Cms

[Electrodynamic]

When Cliff gets back he's going to update the web page quite a bit (color of the text, color of the background, update the Mag and BM web pages, and add a LOT of technical documents and/or links. Anyway, one of the updates to the technical page is going to be a paper on Cms. Here is the plain text, non-PDF, version:

What is Cms?

Cms is a measure of the compliance of a subwoofer. Ok, so what is compliance? By definition, compliance is: a coefficient expressing the responsiveness of a mechanical system to a periodic force. In this case, the force is the motor of a subwoofer. A high compliance subwoofer is one that has a very soft suspension. A low compliance woofer is one that has a very stiff suspension. In small enclosure subwoofer you typically don’t want a really stiff suspension because a stiff suspension is going to rob you of efficiency. Now if you’re after a big SPL monster where you don’t care how it sounds, a stiff suspension is exactly what you want because you want to be able to throw 5,000 watts on a single subwoofer.

Now, lets take a further look into the Cms figure. Cms is more than just a number. Well, it is just a number. But what you really need to look at is the Cms of a subwoofer over STROKE! A wildly varying Cms curve will diminish the effectiveness of the motor and performance of the subwoofer.

Assuming that the motor force is linear (like our XBL^2 subwoofers are!), you don’t want the compliance changing drastically while within the linear limits of the speaker. So when do you want Cms to change? When the driver is approaching its linear limits you want the suspension to tighten up to resist and/or slow down the moving assembly before it gets into trouble (like bottoming the voice coil into the backplate). When you see a Cms curve, the important thing to look at is the linearity of the curve. If the curve is heavily bell shaped, that means that the suspension is changing quite a bit in relationship to what the motor is trying to do.

[hobie1dog]

darn you sure are smart

[M-Special]

How does one make a softer suspension "tighten up" near its limits? Also, doesn't this in effect mean that a driver that takes 2000 watts with a really stiff suspension is going to have roughly the same output as a driver that calls for 500 watts assuming the Xmax and Cone area are similar? Not taking into account enclosure and the other variables....

[Electrodynamic]

Originally Posted by M-Special

How does one make a softer suspension "tighten up" near its limits?

Whether the suspension is progressive or linear and also varying roll width and/or height, and/or glue dips and the area the spider is dipped again and for how long. You can custom tailor a spider and surround to do exactly what you want it to. But throwing all of that out the window - spiders naturally tighten up near their limits anyway. They can only move so far. The crappy spiders and/or crappy designs start tightening up right away. I've seen a LOT of Cms curves that look like the suspension is too stiff for the motor.

Also, doesn't this in effect mean that a driver that takes 2000 watts with a really stiff suspension is going to have roughly the same output as a driver that calls for 500 watts assuming the Xmax and Cone area are similar? Not taking into account enclosure and the other variables....

Up to a certain point, yes. After the 500 watt driver can't take any more abuse (power induced excursion) the 2000 watt driver takes over. And some times the 500 watt driver will be more efficient up to a certain point (depending on the other variables like driver design).

[Electrodynamic]

PS: Micro-cracks in the glue that a spider is coated/embedded with is what softens them up after they've been used for a while.

Scary to think that you're constantly tearing something apart little-by-little the more you use it isn't it?

[hoss]

Originally Posted by Electrodynamic

PS: Micro-cracks in the glue that a spider is coated/embedded with is what softens them up after they've been used for a while.

Scary to think that you're constantly tearing something apart little-by-little the more you use it isn't it?

kind of like that time i asked how the coil leads coming off the former never broke even after all the movement they did?

[M-Special]

So what is the typical life expectancy of a subwoofer you're slowly going to be tearing apart over time?

[Electrodynamic]

A very very long time as long as you don't cook the coil or drive it into suspension failure. Within the limits the driver, subwoofers will last a LONG time. 99.99% of the time it's abuse that kills. It's very rare for subwoofers to fail because of a spider giving way (surround glue issue is another story - it depends on the build house and materials used).

[sundownz]

I have some Sansui woofers from 1973 that play fine to this day, they were "long throw" for the day with probably about 10-12mm of throw So yes, they can last a very long time!

[hobie1dog]

I agree, treated normally, a woofer should last 20 years, other than the occasional rotted foam surround.