Advise to interpret REW measurements

[Marian10]

Hi,
I moved my listening room to basement and spent some time to find the right place for speakers and listening position, using REW measurements.
Now I think about some acoustic treatment of the room and I tried to use some available material (e.g. old matrices) to improve RT60 in front corners.
I measured before and after, and there is quite nice RT60 improvement.
Nevertheless, I am not able to interpret some results
1. There are some ETC peaks at 2, 5 ,9,7 ms. Is it possible to guess the causes?
2. SPL for 80-100Hz is nicely compensating in L+R measurement compared to individual channels, while for 10k Hz it is the opposite, L+R create dip. Any reasons?
3. Any suggestions for acoustic treatment?
I plan bass trap to front left corner and panel to front right corner with doors. Something on front wall and front ceiling. Probably curtain over windows. Not sure what to do with rear wall.

My room is 5x3,7x2,5 meters, most of left side are windows.
Speakers are 60 cm from left wall, 90 cm from front wall, 180 cm aside. Listening position is 230cm from front wall and 150 cm from left wall.

Any help is highly appreciated, as I am not able to find out more, even with AI help :-(

Measurement and photos are enclosed.

Many thanks

Marian

 

[Keith_W]

2. SPL for 80-100Hz is nicely compensating in L+R measurement compared to individual channels, while for 10k Hz it is the opposite, L+R create dip. Any reasons?

I don't have a lot of time right now, but i'll answer this question first. The answer is: measurement artefact from an improperly centred microphone. This is the proof:

This is a comparison of the vector sum of L+R ("before treatment") in purple, and the actual measurement in red. We can see that the vector sum does not have the 8-10kHz dip. This means it is not some weird phase issue in one speaker (e.g. a tweeter wired out of phase in one speaker). So let's look a bit more closely.

If you look at the impulse response of your actual measurement, you can see two peaks, indicating left and right speakers. You can also see that REW has placed t=0 at the dip with the greatest magnitude, meaning that one speaker is "early" and one is "late". The question is which speaker is early.

It's not the right speaker, because the both impulses align at t=0.

So the culprit has to be the left speaker. And sure enough, we can see that the reflection I circled is not aligned. I could also prove the same thing by looking at the main impulse, but there are a lot of chaotic spikes in there and it would be more difficult to see. Nevertheless, after I align them, you will be able to see the main impulse more clearly. Anyway, the left speaker is arriving early.

I measured the time gap between the two peaks in the reflection and determined it was 80us. So I shifted the left speaker (blue) to the left - now it is time aligned. You can see how the two impulse responses follow each other very nicely. Then we sum L+R.

You can see that the summation (blue) is identical to your measurement (red). This proves that the dip is due to improper microphone centering.

You can convert time to distance with the formula d = c * t (d = distance in mm, c = speed of sound, t = time in ms). So an 80us time-gap equals a distance of 27.44mm.

Lesson: centre your microphone properly. REW does not have a microphone centering tool, but I have invented two ways to centre your microphone. See my eBook.

 

[Marian10]

Dear Keith,

Thanks a lot for your fast response. I am reading your ebooks (excellent!) and I will correct my measurements.

If you find some time for other topics, I will very appreciate.

Thanks

 

[Keith_W]

You should be able to answer your own questions by reading that eBook. Re: the ETC peaks, read the section on ETC to see what they mean, determine whether they are a problem, and how to work out what is causing the peak.

As for your other measurements, here is something rather interesting:

This is a comparison of your L/R speakers (green/blue) vs. the vector sum of both (purple). I have staggered the vector sum for clarity.

Look at the area I highlighted. You can see that the L and R speakers individually have peaks in the 60Hz - 120Hz region, yet they sum to a very nasty dip.

... and this is the reason why. You have some kind of nasty reflection corrupting the phase of both curves.

Sure enough, if we extract excess phase and look at the Group Delay, we can see two nasty non-minimum phase peaks. In the left speaker (blue) it's at 70Hz, in the right speaker (purple) it's at 85Hz.

The eBook contains a procedure for diagnosing the cause of a dip. It involves moving speakers and re-measuring. If you are unable to solve your issues with better loudspeaker placement alone, you will need DSP. A time reversed all-pass filter can shift the phase of that narrow band of phase rotation and remove the cancellation. This is why linear-phase DSP has an advantage here.

In the same Google Drive where you downloaded that REW eBook, there are also two free books on how to use Acourate. Download "100. Acourate for DSP Controlled Active Speakers with Subwoofers", and read the section on Acourate's "ICPA" (Interchannel Phase Alignment) feature. I think there's a section in there on how to design your own time-reversed AP filter using Acourate procedures, but it's easily translated to REW + rePhase.

I am sorry I have to use all this jargon, but that's why I wrote the book. So that I don't have to type really long replies explaining what all these things mean all the time! I don't expect you to understand all this at once, I assure you it took me months of reading everything I could get my hands on to understand. Read that book, look at these measurements, and you'll eventually get it.

And BTW, it's probably not my place to criticize, but your TV is mounted way too high. You want your TV to be at eye level.