the truth about bass traps and recording rooms

Posted: Mon Apr 07, 2008 1:56 pm

rockstardave wrote:

if you dont want to pick up excessive bass frequencies , dont put a microphone in the corner !!
right?? or am i missing something?

See, that is a red flag. Don't put a mic in a corner Smile

I can't believe you guys fell for that.

Would it be possible to correct the topic?

It is and has been proven to be wrong or at least incorrect. Maybe just a question mark at the end.

Posted: Mon Apr 07, 2008 3:06 pm

natural wrote:

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.

One more time... sound doesn't "build up" anywhere! Just like those ripples in the pond don't all get together and collect on one part of the pond, leaving another part flat. The waves spread out in all directions.

The ball analogy doesn't really work, because the ball is a solid particle. Sound waves are not particles, they are waves. Particles and waves behave differently. (Let's not get into quantum mechanics, please...) Have you ever tried to catch a wave (and I don't mean with a surfboard, I mean literally catch one with your hand)? Have you ever thrown a ball around a corner?

Also, it is not the number of times that the wave bounces off something that is important (in fact, each reflection will lose some energy to absorption - even without bass traps - so a 3rd or 4th reflection would be considerably weaker than the original). Same with balls here - unless you live in a pinball machine, the more walls the ball bounces off of, the slower the ball will go.

Bottom line - the sound field generated by a source is already very complex on its own. Put it in a 3-dimensional enclosure, with other "stuff" in the room, and now you have a very complicated situation. The acoustic pressure is constantly changing over time and over space, partly due to the source and partly due to reflections and interference with the environment. Broadband bass traps help absorb some of the energy that would otherwise be reflected, thus "calming" the scene somewhat, and keeping the sound in the room closer to the original source.

Incidentally, high frequencies are also a factor, but by their nature they are more easily absorbed and controlled (a la a bit of foam on the walls, which the low frequencies would pass right through).

And yes, the science of acoustics is more complex than this. But a few solid fundamental concepts can go a long way toward understanding it. Thumbs Up

Posted: Mon Apr 07, 2008 3:44 pm

At this point I'm wondering (not doubting, but genuinely wondering) what sets the corners apart from any other point on a wall? I've seen a comparison in the room sound when traps were placed on walls vs. in corners on Ethan Winer's site (I'm pretty sure that's where it was), which basically proved that the corners are in fact a more effective place to trap bass. But I'm just not sure why that is.

I have a feeling the explanation is a little more complicated. Perhaps there's a diagram to illustrate this? Maybe that's a long shot...

Posted: Mon Apr 07, 2008 4:31 pm

danbronson wrote:

At this point I'm wondering (not doubting, but genuinely wondering) what sets the corners apart from any other point on a wall? I've seen a comparison in the room sound when traps were placed on walls vs. in corners on Ethan Winer's site (I'm pretty sure that's where it was), which basically proved that the corners are in fact a more effective place to trap bass. But I'm just not sure why that is....

How about this - some of the lowest frequency wave propagate in only one direction - between two opposite walls. You can damp them by putting traps on those wall, but it wouldn't do anything about a wave traveling in the other two directions. At a two-wall (or wall/ceiling, etc.) corner you can damp two directions. At a three-surface corner you get all three directions.

Posted: Mon Apr 07, 2008 5:01 pm

danbronson wrote:

At this point I'm wondering (not doubting, but genuinely wondering) what sets the corners apart from any other point on a wall?
I have a feeling the explanation is a little more complicated. Perhaps there's a diagram to illustrate this? Maybe that's a long shot...

I couldn't get it either Dan. But I found away to help me understand it better with a visual aid.

Go empty your bathtub of all toys and hand towels and fill your bathtub up with water, about three inches deep.

Let the water settle, still as the, well as something very smooth and not moving.

Get a small handful of change, coins don't ya know.

Drop a dime into the non moving water. Watch the ring as it hits the walls and as the wave "circle" continues to expand, the wave will come in contact with the corner and moves on until it hits another corner and another corner....

Before the wave even gets into the corner, if it hits a side wall, the wave will have already lapped itself and be heading back "outward".

The closer you throw the coin towards the corner the sooner the wave hits the wall and goes into the corner as the wave circle expands.

Start tossing coins at short intervals into the water. Eventually you will have waves going over waves and all of them will hit a wall and at some point will converge in the corner(s).

That is at least one reason to keep a mic out of a corner :)

Posted: Mon Apr 07, 2008 6:02 pm

OK, I'll play

The Truth about Bass Traps and Recording Rooms...

There IS a big confusion here... I addressed this probably 4 years ago... but what the heck, we can all due with a refresher. Several folks helped me get this straight in my feeble brane bucket; Rod, Eric Desart, Steve (Knightfly), Avare, Paul Woodlock, Ethan, Bob Golds and a couple of other notable authorities.

