REW Loopback low frequency issues

F

fftbarros

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Hi
I´m facing probelms to calibrate soundcard (Tascam US-144MK2). There´s a "delay" in lower frequencies below 100Hz causing a non flat whaterfall as well as an increase in RT60. Can someone help me to solve it in order to avoid wrong RT60 in acustical measurements? I´m using loopback as the time reference, but nothing change whatever the timing I set. Below some pictures (whaterfall, RT60 decay and impulse) SPL and phase are preatty flat.

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1728738987042.png
1728738955530.png
 
John Mulcahy

John Mulcahy

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That's an effect due to windowing, different window types can be chosen on the Analysis preferences. It isn't a problem though. RT60 at low frequencies is affected by the group delay of the fractional octave filters. There are options to address that in the RT60 controls, e.g. time reversed or zero phase filtering. Again not usually a problem for acoustic measurements though.
 
F

fftbarros

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Hi John,
Thank you so much for your help!
I have tried some different windowing but nothing changed. My apologies, it´s too technical for me. I read the complete REW manual a long time ago but I will do it again.
So you mean that this effect will not be "added" to my acoustics measurement causing a false RT60?
My target is to measure frequency response and RT60 particularly at low frequencies in my listening room to see the effect of the membrane absorbers for low frequencies. My actual RT60 is about 300ms above 300Hz
Best regards
Luis
 
trobbins

trobbins

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Maybe not related, but the soundcard may have a significant low frequency response roll-off in magnitude and phase, both for signal output and input (which may be different due to internal electronic filters, and the selection of sampling rate on ADC and DAC control). Typical compensation to normalise the response may require large levels of compensation - certainly can be the case below 10Hz.
 
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fftbarros

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Thank you very much John. I will perform some acoustic measurements during the weekend....
 
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