Is there any reason not to pick a driver with a low Cms value? - diyAudio
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Old 18th October 2006, 07:34 PM   #1
Oborous is offline Oborous  Canada
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Default Is there any reason not to pick a driver with a low Cms value?

CMS Calculation from Adire

Hi All,

I'm trying to design a subwoofer, and messing around with different drivers being currently offered.

From the above link, I rearrange the Cms formula to:

Vas = Cms * Sd² * Constants (basically the rest of the formula... I'm more interested in the relation between Vas, Cms, & Sd)

I notice that Vas goes up dramatically with increased surface area, and that a lower Cms value would decrease the Vas for a given surface area.

So, say I want to move a huge amount of air, but not have a 12 cu ft. enclosure, I could pick whichever driver has a lower Vas, which in turn would mean lower Cms (Sd constant for the two drivers).

I'm also looking at this, given everything, I'm probably better off with multiple smaller drivers than one huge driver, if I'm trying to keep a smaller box size...

(And yes, I am fixing all other values that can be fixed, and only changing Cms)
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Old 18th October 2006, 10:10 PM   #2
Svante is offline Svante  Sweden
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If Cms is small, you will need a large cone mass for a given fs. This in turn requires a high Bl in order to keep the efficiency high, ie the driver becomes expensive.

It is interesting to note how small effect the Cms value has on the response of a system. Actually, when the Mms, Bl and Re are kept constant, Cms hardly affects the response at all. In particular, making Cms softer makes almost no change.

Below is a simulation of three BR systems I did some while ago. I don't remember the exact variation of Cms, but I think it was at least a factor of 2 between the curves.

Click the image to open in full size.

The bottom line here is that Cms in itself isn't very interesting. On the other hand, Mms and Sd is.
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Old 19th October 2006, 12:04 AM   #3
Ron E is offline Ron E  United States
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Low Cms or low Vas, means generally higher Fs or heavy cone and lowish sensitivity (less sound per watt or volt)
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Old 19th October 2006, 02:47 PM   #4
Oborous is offline Oborous  Canada
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Default Very interesting curves


Thank you for your replys and curves, that is what I was thinking.

The effect of Cms isn't that large, in comparision to other factors. It's relationship does tie to other important considerations, and so, if I can get a low Cms driver there is an effect, but not large enough to actually specifically seek a driver with a low Cms.
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