In an "ideal" situation, you have 2 rooms for the recording process... a tracking room and a control room.

The control room (CR) is an acoustically isolated room that is as flat in frequency response as possible with a minimal RT60 and free of acoustic artifacts. The CR should minimally impact its own sonic character. That is because the CR is the place to listen to the audio.. not listen to the room.

A tracking room (TR) is quite different. It is desirable that a TR have a coloration and sonic character of it's own. A relatively long RT60 is often sought. This is a performance area, so while an even frequency response is desired, it is sometimes not as critical to the goal of the recording process. Note that I said SOMETIMES, and is not defined as a room with excessive low frequency build up.

So where do bass traps come in?

Right here; Bass traps are areas dedicated to absorbing low frequencies. Duh.

Let's look at wave propagation. EVERY room has a series of standing waves. These are created by the simple fact that the barriers create a node at each barrier. At 1/2, 2x, 3x, 4x, (adfinitum) there are harmonic modes that will create different lengths of these standing waves.

These standing waves will interact with other standing waves... e.g. walls opposite will work with/against walls adjacent.

Corners have the tendency to be the places that these standing waves become additive in amplitude. It's nothing magic... just physics and math.

So, if you stick something non-reflective in the path of the additive process, you tend to interrupt that process and negate the build up. You don't HAVE to put traps in the corners, but they tend to be unused areas of rooms and they are the natural place for fundamental modes to develop, so why wouldn't you put the trapping there?

Posted: Mon Apr 07, 2008 6:12 pm

All your bass traps are belong to us ?

Posted: Mon Apr 07, 2008 6:14 pm

Thanks guys.

Posted: Mon Apr 07, 2008 6:16 pm

But you can buy them now at a reduced price!

Posted: Mon Apr 07, 2008 6:20 pm

Hell... you can build em' cheaper than you can buy em'

Posted: Mon Apr 07, 2008 6:33 pm

as far as a diagram is concerned, sound waves in a room behave like underwater waves in a fish tank. its tough to visualize, but thats how i always think of things.

i'd imagine there arent as many eddies and whirlpools with sound, since it moves via compression / rarefaction instead of a current.

or like an infinte number of rubber balls being shot in every direction simultaneously.

lets hear more thoughts, this is fun!

Posted: Mon Apr 07, 2008 6:34 pm

"or like an infinte number of rubber balls being shot in every direction simultaneously."
In a zero G environment, and where each ball didn't collide with itself.

The water analogy is a good one actually.
What're the fish?

Posted: Mon Apr 07, 2008 8:09 pm

This video shows you visually what's happening with standing waves.

Keep an eye out for what's happening in the CORNERS

Posted: Mon Apr 07, 2008 8:21 pm

natural wrote:

- 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.

Do me a huge favor.

Go stand about 5 feet from a corner and throw a ball into it. I want to see it fall to the floor. Actually, I want you to video it so that when it comes back at you at high speed and random direction... Wink

Also, bear in mind, the ball transfers a decent amount of energy into the wall which in turn excites the wall and sends new soundwaves through the room as well as transferring a bit of that vibration into heat energy. (Remember the law of energy conservation: energy can neither be created nor destroyed...)

Of course, the way the soundwaves work in the room are a little different - first, they hit the wall (corner) in a broader, more uniform dispersion versus the single pin-point of the ball. With this being the case, the more rigid the wall, the less energy is absorbed by the wall and then converted into heat and the more it is reflected. (See why foam and OC is a good trap versus a friggin piece of drywall?? Wink

)

One last point -

Please understand this over all else:

Sound waves do not congregate or collect in corners. They are amplified by the fact that 2 waves of equal wavelength and amplitude hit against adjacent surfaces and thus reflect twice back into the room - this adds up to LOUDER frequencies.

As has already been stated - low frequency standing waves exist in all places within the room, but are amplified by the corners due to the fact that there are two (to three) reflecting surfaces.

My comment regarding the size of the room - larger rooms are less likely to have problems - is based primarily on the fact that:
1 - There is more time/space/distance for moving air molecules to transfer to heat energy over time and space
2 - The fundamental frequency of the standing waves is lowered to the point where it has less of an impact on the critical audible band.

Lots to read here but it's time to go to bed.

Posted: Mon Apr 07, 2008 8:48 pm

MadMax wrote:

This video shows you visually what's happening with standing waves.

Keep an eye out for what's happening in the CORNERS

Just to explain a little more. The dark areas are where the plate is moving. The white areas are the nodes where the plate is not shaking and the salt stays put. As the frequency changes the white patterns become more complex. There are white spots at various points on the walls - but never at the corners. The corners are always black.

Good one, Max

UPDATE: One small warning. This is a plate and because of its stiffness it is modeled by a different differential equation than waves in the air. The standing waves you see have different shapes than acoustic waves in a room. However, because of the similarity in the boundary conditions, the corners have the same general behavior.

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