Thread: reducing Qts
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Old April 6th 04, 12:53 PM
Dick Pierce
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Default reducing Qts

lukasz > wrote in message >.. .
> Hello!!
> Can anybody have an idea how to reduce parameter Qts from 0.75 to 0.5
> for example?

That's quite a hefty reduction, and fairly difficult to obtain.
First, why do you need to reduce the Qts of that particular

Assuming you can't simply use a driver of the right Qts to begin
with, you don't have a lot of choices. As you might know, a
driver's Qts results principally from two other parameters, Qes,
the electrical Q and Qms, the mechanical Q. Both are measures of
the amount of losses, in the former case, from electrical losses,
in the latter from mechanical.

If the driver you has is typical of moderate- to high-quality
woofers, the electrical losses dominate, so that's where you have
to direct most of your effort. While it's generally easier to
increase mechanical losses with little effort, you need large
changes in mechanical loss to get anywhere in reducing the total

For example, take a typical woofer whose total Qts is 0.75, with
an electrical Qes of about 0.9 and a mechanical Qms of 4.5. If you
wanted to drop the total Qts down to 0.5 simply by dropping the
mechanical Qms, you'd have to drop it from 4.5 to about 1.13 or
a factor of 4, an extraordinary feat, to say the least.

On the other hand, to drop the electrical to get there is not so
easy either. In our example, you'd have to drop the electrical
Qes from 0.9 to 0.56. While it may seem that it's easier to drop
the electrical Q by ONLY a factor of 1.6 vs dropping the mechanical
Q by a factor of 1.6, that much of a drop of electrical Q is really
tough as well.

The electrical Q is dependent upon several parameters, such as the
total series DC resistance and the flux density in the magnetic
gap. It's real tough to reduce the total series DC resistance to
less than that of the voice coil's DC resistance without resorting
to one-off techniques such as building a negative output impedance
amplifier, and it's also real tough to increase the flux density
in the gap, because most magnets run fairly close to the saturation
flux density of the pole piece.

So, without the ability to substantially reduce the DC resistance
or substantially increase the flux density, and given that impossibly
large changes in the mechanical damping are required to get you even
close, you're pretty much stuck: there is no practical way to achieve
the substantial reduction in Qts you're seeking.

Which leads us back to my first question: why do you want to do
this? Is there something otherwise extraordinary about a particular
driver that makes you want to try to fix this parameter instead of
of the much easier course of getting the right driver to begin with?