# Suggestions needed on treating a room



## katylied (Feb 24, 2012)

I've recently moved to a new place and the smaller of the listening rooms ist's giving me some big troubles in obtaining good sound. There is a sustained boom around 40 Hz with almost all the speakers I've tried in, especially when positioned on the long wall (better solution in terms of look but sound very bad). Now speakers are positioned along the short wall I've added 2 more DIY bass traps in the corners behind the listener position (in total there are 6 2'X3'X4" and 4 2'X2'X4" in the upper corners) and is much better but I would like to reduce the boominess some more. Room size is L 4,56 mt X W 4,04 mt x H 3 mt.
Here is the waterfall I've took









Can an Helmoltz resonator be useful? Any suggestions?


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## bpape (Sep 14, 2006)

That doesn't look to me like a normal type of resonance in the room. That on the surface looks like a couple of things in the room physically resonating. I've seen it be floors, radiators, cabinets, etc. I would take a serious wander around the room playing those tones and see if you can find what it is.

To only be down about 15db after 800 ms just isn't your normal problem.

Bryan


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## katylied (Feb 24, 2012)

Hi Bryan and thanks for your suggestions. The room is basically empty, just 2 short equipment racks and 2 rattan chairs....
I've tried to find out what can possibly is resonating but I've not found anything beside, maybe, an old door passage that was closed (door was moved) and seems rattling a little bit (just one layer of gypsum board).
I was thinking to eventually create an Helmoltz resonator using that space (about 209cmX89cmX25cm) if will help. It seems that is really a main mode problem because around 38/39 Hz if I stand exactly in the middle of the room the boom goes to almost zero.
Here are the frequency response and RT60 of the same speakers used for the waterfall:
















Your suggestions will be much appreciated!


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## bpape (Sep 14, 2006)

The one appears to be at about 50Hz which could be a ground problem. It's still pretty intense though. 

Move the mic forward or back about 1/2m and measure again please and post the waterfall.

Bryan


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## katylied (Feb 24, 2012)

Here we go: the microphone was at 40, 90 (as originally posted), 140 and 165 cm from the back wall:

































Here are the frequency response. With the microphone at 165 cm from the back wall is much better but too near to the speakers!









Still, the waterfall is only a little bit better......
FWIW, as measuring set I'm using the Behringer ECM800 mike/MIC100 mike preamp/UCA202 soundcard.
What to do?:huh:


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## bpape (Sep 14, 2006)

They're pretty low in frequency but likely, you're going to need to do the back wall in something tuned or very very thick to get down that low.

The 50 still kind of looks like a ground hum problem though it did move very slightly at the farthest most forward position.

Bryan


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## katylied (Feb 24, 2012)

Helmoltz resonator or tuned membrane bass trap? I've now moved the listening position at 155 cm from the back wall and about 160 cm from the speakers and the sound is not bad but still can hear some boomines when a bass note hit the problematic frequency (Massive Attack Protection is unlistenable but Karmacoma is ok.... Just to give an example). Hope to sort it out......:dontknow:,


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## bpape (Sep 14, 2006)

More of the membrane so you get a bit of width to the absorption. The Helmholz's are extremely narrow in range.

Bryan


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## HopefulFred (Jan 20, 2011)

I have no real business butting into this conversation, but if you'll excuse me... shouldn't it also show ringing at a harmonic? (100Hz) Certainly the problems are audible, but why no second harmonic if it's real acoustic resonance? Maybe I'm reading it wrong and you've got 58 and 116, but much lower?

Fred


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## katylied (Feb 24, 2012)

Unfortunately there is no grounding issue. The big problem is the room length resonance mode at 38.5Hz (calculated with the RealTraps ModeCal is 38Hz), confirmed by using the Sine Wave generator included with REW. Is amazing how is easy to see how the resonance go up and down in level with the help of the above mentioned REW tool and the Radio Shack SPL meter.....
As expected, max peak is near the back wall and more so in the corners (that are already occupied by bass traps...). Bryan, a membrane bass trap tuned at 38,5Hz seems not practical to build (way too big!), and reading the MHOA I'm very interested in try to build an Helmoltz resonator, any oher suggestion on how to lower both peak and time decay? (I can't use an equalizer).

Thanks


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## bpape (Sep 14, 2006)

Building a Helmholz is still going to take a considerable amount of space to get down that low and quite honestly, Helmholz at those kinds of low frequencies can introduce their own problems into the equation. 

The 38ish I would agree is likely a mode as it can be seen to drop in intensity and also decay better as you move the listening chair. The 50hz does not. Does it change when moving the mic side to side or up and down?

Bryan


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