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Old 05-14-2007, 09:04 PM   #46
scottdru
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Quote:
Originally Posted by chip mcdonald View Post
Where can he get a broad band absorber that *evenly* reduces 4 octaves at those *specific* frequencies, while not creating a valley elsewhere, that will also fit in a 4x5 room and still leave room for other stuff...?
Chip, broadband absorbers DO NOT create nulls/dips in the frequency response. Rather, they actually REDUCE the severity of, or eliminate, nulls/dips in frequency response, as well as reducing peaks.

The peaks and nulls are BOTH caused by the phase interference of the room reflections with the direct source sound at various points in the room. If you put broadband absorption in a room, you reduce the reflections that cause both peaks and nulls, and therefore your entire frequency response evens out, and the decay time becomes more uniform.

If you can reduce the amplitude of those reflections by even as much as 6 dB or so in relation to the direct sound from source, you will noticeably reduce the severity of both peaks and nulls.

A lot of people get confused and think that putting absorption in a room can cause you to end up with LESS bass . . . but indeed you will get MORE bass where before there were nulls, and you will of course reduce the peaks.

Tuned absorption targets only one frequency or frequency range, and ignores all the rest . . . and therefore does not take care of all the other peaks and nulls that are absolutely there across the frequency spectrum (in ALL small rooms) whether you notice them or not. Most small rooms have peaks and nulls across the entire frequency spectrum somewhere in the range of 30 to 35 dB from top to bottom.

Some of those peaks and nulls occur in very narrow bandwidths, so a lot of times people do not notice them (especially the nulls) when they are listening in a room . . . but you will most certainly notice how much you were missing when you treat the room properly with broadband absorption, and you get back so much of the sound that was previously inaudible due to phase cancellation at modal frequencies. The other thing is that so many of those very narrow band peaks and nulls will get entirely missed by 1/3-octave band pink noise testing which so many people traditionally use for room acoustics analysis/RTA. That's simply not fine enough resolution to get an accurate sense of what's *really* going on in your room.

The bottom line is that ALL small rooms NEED broadband absorption to get anything even remotely close to an even frequency response. That is simply the best and most cost-effective way to treat small rooms, and I think pretty much any acoustician worth his salt will tell you the same.

So . . .

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then maybe that is the time to use a tuned absorber like a Helmholtz resonator or tuned panel absorber.
Sort of like "in a tiny room you'd have to figure out exactly which frequency in the bass is dominant through analysis, and then build a custom-dimensioned Helmholtz resonator to stop it."
My point being that your assertion that bass trapping (i.e. broadband bass trapping) would most likely be "fruitless" is *entirely* incorrect, as is your recommendation for a tuned trap (which, again, only addresses a very narrow band) to the *exclusion* of broadband trapping.

And back to this:

Quote:
Originally Posted by chip mcdonald View Post
Where can he get a broad band absorber that *evenly* reduces 4 octaves at those *specific* frequencies, while not creating a valley elsewhere,
Off the top of my head, I can't think of ANY absorbers/absorption materials on the planet (at least not that could reasonably be used for interior acoustics treatment ) in that have a flat absorption curve (unless you count an open window or doorway), and it is a huge (and unfortunately widely held) misconception that a flat absorption curve is needed (even if in certain instances it might be preferable), because of the principles I've discussed above.

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that will also fit in a 4x5 room and still leave room for other stuff...?
Typical panels made from at least 3-4 inch thickness of OC 703/705 (or the rockwool equivalent), be they commercially manufactured or DIY will offer SUBSTANTIAL improvement across the entire frequency range in a room like that (most definitely *including* even the lowest frequencies I listed as axial modes for that room). This will give you a MUCH flatter frequency response across the entire spectrum than using tuned traps in a small room such as this, and even using tuned traps you don't just use one and call it good . . . you have to have such absorbers evenly distributed throughout the room for best results, and preferably in the corners, where ALL of the room modes are present. This is where you need to break those wave fronts to get the most effective reduction of room modes.

Tuned traps are most often an inefficient use of space in small rooms, because the tuned traps will typically take up as much or more space as a typical broadband panel absorber, and will not lead to an *overall* flatter frequency response in the room unless used *in addition to* broadband traps.

So . . . if you treat a small room such as this with broadband bass traps and you STILL have one very stubborn low frequency peak, then -- and ONLY then -- you might consider adding a tuned trap . . . if you have room.

Last edited by scottdru; 05-14-2007 at 10:19 PM.
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