Damping Factor De-Mystified

[jeffn]

cut and pasted from a web site........easy to understand



Loudspeakers have a mind of their own. You send them a signal and they add their own twist to it. They keep on vibrating after the signal has stopped, due to inertia. That ’s called "ringing" or "time smearing." In other words, the speaker produces sound waves that are not part of the original signal. Suppose the incoming signal is a "tight" kick drum with a short attack and decay in its signal envelope. When the kick-drum signal stops, the speaker continues to vibrate. The cone bounces back and forth in its suspension. So that nice, snappy kick drum turns into a booming throb. Fortunately, a power amplifier can exert control over the loudspeaker and reduce ringing. Damping is the ability of a power amplifier to control loudspeaker motion. It’s measured in Damping Factor, which is load impedance divided by amplifier output impedance. Let’s explain. If the speaker impedance is 8 Ohms, and the amplifier output impedance is 0.01 Ohms, the damping factor is 800. That’s a simplification. Since the speaker impedance and amplifier output impedance vary with frequency, so does the damping factor. Also, the impedance of the speaker cable affects damping. Thick cables (with low AWG) allow more damping than thin cables with (high AWG). The lower the amplifier’s output impedance, the higher the damping factor, and the tighter the sound is. A damping factor of 1000 or greater is considered high. As you might suspect, damping factor is most important at low frequencies, say 10 Hz to 400Hz. High damping factor equals tight bass.

- How It Works
How does an amplifier control speaker motion? When the loudspeaker cone vibrates, it acts like a microphone, generating a signal from its voice coil. This signal generated by the speaker is called back EMF (back Electro Motive Force). It creates a current, which travels through the speaker cable back into the amplifier output, then returns to the speaker. Since back EMF is in opposite polarity with the speaker’s motion, back EMF impedes or damps the speaker’s ringing. The smaller the amplifier output impedance, the greater is the effect of back EMF on the speaker’s motion. An amplifier with low output impedance short-circuits the back EMF, so the back EMF drives the loudspeaker with a relatively strong current that works against the speaker’s motion. When the speaker cone moves out, the back EMF pulls the speaker in, and vice versa

In short, the loudspeaker damps itself through the amplifier output circuitry. The lower the impedance of that output circuitry, the more the back EMF can control the speaker ’s ringing.

 

[TheAnalogman]

Excellent post. Thanks for the info.

 

[BrianC.]

This is undoubtedly The Law for the vast majority of drive units, which have been expressly designed to rely on the amplifier for control. However there are exceptions, usually units from an earlier era. For example Lowthers. These units have intrinsically high self damping (i.e. Qts < 0.3) and can actually benefit from a higher impedance driving source.

An interesting read is at

http://melhuish.org/audio/article5.html

Brian.

 

[fdrennen]

Thanks for the info!

 

[mhardy6647]

Damping factor is about the least mystical thing imaginable. It is the ratio of the speaker (load) impedance to the output impedance of the amplifier. If the amp has a 1 ohm output impedance, the DF into an 8 ohm speaker is 8.

Large amounts of negative feedback may be used to decrease the amplifier output impedance and thus increase the DF. Is this a good trade-off? Listen and decide for yourself.

EDIT: oooh, you might find this interesting, too:
http://www.transcendentsound.com/amplifier_output_impedance.htm

 

[Fisherdude]

Damping factor is about the least mystical thing imaginable. It is the ratio of the speaker (load) impedance to the output impedance of the amplifier. If the amp has a 1 ohm output impedance, the DF into an 8 ohm speaker is 8...

That's true, but this is the first simple (as in, I can understand it!) explanation I've read about HOW damping works. Just a number doesn't tell me anything. It makes perfect sense that a suspended cone would continue to vibrate after a note is struck, and moving a coil of wire through a magnetic field generates an electric current. And, since the speaker wires are still connected to the amp...

Duh! Of course!! Now I understand!

... Also, the impedance of the speaker cable affects damping. Thick cables (with low AWG) allow more damping than thin cables with (high AWG)...

Is this correct? First time, I think, that I've read that specific point, although it also makes sense.

 

[Fisherdude]

The link to Transcendent was excellent. Bruce is always, umhh, interesting, to say the least. This was a very unbiased article.

