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Old July 5th 18, 07:41 AM posted to rec.audio.tech
gregz
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Default Damping Factor in drivers

> wrote:
> On Monday, July 2, 2018 at 1:41:13 PM UTC-4, G wrote:
>> I understand effects of electrical damping of woofers. I'm wondering how It
>> may affect midranges and tweeters. Is it the same, or does it dimish at
>> higher frequencies. Remove any crossover components for direct feed to amp.
>> Tone bursts should Show any difference in DF. Without testing, I'm
>> guessing.

>
> To cut to the chase: forget about "damping factor." It's essentially
> a made-up "spec" that indicates very little about the performance of
> loudspeaker drivers. Really. The important number, if you're worried about
> how well a driver is controlled at its resonance, is it's total Q
> factor: in the case of the driver itself, it's the Qts of the driver.
>
> The Q of any resonant system is a measure, essentially, of the
> ratio of the amount of energy stored in the resonance to the amount
> of energy dissipated. In the case of a driver, you have essentially
> three ways by which that energy is dissipated, or lost or, if you prefer,
> how it is removed from the system. These a
>
> 1. Electrical losses, which, for most drivers, is the largest
> loss mechanism (and is usually specific Qes),
>
> 2. Mechanical losses, which, for most drivers, is secondary
> to the electrical losses (designated as Qms),
>
> 3. Acoustical losses, i.e., the sound that is actually radiated
> into the room which, for almost all drivers, is an insignificant
> loss mechanism (this is why speakers are so inefficient).
>
> Now, #1 might seem contradictory to me declaration that damping
> factor is essentially useless, since we are (I assume) talking
> about electrical damping, but it becomes clearer once you realize
> the fact that it's the total resistance in the circuit that's
> responsible for the electrical Q. That not only includes the output
> resistance of the amplifier (which is where the "damping factor"
> spec comes from), it not only includes the electrical resistance
> in the crossover (not impedance, not inductance, but resistance),
> it also includes the DC resistance of the voice coil.
>
> The problem is, far and away, the single largest resistance in that
> collection is the DC resistance of the voice coil. That resistance
> dominates all others, and attempting to reduce the other resistances
> (crossover, speaker wire, amplifier) won't make a hill of beans
> difference in the damping of the system.
>
> Le's take a typical 8-ohm woofer, with an equally typical DC
> resistance of about 6.5 ohms. Let's assume the mechanical Q
> of the driver is 4, the electrical Q is 0.85, and the resulting
> total Qts is about 0.701 (Qts = Qes*Qms/(Qes+Qts). How is this
> going to perform with an amplifier whose damping fact is, say,
> 1000 vs one whose damping factor is 50. One might be inclined
> to say that there will be a factor of 20 difference in the
> damping of in each case (1000/50=20), but it turns out not to
> be so.
>
> Whatever with a damping factor of 1000, our "highly-damped"
> amplifier has an output resistance of 0.008 ohms, while our
> "not-so-well-damped" amplifier has an output resistance of `0.16
> ohms.
>
> The amplifier output resistance will increase the electrical
> Qes: in the case of the first amplifier, it will increase it from
> 0.85 to 0.851 (Qes' = Qes * (Re+Rg)/Re. In the second case, it
> will increase it from 0.85 to 0.87. NOt much.
>
> But it's the TOTAL Qts that we're interested in. In the first case,
> it will change from 0.701 to 0.702. In the second case, it will change
> from 0.701 to 0.714. In both cases, the actual damping of the system
> changes by less than 2%. This is less than the typical variation in
> these kinds of parameters one finds from one sample of a driver
> to another.
>
> The situation with midranges and, especially, tweeters is even
> more the case where these external resistances have little
> effect, because in these cases, the total Qts is more heavily
> dominated by the mechanical losses, and thus changes in the
> electrical Q have even less of an effect. Further, in many systems,
> the principle resonance is designed outside of the operating
> bandwidth of the driver by the crossover, so it has even LESS
> of an effect.
>
>> Tone bursts should Show any difference in DF.

>
> Tone bursts are very likely to tell you nothing useful at all.
> They're not a terribly useful test in and off themselves and,
> given the very small changes you're likely to encounter in
> the changes you're proposing, a tone burst is not going to
> reveal anything. Further, tone bursts are very hard to measure.
> Bump your microphone a little bit, change its position, and
> you'll measure a different tone burst


That tells me a lot about what I asked. Thanks.

Greg
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