My room's got issues!

[CurtZHP]

I've got traps in all four corners of the room and a couple on the ceiling in front. I also have broadband absorbers in key locations.

Here's an interesting development....
I ran several tests with REW this weekend. I set the room up the way I thought I was supposed to: 38% rule, get the speakers away from the front wall, etc. Took all sorts of measurements. Bottom line - I've got a bit of a low spot in response at around 140Hz.

I tried several combinations of speaker and acoustic panel placement, and I basically got the same result. Sure there were some setups that fared slightly better than others, but not by a very significant amount.

I finally put the room the way I liked it. Taking another measurement, the graph looked almost the same again.

At this point, the only thing I haven't done is pack the right wall with insulation. As I mentioned earlier, that wall was never insulated because the only thing on the other side of it is a solid concrete wall.

According to at least one standard reference text, that's a classic example of a cavity resonator. Who knows what sort of silliness that's introducing?!

I might try one or two other small things, but I've pretty much come to terms with it. If the only real problem with this little home-brew studio is a dip around 140Hz, I can live with it.

I spent part of Sunday afternoon listening to various commercial recordings in there, everything from hard rock to R&B to classical, and everything sounded pretty normal to me.

[Snarl 12/8]

I was running the test tones from the real traps web site today. Holy crap. I don't have a decibel meter, so I set my tapeop omni where my head normally is and ran it through a mixer. I "calibrated" the meter on the mixer to be -9db with the pink noise. Then I ran the tones. I went from one LED away from peaking the meter to it not showing any lights? The meter runs from -21db to +9. And that was just between 60-200Hz. I couldn't stand it to do it longer. I was kindof wondering if I was killing my speakers too.

Anyway, I wanted to tell you. I was walking around while the tones were playing, just digging the very dramatic nulls that I could feel just walking across the room, basically, and I noticed the ceiling above me was making a sound. When I touched it (yes, my ceiling is super low) it was vibrating pretty intensely. It made me think of your situation. Try playing tones at the points where you think your trouble spots are and just feel the wall. You'll be able to tell pretty readily if it's reacting to the sound. Compare the way it feels to the other wall.

Just a suggestion. Oh, and I could make the meter fluctuate by as much as 2 LED's by moving around while the tone was playing. I felt like Heisenberg.

ck

[CurtZHP]

Snarl, you can get a decent sound level meter from Radio Shack for about $50. I've had one for years. The cool thing is it's got an output for the mic, so you can send it to REW via a soundcard.

During the weekend, while I was doing research on this problem, I found this in an article about loudspeaker measurement:

While the very best modern speakers can produce a frequency response flat to ?1 dB from 40 Hz to 20 kHz in anechoic conditions, measurements at 2 m in a real listening room may be considered good if they are within ?12 dB, and efforts to produce anything like a flat response below 100 Hz are likely to provide endless scope for experimentation!

Ain't that the truth!!

I figure insulation is cheap, so I'll go ahead and stuff that wall. Can't hurt.

[Snarl 12/8]

Can't hurt.

Y'know, I'm kindof rethinking this idea. With acoustics, in funky rooms, I don't think you're gonna know how something's going to work until you try it. That vibrating wall (assumption) could be absorbing more harmful frequencies than it's contributing. This shit is COMPLICATED.

[CurtZHP]

Yeah, but if it's resonating, wouldn't it reinforce the frequency at which it resonates?

[Snarl 12/8]

Did you watch the Hodas video? He talks about how you can simply hang a piece of plywood right by a wall and it'll calm certain frequencies depending on the size and thickness of the piece and how you have to place it very carefully.

What if the resonating wall is strengthening a frequency that would otherwise be a null in your room? If it's resonating it'll both absorbing energy to get it moving and releasing energy at a given frequency (or frequencies) and I don't think you can ever know for sure which will be better.

I mean there's no way it can give off more energy than it absorbs, right?

Edit: I'm not at all saying your shouldn't do it. At the very least is should simplify the math. (And therefore the acoustics.) I'm just saying that I'm learning that rules of thumb and "common sense" don't necessarily carry the day here.

[CurtZHP]

I'm just saying that I'm learning that rules of thumb and "common sense" don't necessarily carry the day here.

That's what I'm finding out too.

I figure if I insulate that wall, it will no longer be the only variable (as far as wall construction is concerned).

[blungo2]

Well, so far things are disastrous. The reason for the null at 100Hz turned out to be the simple fact that the width of my room -- 11ft. -- just happens to be the wavelength of a 100Hz tone. I also found a bit of a dip at 160Hz -- which happens to correspond to 7ft, which is -- you guessed it -- the height of my room. Looks like I can also look for trouble at 76Hz as well (the length of the room).

Can i take this opportunity to ask some dumb questions?
How do you find out the wavelengths of different tones? (i know, i'll search on it, but i'd like to know where you find your info...)

Second dumb question, what is the REW software that people are speaking of? I'm on a mac and i downloaded fuzzmeasure (or whatever it's called) and it crashed in snow leopard.

Thanks for any advice. BTW, my big dip is at 70Hz, but i have a feeling that's because it's the current crossover point for my sub...

EDIT: i found a cool wavelength conversion site here

http://www.sengpielaudio.com/calculator-wavelength.htm

but i expect you guys already know about it.

[CurtZHP]

Actually, blungo, I just use this formula to figure wavelength:

L=V/F

Where L=Wavelength, V=velocity of sound in feet per second (1100 ft/sec.), and F=Frequency.

So a 100Hz tone would figure thus:

L=1100/100, so the wavelength is 11 feet.


As for REW (Room EQ Wizard), go to www.hometheatershack.com to download it. You have to join their forum first, but it's free to join. It's basically a program of various audio calibration and measurement tools. It's got a decibel meter, an RTA, and a few other goodies.

[blungo2]

Thank you!