. The problem is, that you need more energy because there's more friction and this produces heat, which can destroy the tweeter. You can do similiar things with a midwoofer, higher mechanical damping means always less efficiency. It's like a electrical (DC) motor that is always under load. It produces less cemf and stops faster, but need more current/energy to reach the speed. Its max. Speed (like the resonance freq of a speaker) is also lower.

So it's more interesting to do some electrical damping: if you have a DC-Motor with very small friction, at the beginning there's no cemf, current I =U/R (R=DC resistance of the wire in the coils, U=Voltage over the motor). When it starts to rotate, cemf becomes higher (voltage over the motor drops), until the power, that goes into the motor (voltage over the motor and it's current) just equalizes the friction. When you stop power supply and let the connection of the motor open it turn very long until it's stops, because there's almost no friction (damping). If you make a short circuit between the connections of the motor it stops very fast, because the energy is burned in the resistance of the wires. Same behaviour for a speaker.

 

[wwenze]

The voice coil resistance is included as part of the total loop impedance when calculating damping. So there's a limit to how far you can improve things with a zero impedance amp and cable.

 

[solderdude]

If the signal from the amp "stops" (sharp transient) best would be, to have a short circuit over the driver-coil, so no voltage is there anymore, and the coil movement immediately stops.

Consider the following.
When a driver is moving and suddenly the current that makes it move is 'stopped' the mass of the moving parts would like to continue its journey.
It then becomes a voltage source (EMF) and has an internal resistance determined by the resistance of the voice coil.
An 'open' circuit will lead to NO electrical damping as no current will flow.

A speaker is fed from a voltage source... NOT a current source. A voltage source has a near 0 ohm output resistance.
A speaker amp is just that, a voltage source, not a power or current source (well... very close to a, limited in voltage and current, voltage source)
This means that after the signal is 'removed' the speaker itself becomes the 'signal source' which has its own (not 0 ohm) source resistance.
Lets say 4ohm.

Electrical damping (breaking) requires current.

Now... a 4ohm source will not magically become 0 ohm when you short it. There will be no voltage across the 'short' because voltage is a parallel thing.
Damping requires current and that is a series circuit. The damping current thus is determined by the source (4ohm speaker) + amp output R.

This means that damping current is not very different between an amp that is 0.001ohm out and 0.1ohm out as the damping current is determined by the whole series chain.
The 4ohm source is the one that determines the actual damping current
It thus does not matter if the whole current circuit is 4.001ohm or 4.1ohm. The damping current will not change much.

The biggest issue with higher output R is voltage division, not so much a damping current.

When you want better impulse response from drivers look into mechanical damping (over-damping is also bad), acoustical solutions or measure the actual impulse response and compensate it (digitally) by applying specific EQ based on measurements of that driver.
This really is the best way to 'improve' the response of drivers/speakers.

 

[Tom79]

right, like the DC-Resistance in the coils of the motor. But in contrary to my DC-Motor example the Speaker Driver normally works above its resonance frequency, and when "stopped" membrane vibrates with the damped resonance frequency. But this doesn't change anything.
Thanks for your inputs!

So what I see, the only possibility to create better electrical damping , is a very short pole reversal that comes from the amp. Sure this would only be possible with closed loop feedback from the speaker (with a sensor on the Membrane, Coil,..). Or perhaps this pole reversal could come from the network from the crossover? Something like a coil or Condensor that is charged always to the oposite direction and unload its energy into the coil of the speaker driver, as soon as the voltage from the amp falls against the cemf of the coil of the driver? Just a tought experiment, don't know exactly how it could work.

 

[wwenze]

An electrical motor that has the position controlled by electrical input + feedback is called a servo. That is probably the term that you were looking for since some subwoofers use it and is not patented since it is a common concept that exists outside of audio.

 

[voodooless]

We all know : for a "thight" bass we need speakercables with very low resistance. So amp impedance is very important.

We don’t all know this. Tight bass is predominately a function of in-room frequency response. EQ and/or room treatment are the best solutions, not thick speaker cables.

 

[fpitas]

And even if we only talk about the box tuning, the electrical damping is just a part of the puzzle.

 

[Tom79]

looks quite inexpensive LOL.

 

[DonH56]

There are several ways to incorporate servo (feedback) control into a speaker. I built one back around 1980 for a subwoofer using a driver with two voice coils. The second coil was used as the feedback sensor. My current subwoofers incorporate a similar scheme. The other common way is to affix an accelerometer to the driver (cone or voice coil) as a sensor. There are pros and cons of either approach; I chose the voice-coil sensor because it should better compensate thermal effects in the voice coil and such. There are patents for some of these, though I did not find any for the voice-coil approach when I built mine back then (lost opportunity). These days drivers with two or more voice coils are fairly common, at least for woofers, so it's easier to do.

For higher frequency drivers things get tougher. Not only are there fewer (if any, I have not looked) smaller (mid/tweeter) drivers with dual voice coils, but it is harder to keep the feedback loop stable due to the lag from the mechanical driver. At typical drive levels the smaller drivers have lower excursion and thus lower distortion so the benefits may (should) be less.

The idea of using a passive circuit in the crossover that essentially compensates the transfer function to reduce overshoot and ringing is interesting, I do not know how practical it is. Literally, I don't know, not something I have really though about and electromechanical systems are outside my area of expertise. My very limited, ancient prior experience and gut urge caution but it is a question for a speaker designer.

 

[audiofooled]

We don’t all know this. Tight bass is predominately a function of in-room frequency response. EQ and/or room treatment are the best solutions, not thick speaker cables.

I agree. Even if a transducer would stop "on a dime", other factors at play most likely will not.

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[fpitas]

As soon as you band-limit the driver in any way, it can't stop (or start) on a dime. The envelope of response is related to the bandwidth.