the truth about bass traps and recording rooms

Posted: Mon Apr 07, 2008 7:13 am

So everyone here is always like "get some bass traps" when it comes to treating a room.

but .. WHY ???

follow me on this one, and then point out where my error might be (if there is one):

1. bass frequencies are easily built-up in corners
2. adding a bass trap in a corner will help with that respective corner's bass build-up.

so far so good right? but who cares. if you dont want to pick up excessive bass frequencies , dont put a microphone in the corner !!
right?? or am i missing something?

i'm all about treatment in "normal use" areas of a room, dont get me wrong. i feel like the bass trap rant is overused and unnecessary.

Posted: Mon Apr 07, 2008 7:18 am

It is the standing waves that becomes the big issue with bass.
I have yet to find it to be an issue for recording, but as far as mixing foes. once the standing waves generate, your mix is going to shit.
the last thing you need if your room to be resonating at some low frequency.

Posted: Mon Apr 07, 2008 7:51 am

A couple things that might help you get started on a long study.

1. Any small room has an uneven bass response - several specific resonant frequencies that are easily excited and predominate the response.

2. These modes have different shapes - different places where they are strong and weak - but the all are excited in the corners. Inside the room will be uneven with one mode strong here and strong excited over there

3. If a room were perfect, all frequencies would be excited evenly, so our goal is to calm down those frequencies that are excited the most.

4. The purpose of bass traps is to damp or calm down any bass frequency that is excited, and the corner is the place where all the excitement is.

Posted: Mon Apr 07, 2008 7:57 am

I've found it to be an issue with recording!

Here's a scenario -

You're recording voice in a small room. You've taken the time to put 2" Auralex foam all over the room (let's say 5' x 6'). But, you've used no traps.

Okay, so the Auralex stuff is effective down to about 200-250Hz, below that, you're going to have significant standing waves. This will be picked up on the microphone and displayed as a sound which is completely lacking in the high-end frequencies (thanks to excessive deadening) and boomy like crazy. In addition, certain low-mid frequencies will likely cancel out. What's worse, it won't be the same ones at all times and is dependent upon minute movements of your head and/or body.

Here's another scenario -

You're trying to record a drum set in a medium sized room. The room is maybe 20'x24' with 9' ceilings.

Without bass trapping but alll other acoustic needs taken care of (really not possible without proper trapping anyway, but that's a book's worth of info), your kick drum and likely floor tom will ring like mad in the room. You'll wonder why it is that your drums sound washed out and distant when it's an overabundance of lower frequency information muddying up the works. As those standing waves fold back over on themselves, you're going to get more cancellation as well as reinforcement.

For example, your 75Hz signal may be extremely boosted while your 100Hz is nearly non-existent (thanks to the fact that these two points on a wave would be at the mid-octave point anyway). When/If this happens, your kick will sound thuddy and dark with no chance of you recapturing the body sound of the drum.

Conversely, if the opposite frequencices were to be boosted/cut due to phasing issues, you'd have too much body to the drum but you'd always wonder why the drum never gets a full thud to it.

The larger the room, the less problematic the bass frequencies are. However, you have to get pretty large before they're not a problem at all.

A 60'x80' room wouldn't need that much trapping as it would need just a decent (but not excessive) amount of broadband absorption.

However, a 10'x12' room should be quite well damped with a decent amount of broad band absorption and a very healthy dose of corner trapping.

Think of it this way -

If you cut the highs with basic absorption (2" studio foam) but do nothing to the lows, your mix will sound dull and lifeless. However, on the other hand, if everything is absorbed equally or darned near it, things will be under control (reflection wise) and won't sound any brighter or darker than the original source.

Does this help?

J

Posted: Mon Apr 07, 2008 8:57 am

nope, none of that helps. i've actually done 20+ page papers on acoustics and countless recordings in countless locations; this isnt my "first step" into any of this territory. i'm just rethinking.

you guys are simply reiterating what we've all read and heard a million times. rethink it with me though.

standing waves will occur in nearly every room, sure, of course. and every location in every room will excited different modes. what i mean is that bass traps are to cut down the standing waves, of which there are more of, in corners. once your microphone is NOT in a corner (99.9% of the time), then who cares how many standing waves there are in that corner.

so Bob, your concluding point (#4) is that "the corner is the place where all the excitement is." my response is "dont put a mic in the corner".

Cucco, your post seems to hit closer at what i'm trying to provoke here. but again, the majority of standing waves is in the corner. once you're out of the corner, then each location in a given room will excite different modes. do you really think that bass trapping will cut down on all bass frequencies (Even outside of the bass trapped corner)? probably not, because no matter where you are, there will be standing waves!!

standing waves are inescapable. especially at low frequencies. we all agree on this point.

what i say is that instead of spending hundreds or thousands of dollars to catch low frequencies in corners - where you'd almost never place a mic anyways ..

instead of doing that, USE YOUR EARS and find the sweet spot in the room. put the mic there.

bass trapping is a myth because there will ALWAYS BE STANDING WAVES especially at lower frequencies.

thoughts??

Posted: Mon Apr 07, 2008 9:26 am

Dave-

Think of it this way. Suppose there are five different standing waves. Their peaks (where you need to put the trap if you are going to damp them) are scattered throughout the room. But there is one place where they all have a peak - the corner. (Neumann boundary conditions if you covered PDEs in your paper.) Put a trap in the corner and you damp all five.

