>These sorts of "bad" motors are ones that have Qms
>that is way high, in attempt to tune the woofer very
>low. This is done to encourage maximum bass extension
>and motor strength is necessarily weakest.
From the time you start to mention motor strength, you're talking about Qes, not Qms.
>Often though, they'll have very low resistance and
>the value of Qes is "shunted" across the mechanical
>Qms, which is much higher. Qts is then made
>relatively low by virtue of the low Qes shunted
>across what is a fairly high Qms. In this case it
>needs motor braking in the extreme.
Which is exactly what will be happening if Qes is low. There is a direct correlation between Qes and the ability of the motor to stop and start cone motion - the lower the Qes, the greater this ability. Therefore, in this case the output characteristics of the driver are primarily dependent on Qes.
>But with motors with very high Qms, say 20, they are
>just real bad, and could be desribed as a "mudmotor".
I haven't come across any drivers with Qms that high. The highest I've seen is Qms=12. In any case, you're likely to find that, even with those drivers, performance is more affected by Qes, which is usually an order of magnitude smaller than Qms.
>What we have in this situation with the shorted
>terminals, is what would look like extremely low
>output impedance of the amplifier - but of course,
>theres no amplifier there, its just a wire shorting
>the terminals which also has very low resistance.
If you repeat the test, this time instead of using a cable, you use an amplifier that's turned on but no input signal, you'll get a similar response to the tap test. Good amplifiers have high damping factors (30 and above is typically the case).
We can even work out an estimate of the amplifier's DF on the Qes of the amplifier/speaker system:
Qes' = Qes * (1 + 1/DF)
For example, a driver with Qes of 0.30 connected to an amplifier with a DF of 30 (at the driver's rated impedance) will result in a system with Qes = 0.30 *(1+ 1/30) = 0.31.
>Shorting the terminals essentially provides infinite
>motor braking to the driver.
Nope. The motor braking won't be infinite - it's dependent on the driver's Qes :-).
>But you've made a good point though, and motor
>braking is typical of drivers having high Q and low
>resonant.
Motor braking is typical of all drivers. The ability of a driver's motor to control the motion of the cone is reflected in Qes. The greater the ability, the lower the Qes figure.
>But we cannot say that Qms is not relevant, because
>it describes a large part of how the driver is damped
...only if there's nothing connected to the driver. Once we connect an amplifier (or anything else across the driver's coils), Qes dominates, unless of course Qes approaches Qms in value (in the vast majority of drivers, there's an order of magnitude of difference between the two).
There's one factor that Qms does affect significantly - the driver's impedance response. The greater the Qms, the high the impedance peak at resonance. As for how this would affect the driver's frequency response or distortion characteristics, I'm not sure.
>Total Q and the figures that make it up, Qes and Qms,
>are not the only factors of distortion. Having a
>perfectly symmetrical magnet field, underhung voice
>coils, voice coil windings, magnet material, flow of
>magnetic flux, magnetic geometries, copper shorting
>rings, cooling techniques, and more, all are
>contributors to distortion, and all primarily arise
>from the magnetic structure
No argument there.
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