So, if I understand the differences between DF's for tube vs. SS gear, tube gear generally will have a lower damping factor due to the higher output impedances. Which, if not dealt with, such as by increasing negative fb in pp designs, the result would be weakness in the lower frequencies, i.e. muddy bass.

Do I have that right?

Obviously, though, it's not going to be generally possible to make DF's for tube and SS gear identical because the differences in amp output impedances are just too wide to fully adjust for. Which leads to the most commonly accepted explanation as to why some speakers "like" tube gear better than SS, and vice-versa.

And, two different speakers with identical nominal impedances might still behave very differently, even when connected to the same amp, because their actual instantaneous impedances at differing frequencies might be, (I think the technical phrase here is...), all over the place.

How am I doing? I think it's starting to click!

 

[totem]

I would like to welcome Brian C to AK. Brian is very much involved with early
mono gear and has an interest in Classic speaker designs. Nice to see you on this side of the pond! Back to the damping factor questions.

 

[FockeWulf190D]

One little but no less important thing!!, All said is true BUT, there is a xover between that driver and the amp in the vast majority of passive home speakers, SO ... for bass frec. you'll find an Inductor, always a big one in series, as result those magic numbers are less meaningful because the driver is unable to "see" very low output impedances from the amp.

 

[the-real-mandak]

There is also a good article here: http://www.classic-audio.com/marantz/mdampingfactor.html

and of curse this link has to be in this thread: http://en.wikipedia.org/wiki/Damping_factor

 

[geaugafletcher]

and of curse this link has to be in this thread: http://en.wikipedia.org/wiki/Damping_factor

Curses! Foiled again!

(Thanks for the info & links fellas, I've wondered about damping factor.)

 

[Fisherdude]

One little but no less important thing!!, All said is true BUT, there is a xover between that driver and the amp in the vast majority of passive home speakers, SO ... for bass frec. you'll find an Inductor, always a big one in series, as result those magic numbers are less meaningful because the driver is unable to "see" very low output impedances from the amp.

Dang. I knew it couldn't be that simple!

 

[soundson]

Thanks alot for the post. I was planning to do a little research to find out what damping factor was. Now I don't have to.

 

[FockeWulf190D]

Fisherdude, that's a good start to love well damped Speakers, to name a few: Acoustic Suspension, Transmission Line, Open Baffle Dipoles. That secuence is a valid sonical evolution for Dynamic Drivers but is in reverse relasionship considering SAF and placement difficulty. Rounding concepts ... Followers of Single Speakers Drivers have a good point: No Xovers!!!

 

[Columbo]

Wasn't there a speaker company on the '70s called LWE that had some sort of "motion feedback" system???

 

[mhardy6647]

Philips made some pretty cool "Motional Feedback" speakers as well.

http://www.stumpie.com/tech/mfb/

 

[onplane]

Further confusing things is the output impedance of an amp varies with frequency. Usually, the impedance rises with frequency.

One theory on bi-amping is that where we really need damping is in the lowest frequencies (i.e. close to the woofer's resonant frequency). So by splitting the load (i.e. the woofer usually has it's own amp), we end up with lower output impedance and higher damping where we need it the most.

Again, this is one theory, but I've seen similar logic describe how bi-wiring can be beneficial when a single amp with a high damping factor is used.

Basically the woofer generated EMF has to travel back to the amp (via speaker wires) BEFORE it can get to the mid and tweeter. Once at the amp, it's dissipated by an amp with low output impedance and never gets to the mid/tweeter. That is, the path of least resistance is dissipation in the amp as opposed to traveling again down the speaker wires to the mid/tweeter.

Regards,
Jerry

 

[Gibsonian]

One little but no less important thing!!, All said is true BUT, there is a xover between that driver and the amp in the vast majority of passive home speakers, SO ... for bass frec. you'll find an Inductor, always a big one in series, as result those magic numbers are less meaningful because the driver is unable to "see" very low output impedances from the amp.

Also, the impedance of the speaker cable affects damping. Thick cables (with low AWG) allow more damping than thin cables with (high AWG).

1. High DF amplifier(s)
2. Shortest runs possible of large gauge speaker wire
3. Active crossover (no passive X components, speaker wire connected to output of am to speaker terminals)

Put 1 thru 3 in your system for very tight/clean bass and mids. With these in place you can really hear the effect that superior cone control can acheive, of which many have not tried/heard all together.