[ACHiPo]

I'm getting very confusing REW measurements and need some help sorting them out, as it looks like there is some sort of high-pass filter on my mic pre or in REW itself that is rolling off the bass measurements.

After messing around with Studio Six and my iPad for a few weeks, I figured I'd get serious and picked up a used calibrated condenser mic and Focusrite Scarlett 2i2 USB preamp to use with REW, since that software seems to be the standard here and on other acoustic forums.  As an aside StudioSix indicated that I had pretty flat (+/- 10 dB) in-room frequency response down to 30 Hz, and decent decay time at about 250 ms.



I took a bunch of REW measurements at different points in my room and starting trying to figure out seating position and speaker placement.  Well while I could see the modes flatten and move depending on position, there was a serious (~30 dB) roll off from 200 Hz to 20 Hz that I didn't see with StudioSix and I don't hear.

Last night I had some time so I took two full-range sweep measurements, one directly in front of my left woofer (my Kef 207/2s go down to the low 30s, so they definitely produce bass), and I still saw the roll off.  I took another measurement in the listening position just for comparison, and sure enough I see the same roll off.  I then did a manual frequency scan using the SPL meter in REW and sine signal generator at different frequencies and I saw and heard what I'd expect--solid bass down below 40 with nulls and audible and measurable bass down to the low 20s.



The data are consistent with some sort of bass filtering on either the sweep generator or analyzer.  Can anyone suggest things to look at before I give up and just order a calibrated USB mic?

Thanks,
AC

[HAL]

Check to see if the MIC pre has a filter switch for high pass.  It should be set to flat.

[ACHiPo]

Hal,
Thanks, but the Scarlett 2i2 has no filter switches.  Each channel has a line/instrument switch and a gain pot.  There is a phantom power button for both channels (set to on), a switch to turn on monitoring (set to off), and monitor volume knobs.




Is it possible that there is a high-pass filter somewhere in REW that I haven't been able to find?  I tried changing calibration curves (the mic comes with 6 cal curves depending on orientation) with no effect.

AC

[JohnR]

I'm not sure I'd call REW a standard, it is popular though because it has a lot of analysis tools, is cross platform, and has constant support from the author (JohnM) on the REW forum. And it's free.

With that said... your measurement looks to me like it might be from the laptop mic.

[ACHiPo]

I'm not sure I'd call REW a standard, it is popular though because it has a lot of analysis tools, is cross platform, and has constant support from the author (JohnM) on the REW forum. And it's free.

With that said... your measurement looks to me like it might be from the laptop mic.

That would make sense 

Sure enough the internal mic was still set as default.  I'll re-run tonight to verify that was the problem.  Thanks!

[HAL]

There are settings in preferences in REW for response bandwidth.  Check those to see if is setup correctly.  You should also check the calibration file is loaded. 

[HAL]

REW has separate settings for the input and output devices under it's settings. You need to change those. 

[ACHiPo]

There are settings in preferences in REW for response bandwidth.  Check those to see if is setup correctly.  You should also check the calibration file is loaded.

I confirmed the cal file is loaded, and even tried a different cal file with no effect.  Pretty sure the main problem is that I was using the wrong flippin' mic.  Will confirm tonight.

[ACHiPo]

Yep, I'm (as my grandson would say before being scolded) a dummy-head-face.  The consolation from this character-building experience is that at least I suspected something wasn't right. 

I changed the default mic in Windows and that fixed the problem.  Now I'm back to sorting out peaks and nulls, but at least they make some sense.

Thanks for all the help!

[JohnR]

Glad you got it sorted

[ACHiPo]

Two steps forward, one step back (or maybe spinning around with a blindfold on trying to pin the tail on the donkey)...

