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cT equals piD 7th October 2012 12:50 AM

electrical damping freq. range
 
It would seem that the majority of speaker talking heads agree that a "high" damping factor, maybe 10, is needed throughout the operating frequency range of a driver. Usually that includes full frequency response not in the region of resonance magnification of the driver. Thus, for that majority, passive crossover networks (between the power amplifier and the driver), are seen as problematic. The problem is that near the crossover frequency, the damping factor is reduced by the impedance of low-pass and high-pass filters. For example, see

Active Vs. Passive Crossovers

Vance Dickason in his Cookbook (5th ed.) claims improved transient response by replacing a passive crossover with an active one, citing a paper in JAES, Sept. 1971.
It would seem that Colloms in his book High Performance Loudspeakers, sixth ed. p. 325, takes the view that the damping factor (df) is only relevant to the region of resonance magnification as he discusses how the damping factor affects Q of the driver.

As back EMF as a percent of the driving voltage is very diminished except in the (frequency band) region of resonance magnification, or around the resonant frequency of the driver, I would argue that electrical damping or the df is irrelevant for example at a crossover frequency that is far removed from the resonant frequency of the driver. That is, diminished electrical damping of the driver has no affect on transient response of the driver at frequencies far removed from the resonant frequency of the driver.

Would anyone care to refute this. Not being an engineer, I'm open to being mistaken, but if so, then what is a logical explanation?

Regards,
Pete

sreten 7th October 2012 02:12 AM

Hi,

The so called "damping factor" is increased by driver impedance peaks
and impedance peaks caused by passive crossover section interaction.

These peaks only cause problems with low damping factors, as they
increase the damping factor causing variation of the voltage response.
(For higher damping factors the DF is still increased but it makes little
difference to the voltage response as long as DF is above about ten.)

Damping factor is relevant, not irrelevant, but not in the way you describe.

rgds, sreten.

Dissi 7th October 2012 02:19 AM

This is an interesting question. I'm writing a speaker simulation program and i already had the idea, to display the resulting damping factor of a driver combined with a passive crossover. It is obvious, that even order high- or low-pass filters will perform much better than odd order filters, because they have a shunting component.

Without thinking about it, i did assume, that a high damping factor is preferable in the whole pass-band of a driver. But is that really true?

I would say yes. As you mention, the back EMF induces a voltage in the voice coil. The major part is at the resonance frequency, but there are also other parts. Every irregularity in the impedance plot of a driver is caused by induction. A standing wave at 100 Hz, a surround resonance at 800 Hz or a cone resonance at 5 kHz, they are all visible in the impedance.

The other question is, will electrical damping really reduces these resonances? I guess, the effect is very limited, maybe a reduction of 1-2dB can be achieved.

john k... 7th October 2012 02:49 AM

Some simple truths can sum it up. Taken as systems the combination of active crossover/amp/driver and amp/passive crossover/driver represent predominately linear systems. Since linear systems with the same frequency response have the same transient response it follows that if the active and passive crossovers systems are designed to have identical frequency response then they must have the same transient response. Any difference is damping due to series impedance in the passive crossover simply means that the transfer function of the passive crossover will have to be somewhat different that that of the active system to account for the difference in damping.

The real difference between passive and active systems is insertion loss and the way the response to changes in VC temperature.

john k... 7th October 2012 02:53 AM

Also, it should be understood that damping falls off at a rate of 6dB/octave to both sides of fs. In the flat band region of a driver, above fs, damping plays essentially no roll in determining the response of the driver to a given input signal which is why this region is referred to as the mass controlled region.

cT equals piD 7th October 2012 03:33 AM

Quote:

Originally Posted by john k... (Post 3192379)
Also, it should be understood that damping falls off at a rate of 6dB/octave to both sides of fs. In the flat band region of a driver, above fs, damping plays essentially no roll in determining the response of the driver to a given input signal which is why this region is referred to as the mass controlled region.

So you would agree that the presence of a passive crossover network (between power amp and driver) doesn't degrade transient response of two drivers attached to the network, assuming the cross-over frequency in the flat band region of the drivers?

tvrgeek 7th October 2012 08:23 PM

"Damping factor" is an attribute of the feedback in the amp stated as the inverse of the output impedance. Overrated in any case. This is totally different from the Q of the box and how it effects the drivers resonance.

cT equals piD 8th October 2012 12:05 AM

Quote:

Originally Posted by tvrgeek (Post 3193025)
"Damping factor" is an attribute of the feedback in the amp stated as the inverse of the output impedance. Overrated in any case. This is totally different from the Q of the box and how it effects the drivers resonance.

The damping factor tells you what the internal resistance of the output stage of a power amplifier is. For the vast majority of solid state power amplifiers in existence now, that internal resistance is usually so low that it doesn't factor into Q of the speaker system connected to it. You are correct in saying that df strictly only applies to the amplifier. So but the electrical damping and thereby also Q of the driver certainly are affected by any substantial resistance in-series with the driver. There is the equation,

Qes' = Qes*[(Rg + Rx + Re)/ Re]

where
Qes' = modified electrical Q
Qes = electrical Q of the driver (specified Qes)
Rg = amplifier source resistance
Rx = resistance in-series with the driver
Re = voice coil resistance

If Rg is comparable to Re, then the damping factor also affects Q of the driver.

Regards,
Pete

bentoronto 8th October 2012 04:18 AM

I may be ignorant, but I think some of these seemingly sophisticated analyses are out to lunch. The issue isn't how the system works right but how it works wrong.

As the OP asks, when and for what ever reason there are false perturbations in the driver (as reflected in the voice coil), will the hook-up damp them? And the OP is right in the way he is looking at it. A good reason (among many other good reasons) never to consider passive crossovers unless you are a manufacturer who needs to sell integrated speaker systems in a box.

Frankly, for REAL damping, you want a negative output impedance amp AKA motional feedback.

Ben

john k... 8th October 2012 02:41 PM

Quote:

Originally Posted by cT equals piD (Post 3193247)
The damping factor tells you what the internal resistance of the output stage of a power amplifier is. For the vast majority of solid state power amplifiers in existence now, that internal resistance is usually so low that it doesn't factor into Q of the speaker system connected to it. You are correct in saying that df strictly only applies to the amplifier. So but the electrical damping and thereby also Q of the driver certainly are affected by any substantial resistance in-series with the driver. There is the equation,

Qes' = Qes*[(Rg + Rx + Re)/ Re]

where
Qes' = modified electrical Q
Qes = electrical Q of the driver (specified Qes)
Rg = amplifier source resistance
Rx = resistance in-series with the driver
Re = voice coil resistance

If Rg is comparable to Re, then the damping factor also affects Q of the driver.

Regards,
Pete

Actually, Q of the driver never changes. What changes is Q of the system composed of the driver, any series resistance, and Rg of the amp.


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