Mmd Vs Mms

Elbert · 5th October 2008, 07:19 PM

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?

Rybaudio · 5th October 2008, 10:31 PM

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

David McBean · 6th October 2008, 07:20 AM

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

Elbert · 6th October 2008, 07:20 PM

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?

Rybaudio · 7th October 2008, 02:16 AM

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).

Elbert · 7th October 2008, 07:54 PM

Thanks for that last clarifying answer, makes good sense!

5th October 2008, 07:19 PM	#1
Elbert diyAudio Member Join Date: Mar 2008	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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5th October 2008, 10:31 PM	#2
Rybaudio diyAudio Member Join Date: Mar 2003 Location: State College, PA	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)rhor^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

6th October 2008, 07:20 PM	#4
Elbert diyAudio Member Join Date: Mar 2008	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?

7th October 2008, 07:54 PM	#6
Elbert diyAudio Member Join Date: Mar 2008	Thanks for that last clarifying answer, makes good sense!