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Acoustic Tests
Old 3rd November 2005
  #1
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DeeDrive's Avatar
 

Acoustic Tests

I just did an acoustic test of my mixing room, which I KNOW has some acoustic problems. Pretty simple test, I just fed some pink noise through my speakers (HR824's), and recorded in with a large diaphragm mic (AT4050/CM5). When looking at the recorded sample on a spectrum analyzer, I don't really see the dips and peaks I was expecting to see. It pretty easy to tell just from listening to the pink noise that this room has big problems, but I was hoping to get some input from the spectrum analyzer as to what the specifics might be. Do I need to be using a program specially made for testing room acoustics? Is this test not sufficient? I'll attach the files I recorded, maybe someone who has a better analyzer can see what's going on here.
Attached Files

pinknoiseomnimic.wav (760.1 KB, 176 views)

Old 3rd November 2005
  #2
Gear Guru
 

another test you could try would be to play sine waves - say 40 Hz to 300 Hz. Go up in 1 Hz steps.

play the tones through your monitors and record the output with a mic at the mix position or any place else in the room. I guarantee you will see the peaks and dips!



You could automate a tone generator plug-in. I didn't think of that at the time, so I cut and pasted hundreds of sine waves.

Old 3rd November 2005
  #3
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r0ck1r0ck2's Avatar
 

sooo
the ideal is a totally flat responce across the spectrum......
....
this is one of those eye openers for me....
i love a good simplistic (sp?) method...
thanks gents....
Old 3rd November 2005
  #4
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Berolzheimer's Avatar
 

did you have your mic in omni? If not it's going to be very deceptive, because the off-axis response is not flat.
Old 3rd November 2005
  #5
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Glenn Kuras's Avatar
joeq is right on the money and glad to see people are really starting to think about there room. You may want to run your test up to 600 hz. Higher is better, but I see a lot of problems in the 400 hz to 500 hz area also.


Glenn
Old 3rd November 2005
  #6
Gear Guru
 
Ethan Winer's Avatar
 

Lightbulb

DD,

> When looking at the recorded sample on a spectrum analyzer, I don't really see the dips and peaks I was expecting <

As Joe explained, you really need to measure at a resolution approaching 1 Hz. Most spectrum analyzers resolve only to 1/3 octave, and sometimes even 1/12 octave, but even that is not fine enough to see the true response. I use the ETF program which resolves to 0.7 Hz.

All of that said, the main reason to measure is simply to convince yourself there's really a problem. Hint: All small rooms have bass problems. heh

Regardless of what you measure, the solution is always the same: Broadband bass trapping that works well to as low a frequency as possible.

--Ethan
Old 3rd November 2005
  #7
Gear Guru
 

Quote:
Originally Posted by Berolzheimer
did you have your mic in omni? If not it's going to be very deceptive, because the off-axis response is not flat.

the result I posted above a 414 in omni was used. This picture is actually not at the mix position, but at another place in the room where the nulls were more dramatic.

the real freakout is to sit quietly in the room feeling the nulls "pass over" you as the frequency changes, or feeling the big difference moving your head a few inches makes.

I also did the test with gaps of silence between each tone so see how the room decayed.

I remember reading somewhere that sine waves don't tell the whole story either- I forget why, but as Ethan says, it certainly is a wake-up call.
Old 3rd November 2005
  #8


Sine waves can mask transients (like ringing and flutter). But, if you know what to look for, the sine sweep will tell you need to know.

The trick is: do the sweep with the mic in several positions. Move it left to right, front to back, and top to bottom. A cube that's 2 feet on a side cetered where your head is will tell you a lot. Measure at least the corners and the center, adding center points (so you have 27) is better, but may not be worth the trouble.



-tINY

Old 4th November 2005
  #9
Gear Guru
 
Ethan Winer's Avatar
 

Lightbulb

Joe,

> I remember reading somewhere that sine waves don't tell the whole story <

Yes, that will give you the frequency response fine, but not decay times which can be at least as big a problem.

--Ethan
Old 4th November 2005
  #10


Ethan,

If you get a pronounced peak in several locations it usually indicates a ringing problem. At least that's what I've seen. You may have different experiences.

In big rooms, the waterfall plot is invaluable, though....



-tINY

Old 6th November 2005
  #11
Gear Guru
 
Ethan Winer's Avatar
 

Lightbulb

tINY,

> If you get a pronounced peak in several locations it usually indicates a ringing problem. <

Of course you are correct. I'm spoiled by the ETF software because it shows everything, making it easy to identify all of the problems.

--Ethan
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