RT60 - Missing 500 Hz data.

[narut titayangkurn]

HI there.
I have a question about the missing value in the 500Khz in the T20 and T30 graph.
I tried to change a sound source location and mic positions.
But the result of measurements was more or less the same. Some graph were missing 1kHz, some missing 800Hz. but the one show was missing 500 Hz
The room size is about 10 x 10 x 4 meter. Sound source was 90dB C-slow and mic was calibrated.
I have no clue at all how to interpret this info.

Could somebody help explain to me if I somehow there was something wrong in setting measuring tool or
this graph really showed the true acoustic character of this room?
I didn't use DODEC speaker in this one just a simple single-direction sound-source.

Thank you a lot

 

[John Mulcahy]

Missing values mean REW wasn't able to calculate them. To understand why, you would need to look at the filtered impulse response. If you attach one or more of the mdat files that show the problem I can investigate.

 

[narut titayangkurn]

Dear John
I attached the .mdat file to the link below


From time to time, I found missing RT Value in certain Hz but I have had no clue how to fix it.
I tried

- varying number of sweeps and file length combinations,
- increasing the sound source making it around 90dB.
- swapping between Behringer ECM8000 and MINI_DSP_USB Mic

The input signal, Ref signal both were not clipped.

I appreciate it.

 

[John Mulcahy]

Thanks for that. The problem is that REW's default setting for how much of the IR to keep is too short for a large space, so the response is truncated before the IR has decayed to the noise floor. That stops the RT60 calculation from working properly. To fix it, go to the Analysis preferences in the Impulse Response Calculation section and change the "Truncate IR after 1.7s" setting to "Truncate IR after 9.9s" (or to "Keep full IR"). The new setting will apply for new measurements.

 

[narut titayangkurn]

Dear John. Thank you a lot for your help