I took a bunch of waterfall measurements this morning playing around with different room treatment arrangements.  I focused on 20 Hz - 300 Hz, but took full-range sweeps from 15 Hz to 20kHz.  What I find surprising is the lack of difference I see in decay time with and without panels.  The most extreme test I did was comparing the listening position with all treatments removed vs. 2 monster traps and a 4" 703 + mass loaded vinyl DIY trap stacked behind the listening position and soffit traps around the room at the wall/ceiling interface.

Here's the waterfall with no treatments...




...here's the listening position with my planned treatment (MT on wall behind listening position, trap in closet, soffit traps at wall ceiling boundaries, DIY trap in L rear corner...




and here's the listening position with all soffit traps and 2 monster traps plus 4" MLV DIY trap for 18" of acoustic insulation plus vinyl...



The last graph looks pretty good, but that's a LOT more trapping than I thought I'd need.  Also, if I blow up the waterfall for full-range, it looks like I'm getting some sort of weird frequency oscillation.  Is this comb filtering?



I guess the good news in all of this is that it seems like I did a decent job finding my listening position, as it seems to have the flattest response I found.  Here's the frequency response curve with both left and right speakers (may be why I'm seeing comb filtering?).  What's interesting/confusing is the big suck out I see at 50 Hz and ~900 Hz.

[HAL]

Yes, that is comb filtering, usually due to floor, sidewall and ceiling bounces. 

Limit the REW response plot to 300Hz to help eliminate that from the measurement.

[Big Red Machine]

I like the blue chart.  Starting to look better.  YOu have those multiples of 50 hz and 100 hz hitting you.  Must be hitting room dimensions in multiples of 5.6 feet (200 hz).  What are your room dimensions?

[ACHiPo]

I like the blue chart.  Starting to look better.  YOu have those multiples of 50 hz and 100 hz hitting you.  Must be hitting room dimensions in multiples of 5.6 feet (200 hz).  What are your room dimensions?

Yes, the blue chart looks decent, but to get that I'm going to have to add a whole lot more trapping volume--that's 18" of high density fiberglass with mass loaded vinyl 24" behind my listening position and the response elsewhere is still not great, so it's just a localized solution.

My room is irregularly shaped, but I ran a modal analysis on the major dimensions.  Here's a plan of the room:



Here are all the permutations I modeled (in feet):
   Dimensions in feet                     
Length   17.33   17.83   
Width   15.50   18.00          20.75         25.75
Height   9

And here's a histogram of the modes:



There's some correlation with the measurements, but not a lot.  Note there are no theoretical modes at 50 or 100 Hz.

[Big Red Machine]

When I look at your 'dips" they are between 50 and 60 hz and all multiples of that.  So that is between 5 and 6 feet which lines up as multiples of both the depth and width.  What that says to me in my experience is you cannot defeat it. It is what it is and the possible solution is offset the speakers side to side and front to back slightly.

[ACHiPo]

When I look at your 'dips" they are between 50 and 60 hz and all multiples of that.  So that is between 5 and 6 feet which lines up as multiples of both the depth and width.  What that says to me in my experience is you cannot defeat it. It is what it is and the possible solution is offset the speakers side to side and front to back slightly.

Ahh got it.  Was thinking peaks, not nulls.  Yeah, I can jiggle the speakers a bit--might help smooth things out a bit.  Thanks!

[*Scotty*]

RPG or QRD type diffusion panels would help with the comb-filter problem and are much more desirable than turning your room into an anechoic chamber. See link http://arqen.com/sound-diffusers/
Scotty

[ACHiPo]

RPG or QRD type diffusion panels would help with the comb-filter problem and are much more desirable than turning your room into an anechoic chamber. See link http://arqen.com/sound-diffusers/
Scotty

Thanks.  I'm not worried about the comb filtering--it's a measurement artifact.  I'm trying to smooth the bass below 300 hz at this point, and it's proving challenging.  I've got enough glass and wood in the room that it's not dead.  I'd just like to have smooth bass response.

That being said, I'm interested in diffusers to improve imaging once I get the bass tamed.  Just not sure where I'd put them.