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Proper use of equation 17.8?
April 17, 2022 - 7:07 am
Member Since: July 17, 2021
Forum Posts: 6
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Hi all,

I have a question regarding the use of equation 17.8. 

I am trying to generate a reverb effect (in MATLAB) with specified reverb times using comb filters and I am struggling to get correct reverb times out of the equation. 

Is this correct use of the equation?




15 = delay time in msec

500 = RT60 in msec

This returns a gain coefficient of 0.0001073 which does not generate an RT60 of 500 msec.

I suspect I am plugging in the sample rate wrong but just plugging in 44100 returns 0. Confused Or that is correct but there may be something wrong with my comb filter algorithm. 


Thank you. 

April 24, 2022 - 12:26 pm
Member Since: January 1, 2020
Forum Posts: 107
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Hi Jordan,

You have the rate there correctly in kHz since you're using ms and not s. Not an issue as long as you keep that consistent.

I think that equation requires the sampling period though, whereas 44.1kHz is the sampling rate.

I'd say that's a typo in the book claiming Ts to be the samplerate there. Justification - Samplerate was already defined as fs in equation 17.7 and in 17.9, Jot's equation, Ts is reused and explicitly named as the sample period.

If we take Ts as the sampling period - 1/44.1, that now gives a gain value of 0.9953 which is way too long.

So it then seems that D has been given in samples and not seconds. And so the equation reduces from

= (-3*D(smpls)*Ts)/T60(s)

= (-3*(D(s)*SR)*(1/SR))/T60(s)

= (-3*D(s))/T60(s)

D and T60 can be independent of SR in this equation as we account for that when calculating the delay time in samples from ms.

And this gives a gain value of 0.81283 which seems reasonable from my testing.


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