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Old 5th October 2008, 07:19 PM   #1
Elbert is offline Elbert  Norway
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Default Mmd Vs Mms

Just a quick question.. Does anybody know if it is possible to deduce Mmd From Mms?

Hornresp requires Mmd, but most spec-sheets only quote Mms.. or is the difference really insignificant?
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Old 5th October 2008, 10:31 PM   #2
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Yes this is possible. First it is important to understand what Mmd and Mms are. Mmd is the effective moving mass of the mechanical system. Mms is the effective moving mass of the mechanical system plus the air on the front of the driver. Thus to get Mmd from Mms we need to know what the effective mass of the air in front of the driver is.

What's going on here is that the cone sees an impedance from the outside world called the radiation impedance. For a piston, at low frequencies the reactive part of the impedance looks like a mass- this is the mass added to Mmd to get Mms, and it is given by:

m = (8/3)*rho*r^3

where

rho = density of air
r = piston radius

Thus to get Mmd from Mms you need to compute this mass and then subtract it from Mms to get Mmd.

Source: Fundamentals of Acoustics by Kinsler and Frey pages 184-187
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Old 6th October 2008, 07:20 AM   #3
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Default Re: Mmd Vs Mms

Quote:
Originally posted by Elbert
Hornresp requires Mmd, but most spec-sheets only quote Mms.
Hi Elbert,

Simply double-click on the Mmd text box in edit mode, and enter the fs value given on the specification sheet instead.

Kind regards,

David
__________________
www.hornresp.net
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Old 6th October 2008, 07:20 PM   #4
Elbert is offline Elbert  Norway
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Hey, this is great, two definitive answers!

One shortcut for the lazy (me) in Hornresp, and one formula for general application!

What more could one ask for!

Thank's guys, much appreciated!

PS:

Regarding the formula; should the answer be multiplied by two since the cone has two sides, both of which I anticipate will se a mass loading from the air?
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Old 7th October 2008, 02:16 AM   #5
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Quote:
Originally posted by Elbert
Regarding the formula; should the answer be multiplied by two since the cone has two sides, both of which I anticipate will se a mass loading from the air?
That's a good question. The answer is usually no, because the impedance on the rear of the cone is that of a box and not a piston radiating into free space. This is similar to horn modeling, where instead of using Mms you use Mmd because the impedance seen by the front of the cone is the horn's impedance and not that of a piston radiating into half space.

However, if the driver is in an infinite baffle radiating into half space on both sides, the answer is yes (at least as an approximation).
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Old 7th October 2008, 07:54 PM   #6
Elbert is offline Elbert  Norway
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Thanks for that last clarifying answer, makes good sense!
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