How does REW calculate Q? - Home Theater Forum and Systems - HomeTheaterShack.com

 
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post #1 of 6 Old 04-03-15, 06:58 PM Thread Starter
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How does REW calculate Q?

I've taken some measurements of an Infinity 1260w both to calculate its TS paramters, as well as measuring the Qtc in the 38.6L sealed box I made for it. REW's calculation for Q of the impedance sweeps is close to but not exactly the same as what I get calculating Qes/Qms/Qts by hand, using the REWs impedance graph as my source for data. I'm using the forumals from my copy of Introduction to Loudspeaker Design 2nd edition, which is the same as the method described here: http://sound.westhost.com/tsp.htm

I uploaded the mdat, the spreadsheet with my Q calcs, and some screenshots from REW. Qms from REW is pretty close to what I calculate, but Qes and Qts differ enough that I'm doubting my methods. I'm also trying to correlate to the modeled Qtc using the TS parameters in WinISD. Nothing quite lines up perfectly.

So with that said, can anyone comment on what method REW uses to calculate Q? I'd like to validate the math against what I'm doing.

Thanks.
Attached Thumbnails
How does REW calculate Q?-rew-graph.jpg  

How does REW calculate Q?-ts.png  

How does REW calculate Q?-ts_empty.png  

How does REW calculate Q?-ts_stuffed.png  

Attached Files Infinity_1260w_Measurements.mdat (3.52 MB)  1260w_Q_Calcs.xls (27.0 KB) 
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post #2 of 6 Old 04-03-15, 07:01 PM Thread Starter
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Oh, and a screenshot of the spreadsheet.
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post #3 of 6 Old 04-04-15, 05:16 AM
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Re: How does REW calculate Q?

REW uses a curve fitting process to determine the model parameters and derives the Q values from those. Curve fitting provides more accurate values than bandwidth estimates. There is more information about the model in the REW help, you may also find useful info in Claus Futtrup's help for his Driver Parameter Calculator software at DPC manual, but note that the software is from 2004 and pre-dates the work on more elaborate models.
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post #4 of 6 Old 08-17-16, 06:30 PM
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Re: How does REW calculate Q?

Hi John,

May I know more the curve fit used in REW? Is the solver similar to that of Excel for nonlinear fitting? I think REW tries to fit the models (e.g. FDD model) to measured data. Correct? How REW selects initial values of unknown parameters?

Thank you!
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post #5 of 6 Old 08-18-16, 02:09 PM
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Re: How does REW calculate Q?

Initial estimates for the low frequency parameters are based on the peak frequency and the -3dB bandwidth of the peak in the classical manner. The parameters are optimised using a version of Uncmin, the optimiser operates on converted forms of the model parameters to allow bounds to be imposed where appropriate and non-linear scalings to be applied.
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post #6 of 6 Old 08-21-16, 08:07 PM
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Re: How does REW calculate Q?

Thank you, John.

I have a personal project using excel to extract parameters from impedance curves. I am using excel solver to perform fitting/optimization.
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