Estimating leakage, absorption and port losses

bmansz · 30th October 2008, 06:17 AM

Does anyone know of any guidelines about estimating the leakage (Ql), absorption (Qa), and port (Qp) losses that need to be factored into a box design? I know that they can be changed in the programs I have evaluated like WinISD Pro and Speaker Workshop. I am not sure if the port loss is a function of its size (diameter and length) or not.

Anyone know where I could look? Not sure if these could drastically change the size and/or performance of the system if not estimated accurately. Thanks.

speakerdoctor · 30th October 2008, 12:04 PM

Vance Dickason's "Loudspeaker Deesign Cookbook" devotes some ink to your question. Pick up a copy. It's a great reference.

bmansz · 30th October 2008, 10:01 PM

Thanks speakerdoctor.

I blew the dust off of my 5th edition of the Loudspeaker Cookbook and read up on it.

According to Vance, in practice, Qa and Qp tend to be so low that they are not significant.

Ql is assumed to be 7. If one were to test further for it, a box would be constructed to proper size and an impedence test would be run. With a few select datapoints from this test, an actual value of Ql can be calculated from the equations he presents.

noah katz · 30th October 2008, 10:51 PM

"According to Vance, in practice, Qa and Qp tend to be so low that they are not significant."

He devotes a section to losses due to port compression, which obviously means they aren't insignificant.

Ron E · 31st October 2008, 12:02 AM

Qa, Qp and Ql can be measured easily only if one dominates such that the others can be neglected. If you curve fit the impedance curve, you might be able to extract useful values.

The best reference on linear T/S theory I know of is J.E. Benson's book "Theory and Design of Loudspeaker Enclosures" - which is out of print, but available at very high cost online. This book is written for engineers (very mathematical) and doesn't really have worked out examples, just hints at how to do it. If you contact TAB books, they might print one on demand for you if they still can legally.

As you said it (Qp at least) changes with level....so for your model to be useful it would have to be predictive of the changes with level.
The SoundEasy manual (available online) has a model that was cooked up to predict changes in port losses with level.

Once you go nonlinear, you might as well start modeling other nonlinearities as well, which can get complex very fast unless you are just fudging.... To avoid all that effort, I would just stick with the 17 meter per second (5% of the speed of sound) rule on port velocity and be done with it.

bmansz · 31st October 2008, 06:47 AM

noah,

About the only section I see is titled Vent Resonance and Mutual Coupling. It doesn't deal with specific losses due to the vent (Qp).

noah katz · 31st October 2008, 10:08 PM

IIRC I have editions 4 and 5, and it was in both of them.

If feasible with your box size and Fb, I'd take Ron's advice.

bmansz · 1st November 2008, 01:18 AM

Noah,

Yeah, he does offer some good advice.

30th October 2008, 06:17 AM	#1
bmansz diyAudio Member Join Date: Oct 2008	Estimating leakage, absorption and port losses Does anyone know of any guidelines about estimating the leakage (Ql), absorption (Qa), and port (Qp) losses that need to be factored into a box design? I know that they can be changed in the programs I have evaluated like WinISD Pro and Speaker Workshop. I am not sure if the port loss is a function of its size (diameter and length) or not. Anyone know where I could look? Not sure if these could drastically change the size and/or performance of the system if not estimated accurately. Thanks.

30th October 2008, 10:51 PM	#4
noah katz diyAudio Member Join Date: Jul 2004 Location: Mountain View, CA	"According to Vance, in practice, Qa and Qp tend to be so low that they are not significant." He devotes a section to losses due to port compression, which obviously means they aren't insignificant. __________________ ----------------------------------------- Noah

31st October 2008, 12:02 AM	#5
Ron E diyAudio Member Join Date: Jun 2002 Location: USA, MN	Qa, Qp and Ql can be measured easily only if one dominates such that the others can be neglected. If you curve fit the impedance curve, you might be able to extract useful values. The best reference on linear T/S theory I know of is J.E. Benson's book "Theory and Design of Loudspeaker Enclosures" - which is out of print, but available at very high cost online. This book is written for engineers (very mathematical) and doesn't really have worked out examples, just hints at how to do it. If you contact TAB books, they might print one on demand for you if they still can legally. As you said it (Qp at least) changes with level....so for your model to be useful it would have to be predictive of the changes with level. The SoundEasy manual (available online) has a model that was cooked up to predict changes in port losses with level. Once you go nonlinear, you might as well start modeling other nonlinearities as well, which can get complex very fast unless you are just fudging.... To avoid all that effort, I would just stick with the 17 meter per second (5% of the speed of sound) rule on port velocity and be done with it. __________________ Our species needs, and deserves, a citizenry with minds wide awake and a basic understanding of how the world works. --Carl Sagan Science is a way of thinking much more than it is a body of knowledge. --Carl Sagan

31st October 2008, 10:08 PM	#7
noah katz diyAudio Member Join Date: Jul 2004 Location: Mountain View, CA	IIRC I have editions 4 and 5, and it was in both of them. If feasible with your box size and Fb, I'd take Ron's advice. __________________ ----------------------------------------- Noah

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30th October 2008, 12:04 PM	#2
speakerdoctor diyAudio Member Join Date: Sep 2006 Location: New England	Vance Dickason's "Loudspeaker Deesign Cookbook" devotes some ink to your question. Pick up a copy. It's a great reference.

30th October 2008, 10:01 PM	#3
bmansz diyAudio Member Join Date: Oct 2008	Thanks speakerdoctor. I blew the dust off of my 5th edition of the Loudspeaker Cookbook and read up on it. According to Vance, in practice, Qa and Qp tend to be so low that they are not significant. Ql is assumed to be 7. If one were to test further for it, a box would be constructed to proper size and an impedence test would be run. With a few select datapoints from this test, an actual value of Ql can be calculated from the equations he presents.

31st October 2008, 06:47 AM	#6
bmansz diyAudio Member Join Date: Oct 2008	noah, About the only section I see is titled Vent Resonance and Mutual Coupling. It doesn't deal with specific losses due to the vent (Qp).

1st November 2008, 01:18 AM	#8
bmansz diyAudio Member Join Date: Oct 2008	Noah, Yeah, he does offer some good advice.

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