Please help me to fix that 150Hz dip

[sxx]

Hello all,
I'm really struggling with that dip at 150Hz and can't get rid of it...
I realized that if I move the insulation on the front ceiling, the dip will move (between 130 and 180 Hz) but I couldn't manage to fix it.

Here some REW results with different takes while moving the insulation on top of the listening position + front ceiling:
https://drive.google.com/open?id=1RXzk6 ... 5EU27oM0i6

And pictures of the control room:
https://drive.google.com/open?id=1-fOAB ... RhwhwEtCOH

Room dimention :
Length = 503.0 cm
Width = 472.0 cm
Height = 359.0 cm



Thanks in advance for help ;0)

[sxx]

Got some improvement by adding 15cm of glasswool under the ceiling (with 25cm air gap)...

Could someone please explain what could be those comb like dips between 6Khz and 20Khz?

[Soundman2020]

Hi there "sxx", and Welcome!

Could someone please explain what could be those comb like dips between 6Khz and 20Khz?

Do your testing with just one speaker at a time, not both at once! That comb filtering does not actually exist when you are listening at the mix position. It is just an interference pattern between the two speakers, which you would never hear because you don't have one single ear located in the center of your head: you have two ears, located out on the sides of your head, so your ears will never "see" that.

To get a more realistic measurement, do three tests: one with just the left speaker on, one with just the right speaker on, and one with both speakers on. The one with both speakers will show you what the low end of the spectrum is doing, up to about 1 kHz. The other two will show you what the high end is doing, and will also show you if there are any significant differences between the two channels, which is important.

If you haven't already seen it, you might find this interesting: How to calibrate and use REW to test and tune your room acoustics

Your 150 Hz dip could be SBIR, or maybe a floor or ceiling bounce, or it could be a modal null. You'd have to play around with mic positions to be sure.

Also, your room is a bit too dead: you have too much absorption in there, or rather, you have insufficient reflection for the amount of absorption. Your overall decay time is around 170 ms, but for that size room it should be around 250 ms. So, you'll need to add some reflective surfaces over your bass trapping, but in such a way as to not cause specular reflections back to the mix position.

Strangely enough, despite the above, you do seem to already have a lot of strong early reflections arriving at the mix position... so those are going to need damping.

What does the rear of the room look like?



- Stuart -

[sxx]

Hi Soundman2020,
Thanks for your helpful reply!

Your 150 Hz dip could be SBIR, or maybe a floor or ceiling bounce, or it could be a modal null. You'd have to play around with mic positions to be sure.

In that regard, I removed the ceiling treatment that I added in previous post then:
I took more measures from the listening position, then moved the mic. 30cm froward and also 20, 50 and 100cm back from the original listening position...
https://drive.google.com/open?id=1rmdmX ... r2O9uDTWQT

The the dip is "moving" @130Hz and 175Hz getting deeper and deeper while moving the microphone back or ahead of the L.P.
I also did some separate L - R speaker measurement...

Does that gives more information to identify if it's SBIR or modal and how to address it?

So, you'll need to add some reflective surfaces over your bass trapping, but in such a way as to not cause specular reflections back to the mix position.

Strangely enough, despite the above, you do seem to already have a lot of strong early reflections arriving at the mix position... so those are going to need damping.

Sure, I'll look after those reflections I think they're mostly floor bounces...

For the back wall, I planned to build 2 Prime 523 2D Primitive Root Diffusor like those, do you think they could add the missing 50 - 60ms of missing "liveness"?
https://www.jhbrandt.net/wp-content/upl ... fusors.pdf

Here's a picture of the backwall, sorry for the mess. (I already added a plastic sheet to add some high frequencies back in the room)
The MDF boxes are DIY BDA traps:
https://www.acousticfields.com/diy-broa ... orber-bda/
Plus some mineral wool (8300k/Pa) insulation.

Cheers,
Sam

[Soundman2020]

The the dip is "moving" @130Hz and 175Hz getting deeper and deeper while moving the microphone back or ahead of the L.P.
Also did some separate L - R speaker measurement...

Does that gives more information to identify if it's SBIR or modal and how to address it?

Yes it does! But without seeing the actual data, I can't tell you what it is...

I'm not sure if you have read that tutorial I wrote on how to use REW. From looking at your setup and what you are saying, I think there's a few tips in there that you might find useful. Here's the link again: How to calibrate and use REW to test and tune your room acoustics

Sure, I'll look after those reflections ... I think they're mostly floor bounces...