Ever make harmonics on a guitar? By lightly touching the string you damp certain modes. You don't just damp them at the place you touch. You kill the entire mode. If you make a harmonic at the 5th fret, the string is still (has a node) at the 12th and halfway between the bridge and the 12th fret. Similarly, when you damp acoustic modes in the corner, they are damped throughout the room.

Otherwise, what you are saying is perfectly correct - that a corner is a bad place for a mic. But that doesn't mean the rest of the room is great if the waves are not damped. What you are saying is also "what we've read a million times" - that proper positioning of sound sources and mics within the room is crucial. That's true with bass traps or without. It's just a lot harder to do without bass traps.

Sorry, not a myth. Solid science. Lots of empirical evidence to back up rigorous mathematical theory that's been around for a hundred years.

Posted: Mon Apr 07, 2008 9:32 am

I will address this in a few, but in a session right now.

Dave, the biggest problem or mistake you're maiking is that you're assuming that a corner is where standing waves reside. This is not correct.

Corners are where modes propogate and are amplified and then sent back into the room.

Posted: Mon Apr 07, 2008 9:46 am

Not sure if these responses help Dave out, but I found it helpful. Thanks!

Posted: Mon Apr 07, 2008 11:15 am

I think Cucco and BobRogers are onto the answer...

As Bob said, you are correct that the corner is a bad place to put a mic. But it is not the only bad place to put a mic. When significant standing waves occur, almost everywhere is a bad place to put a mic (or your ears).

Sound does not accumulate in places, it is always propogating. What we perceive as "standing" waves are really not static at all - they are just an interesting synchronization of overlapping incident and reflected waves.

Low frequency sound does not "gather at the corners" as if it were drawn there by gravity (like spilled water would "gather at the lowest point").

Standing waves exist everywhere in the room, not just the corners. They are a problem because depending on where your head is (or your mic), you will hear anywhere from a node of zero to an antinode of almost twice the original amplitude [Edit: actually, this could be even more than twice the original if you allow for mutliple reflections. The 2x factor jsut represents the superposition of a single incident and single reflected wave.] Keep in mind that the original amplitude is what you want to hear for an accurate mix - it is the sound coming out of your monitors before any reflections or other interference.

Bass traps work because when the sound hits an interface (wall, corner), less of it is reflected and more is absorbed. "Trapping" is probably an unfortunate term, because some might think that you could then release that sound from the trap and listen to it again. But once it is absorbed, it is not really sound anymore - the sound energy is dispersed as heat energy.

By taking some of the sound energy out of the room, there is less to reflect and interfere with the pure original sound everywhere else - not just the corners.

Posted: Mon Apr 07, 2008 11:54 am

Alot of good info here but I think you have to look at bass traps for both recording and mixing purposes.

For recording they may not be required as such as you will tend not to place mics in the corner.

From a mixing point of view your track will end up too bass heavy when listening to it another sound system outside of your control room as you most likely will have boosted the lower frequencies to suit the room you mixed the track in.

My thoughts anyway...

Posted: Mon Apr 07, 2008 12:04 pm

tobacco_slammers wrote:

For recording they may not be required as such as you will tend not to place mics in the corner.

No, for recording they will be just as important. Again, it is not just the corners where the "bass lives". Standing waves are everywhere in the room. If you are using a mic to record a sound source in the room, then standing waves caused by reflections will interfere with the sound you are trying to capture, no matter where you put the mic! The exact effect of this interference, though, could mean either highly amplified (louder) or reduced (softer) frequencies depending on whether you move the mic an inch to the left or right. Broadband bass trapping reduces the amount of energy that is reflected, and minimizes/reduces all of the interference effects. The result is that you record a "truer" version of the original sound, not a version of it that has been processed so much by the room.

Posted: Mon Apr 07, 2008 12:04 pm

tobacco_slammers wrote:

For recording they may not be required as such as you will tend not to place mics in the corner.

No - this is not correct. If you had read what has been posted so far, you'll understand why.

It has NOTHING to do with placing your mics in the corner. Bas doesn't just congregate in the corners. It propogates from the corners into the rest of the room.

Posted: Mon Apr 07, 2008 12:14 pm

Cucco wrote:

....No - this is not correct. If you had read what has been posted so far, you'll understand why....

But he got to not listen to me for free. I have to go to class and talk to 35 kids who have paid thousands of dollars to not listen to me. Very Happy

Posted: Mon Apr 07, 2008 12:29 pm

Send him a bill, Bob! Smile

Posted: Mon Apr 07, 2008 1:46 pm

Ok- let me see if I can put a finer point on Rockstar's thought.
- Speakers put out a 100hz wave.
- This ripples through the room like a ripple in a pond.
- It hits the side and back walls. (and corners)
- It then begins to bounce back into the room, but now at different times depending on when it hit the other surfaces.
- EXCEPT for the corners. Here, the sound bounces repeatedly against the 3 close together surfaces, resulting in a buildup within the corner.

I'm guessing that this is his thought.
Try it with a ball- Hitting a wall, it bounces back into the room. Hit the corner, and it usually bounces off 2 to 3 of the surfaces there and falls to the floor.

Thinking of it in this way, I can see his point.
But I'm sure the science of acoustics is more complex than this?
Acoustics and Grounding are always meta-cerebral for me.

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