Big differences for RT60 over frequency on REW 5.16

[gerardochile]

Dear All,
I am using REW 5.1 on Mac El Capitan with a Scarlett 2nd gen 2i4 sound card and a Dayton Mic, both of them with a cal file.
I am measuring RT60 on an empty room that is about 58K2 cubic feet. ( 1650 m3 ). I used two speakers and one sub to generate the sweep. I tried sweep ranges from 256 to 1M on REW. I used different mic positions and different speakers positions and pointing. All the frequency responses and all the RT60 graphs on REW are very similar among them, and all of them have a big peak of about 2s. on 100Hz and 2KHz. but only near 1s. from 200Hz to 1KHz. My manual calculations are near 1s. for the space. Materials on the walls are glass and concrete. The ceiling has a perforated acoustic cover. Floor is concrete also. Has chairs and tables. All of those materials have no big absorption variations on frequency, so I guess RT60 should be very similar over all frequencies. Question is, is it RT60 valid at 100Hz and 2KHz? is this real or is some kind of mistake or issue? May I trust the whole curve from 60Hz to 8KHz with this huge peaks or should I just believe on the 200Hz-1KHz range?
Thanks in advance for your feedback.
My best regards,
Gerardo Ruiz.

 

[John Mulcahy]

It looks valid and sounds plausible from the description of the space. If you post the mdat file it will be easier to check. REW has moved on a bit since 5.16 by the way. the latest beta releases are here.

 

[gerardochile]

Dear John. Thanks for your feedback, I really appreciate it!. Point is that all the materials have a similar absorption coefficient over the range of frequencies, make some noise to me to have more than 2 sec. at 100Hz and 2KHz but near only 1 sec. in the middle. Attached you will find one of the mdat files of that measure ( all of them are pretty similar ). Could it be some "resonance" of the room at 2KHz? Do you think that I have to measure again with a newer version? Regards.

 

[gerardochile]

Dear John. Thanks for your feedback, I really appreciate it!. Point is that all the materials have a similar absorption coefficient over the range of frequencies, make some noise to me to have more than 2 sec. at 100Hz and 2KHz but near only 1 sec. in the middle. Attached you will find one of the mdat files of that measure ( all of them are pretty similar ). Could it be some "resonance" of the room at 2KHz? Do you think that I have to measure again with a newer version? Regards.

Actually I am concerned about the 2KHz peak since the one at 100Hz is more obvious since the absorption coefficient is lower at that frequency for the rigiton material on the ceiling.

 

[John Mulcahy]

The measurement looks fine, and the decay time at 2 kHz is 2 seconds. That's too high to be attributed to a modal resonance, the mode density is far too high by that frequency, but you might be seeing the effect of flutter echoes. The decay in the 2 kHz band does have an odd hump.

 

[gerardochile]

So, you think that is the real value of RT60 and I can trust it? theoretic calculator gives me 1.6 sec. for 2KHz.

 

[John Mulcahy]

Yes.