# Standing waves at 40, 400Hz



## Jarrod (Sep 29, 2009)

I am trying to optimize the sound in my family room.

With my large subwoofer in the only WAF-friendly location, I found a large cancellation at around 40Hz in my listening position. I was able to compensate for this by integrating a second small sub next to the center channel and applying a little EQ. I now have a much smoother subwoofer response at the primary listening position.

Next up was to do full-range measurement of the response at the listening position. Below is a REW plot of subs plus center channel from 15-20k (black) and subs plus L+R mains (magenta). I also measured L and R separately and they both overlay onto the magenta L+R graph (data not shown).









Yikes!! :yikes:

As you can see, my previously identified mode of 38-40Hz appears to also rear its ugly head at 10 times the frequency. To map the 40Hz mode spatially, I played a test tone and walked around the room with my SPL meter. There is a standing wave across the width of the room, with its null at the listening position, and a peak along each L/R wall. Spatially, it is one giant cosine wave with the left wall representing x=0 degrees and the right wall representing x=360 degrees. It doesn't matter where you stand in the length dimension - you can walk the length of the room in a straight line along the null or one of the peaks and the sound meter doesn't change.

The room is shaped kind of like a barn with the back torn off - in back, there's an 8ft x 10ft opening to the kitchen and beyond to the rest of the house. The ceiling is vaulted, but symmetrical. The right wall has a fireplace and two windows so I can't do much there. The left wall is solid sheetrock - I could put some absorptive devices there but I know that 400Hz is close to the lower limit of what typical absorptive panels (like OC703) can handle.

What about diffusors? Is this hopeless?


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## bpape (Sep 14, 2006)

Good job of identifying the problem. Now we just need to dig a little deeper. Does the dip stay consistent regardless of height also?

A 2" panel of OC703 is certainly capable of reaching way deeper than 400Hz. 2" with 1" gap behind has a coefficient of over .5 at 125Hz and 1 to 1+ depending on which specs you read between 250 and 500Hz.

Bryan


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## Jarrod (Sep 29, 2009)

I checked and no, the SPL at 40Hz doesn't change along the height, only along the width of my room. Furthermore, the L/R walls are the only two parallel boundaries of the room, and they are 13' apart. If you do a simple axial room mode calculation you get:

565ft per sec / 13ft = 43Hz

My REW measurements for speakers and subs placed on the left wall clearly show dips in the center of the room (my listening position) at ~39Hz, ~115Hz (3x), ~230Hz (6x), ~370Hz (9x). So it looks like the 3-multiples of this main axial mode are the ones giving me the problems, and for whatever reason (reflectivity of the sheetrock at this freq?) the one at 370Hz is the worst.

Thanks


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## bpape (Sep 14, 2006)

OK. Just making sure. Predicting and real world many times don't really match up due to interference from tangential and oblique modes, walls not being 100% rigid, etc. If height doesn't matter, then you're correct in that it's purely a width thing.

To deal with that, you can use absorption on the side walls beside your listening seats to minimize those issues.

Bryan


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## Jarrod (Sep 29, 2009)

I have three spare 2" OC703 panels in my garage. So you recommend the 2" panel with 1" gap configuration that you mentioned above? I've made DIY panels before for my front wall (that's why I have spare panels) but didn't leave a gap, they are flush with the rear of the frame. What does the gap do for you?

Unfortunately my mic preamp (Eurorack UB802) died yesterday  so I can't do any more measurements until I get it squared away.

Thanks for the advice.


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## Jarrod (Sep 29, 2009)

This is how I made my current panels:










Taken from my DIY 703 panels web page.


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## bpape (Sep 14, 2006)

For 350-400hz, you don't even need the gap but you can if you want. 2" 703 is over 1.0 coefficient at 2" on wall (A mount)

Bryan


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