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Old 11-24-2008, 05:54 PM   #21
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Part IV - principles of down and dirty acoustical treatment

First off, you CAN make great pop recordings in a low-ceilinged space. And don't let anyone tell you different. Every single Motown record in the Detroit era was made in the low-ceilinged basement of Berry Gordy's house (originally with a dirt floor, no less). Blaming the room is just excuse-making. The list of brilliant records that were cut in lousy spaces is even longer than this thread. If you plan to record the Boston Symphony Orchestra in your basement, you might need to work a little harder, but if you were able to book them for the gig then you're already beating the odds.

Having a vaulted hardwood ceiling at your disposal for drums and pianos is a neat convenience, but the really critical space is where you listen and mix. The control room (or just the spare bedroom where your computer is) is where you make the critical decisions not just about mixing, but also where you evaluate your afore-mentioned walking-around recordings and everything else. So it's pretty important to minimize the most drastic effects of room resonances and standing waves in this space, even if you decide to cut your tracks in the laundry room.

Fortunately, you can treat this room in an afternoon for less than $100 (even less if aesthetics are no object). Will it be a world-class audiophile listening room? Probably not. Will it be a vast improvement over almost any listening space that non-professionals have ever been exposed to outside the movie theater? Probably so.

Here's the deal. What we want to do is to "trap" or absorb the standing waves (convert them into heat energy, to keep a pretense of science). I'm going to start by talking about the stuff that you *don't* want to do, because there is all kinds of misinformation out there.

We want to do this without creating a totally dead space. Especially if you are recording in the same room. In any case, an anechoic chamber is both nigh-impossible to create, and also a very unpleasant place to spend time in. Trust me.

First off, covering all the walls with 2" foam is one of the worst things you can do to create a pleasant, even, natural acoustic space. Acoustical foam has its place, which is in marketing brochures. (this is an exaggeration, but the over-use and misapplication of foam and egg-crates and egg-crate foam does so much more harm than good that it's not a bad starting point to think of it is as bad, and then find exceptions to the rule).

Secondly, you do not want to get into helmholtz resonators (proper "bass traps") such as panel traps or tuned clay pots or any of that. You can easily find resources on the web that will help you calculate your room modes and so on, but honestly, you do not even need them. In a small residential space, there is basically no such thing as good bass resonance. Getting all precious about calculating the frequencies you want to corral is not just over-kill, it's counter-productive. We just want to scoop up everything in the way of low-frequency standing waves and kill it. And you don't need to understand much in the way of theory to do this.

A little more on the topic of "tuned" solutions and why they are inappropriate for a home studio: In an acoustically-designed space, such as a classical concert hall, the designer has some very specific goals, typically contractual ones. For example, their design criteria might be even frequency response measured in 1/3-octave intervals, +/- 6dB in 80% of the seats, with a reverberation time of 60dB of loss after 2 seconds, and no more than 12dB loss of intensity from the front row to the back, and so on. These criteria mean that any additional absorption is potentially a deal-breaker. So if they have a 15-foot high balcony, that is going to create standing waves at say 40Hz and intervals thereof below the balcony, and they are trying to design the space to spec, which would include finding a way to tame that standing wave without soaking up any more volume intensity.

There are ways to do this by creating tuned resonant spaces with a certain proportion of size open to the treated space. If you have no life you can read up on it in McGraw Hill's Master Handbook of Acoustics or probably find some stuff on wikipedia or whatever. But unless you are designing a space with a million dollars hanging on whether row 28, seat 12b hears the unamplified orchestra at -8dB with +/- 3dB frequency response, it doesn't matter.

in the small, amplified, sweet-spot focused world of the studio control room, even in a million-dollar studio, we are perfectly free to just kill ALL the bass resonance and call it a day. In fact, this is not just a shortcut, but is actually the best approach to take in a small room with amplification, acoustically speaking.

One of the really cool things about random is that it is still random even if you randomize it some more. E.g., if you have a beach that is made of sand with some rocks in it, and you scoop out all the rocks and some of the sand, then it is still a beach made of sand, and is indistinguishable from a beach where someone forensically picked up each rock and carefully brushed every grain sand back onto the beach.

So it is with acoustics. What we want is random. Random resonance, random decay, for a nice, even, natural reverberation, and preferably a fairly short one. What we don't want is the non-random, localized, frequency-specific resonances that room modes produce. If we can scoop out all of the non-random stuff, and we get some of the random stuff as well, then who cares? The random stuff that is left over is still random. No more and no less random than it was before.

Moreover, it's not only easier but also a lot more effective to clear a beach of rocks with an overkill approach. We don't need a bunch of geologists on their knees with calipers and toothbrushes to sift the beach, and we don't need any calculations or fancy resonators to soak up problematic standing waves. What do we need?

BROADBAND ABSORPTION. How do we get it? To be continued...

Last edited by yep; 11-24-2008 at 10:04 PM.
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