You only get one floor bounce... There's only one location on the floor that can cause a floor bounce. And the reflections in your REW data are not consistent with a floor bounce.

For the back wall, I planned to build 2 Prime 523 2D Primitive Root Diffusor like those, do you think they could add the missing 50 - 60ms of missing "liveness"?

Your room is only 5m long, so it's too small to be able to use a numeric-series diffuser. You need at least 3m between the face of such a diffuser, and your ears, which isn't possible to achieve in a room just 5m long. It needs to be over 6m long to do that comfortably. Yes, I know that you do see lots of photos of small rooms with QRD's, PRD's, Skylines, and other similar diffuser on the rear wall, just a meter or so behind the engineers head... but just because you see it in a photo doesn't mean it is good!

The MDF boxes are DIY BDA traps:

Let's just say that the advise offered by those guys is not exactly well regarded among real acousticians ... There's solid acoustic theory... then there's actual real-world acoustic practice... and there's also the distant fringe on the edge of the twilight zone, based mostly on wishful thinking... So do be careful with that! Also, you have it in the middle of the wall, which isn't the best spot, and it isn't very big.

Did you do REW tests on your "BDA" device, to see if it really is working? The only way to be sure, is to do one REW test of the room with the mic at the mix position without the device in place, then another test with the device in place, but not changing ANYTHING else in the room. That will show for sure what it is doing.

Plus some mineral wool (8300k/Pa) insulation

I think there's a typo there: acoustic impedance is measured in rayls, or pascal-seconds per meter (Pa·s·m−1), not k/Pa. Are you talking about rayls? The usable GFR range for studio acoustics is around 2,000 to maybe 50,000 rayls. If so, 8,300 rayls is fine for that.

sorry for the mess.

All studios under construction are messy! Most are WAAAY more messy than yours...


- Stuart -

[sxx]

I followed your prcedure.
I use a Minidsp Umik-1 USB calibrated measurement microphone. it’s provided with a calibration file and connected to a long usb cable.
Is it ok or should I buy an other xlr microphone?

I get very different results while doing 2 256k sweeps instead of 1 single sweep!
Could you explain that?
Now I'm worried about that null @390Hz
And when I play a “static” 390Hz sine wave using REW sound generator, I don’t get such big null.

https://drive.google.com/open?id=1v2iev ... ZYfShEyokw

[kominak]

I get very different results while doing 2 256k sweeps instead of 1 single sweep!
Could you explain that?

I had the same problem when using MiniDSP Umik measurement mic. You have to stick with just 1 sweep, when using USB measurement mics, here's the excerpt from REW help page:

REW allows multiple sweeps to be averaged, although best results are generally obtained by using single, longer sweeps rather than multiple, shorter sweeps. Do not use multiple sweeps if the input and output are on different devices (for example, if the input is a USB mic). If Sweeps is more than 1 REW uses synchronous pre-averaging, capturing the selected number of sweeps per measurement and averaging the results to reduce the effects of noise and interference. The pre-averaging can improve S/N by almost 3 dB for each doubling of the number of sweeps. Averaging can be useful if the measurements are contaminated by interference tones, whether electrical or acoustic, as they typically will not add coherently in the averaging and hence will be suppressed by the process

Warning: some soundcards do not maintain sample synchronisation between the successive sweeps which produces a corrupt measurement that has multiple, closely-spaced peaks of approximately the same level in its impulse response, 1 peak for each sweep. This can also happen if the input and output are on separate devices. If the frequency response with multiple sweeps is significantly different from the response with a single sweep, stick with single sweeps

[sxx]

If the frequency response with multiple sweeps is significantly different from the response with a single sweep, stick with single sweeps

Pfew, I'm happy to read that, I was quite worried to be honest, thank you kominak!
Took those measures again:
https://drive.google.com/open?id=1iSHEa ... hhvQS8_0IX
Also included some measures in front of and behing the default listening position.

[sxx]

I pushed further my investigations and here is what's been figured out so far (please correct me if anything sounds strange to you):

Did you do REW tests on your "BDA" device

I did a test with (bloe measure) and without (green measure) those BDA in the room.
(but took them out of the room for now).

[sxx]

About the issue @150Hz we tried to use an other pair of speakers (Yamaha HS80m), to place them in front of the mains and take a reference measurement...
We got similar results/issue @150Hz as before.
Moving those speakers about 50cm higher and tilting/pointing them to the listening position gave us the following results (better response @150Hz and a little dip @128Hz):
https://drive.google.com/open?id=1PytzW ... 1P90LTFBlE