Bass Traps (Room corners)

M

mjbee

New member
Hi.

I've been looking at some of the "guide plans" for studio builds on the SAE site, and I have some 3-D implementation questions! The more I think about it, the more confused I get!

The corner bass traps shown in the control room layouts are usually shown as a triangle that knocks the corner off the room.

Do I get this correctly? :-

1) They need to be a sealed, top to bottom, "Toblerone" shape ?

2) 2 faces (can be/usually are) the outer walls of the floating room, i.e. just box in a piece and seal it? Rather than building a triangular structure inside the corner. Does the trap go floor to ceiling, or only part of the way?

3) The insides are lined with? Or contain? (I've seen some lined with rockwool/fibreglass, others with fibre glass spaced off etc.)

4) The outside face: Is it good to leave this as hard material (gyp? plywood?) or should it be cloth covered, draped, padded?

5) Is the triangular shape important, or can you use an "M" shape (think the M-Audio logo) to make a bass trap around an existing brick pillar corner. Effectively making 2 connected triangular bass traps per corner?

6) As some "bass traps" seem to be open at the base or top, yet others seem to be totally sealed boxes, what should one be aiming for? If "ported", how big would the port need to be ...?

7) Is there are a way to "tune" these, i.e. mathematical models that will show what frequency range they trap?

I've looked at lots of pictures of the finished constructions, and I'm still not happy I understand what's going on there. Can someone provide a clue by four? ;) Or a site with some 3-D views of this stuff in detail?

Mike.
 
1. they need to be sealed. See explanation on how bass traps work, later this post.

2. Either way will work. Sealing - yes. Floor to ceiling - optional, depends on how much bass you need to cut.

3. Depends on the type of trap. If making a panel absorber, any material inside should either NOT be touching the face panel, or it should be BONDED to the panel. Any mass added to the panel will change (lower) the resonant frequency. For lower bass/more absorption, larger traps with sheets of hanging rigid fiberglas (703, Knauf) can be used.

4. The outside face can be a (relatively) thin (hard) panel, which someone like John will have to explain (haven't studied much on those yet) or a more porous (such as Owens Corning 703, or the Knauf equivalent) rigid fiberglas insulation board. The rigid fiberglas needs an open weave cloth cover to avoid shedding and improve appearance.

5. The triangular shape isn't mandatory - what it does, is widen the frequency range the trap works at. See explanation at end of this post.

6. Closed. When you start porting, you get into Helmholz calculations. Doable, but not as simple...

7. Tuning - check the SAE site under absorbers, midrange - There is a formula, AND an Excel spreadsheet for calculating slat absorbers.

For most of your questions, you need to go back to the SAE site

http://www.saecollege.de/reference_material/index.html

click on absorbers (left side panel), then click on the top tabs for high, mid, low freq and absorption chart.

The only thing I didn't see there was a basic explanation of HOW a trap works -

In a room, walls/floors/ceilings are referred to acoustically as "Boundaries" - each room has its own set of characteristic resonances, caused mainly by dimensions and modified by wall construction and other material properties.

Using a rectangular room as an example, there will be 3 types of resonances, caused by the distance between pairs of walls (axial modes) sets of 4 walls (tangential modes) (think 4-cushion bank shot, if you're a pool player) and modes involving all 6 surfaces of the room (oblique modes)

Axial modes are the strongest resonances, Tangential modes are half as strong as Axial, and Oblique modes are half as strong as Tangential - This is why some designers concentrate mainly on the Axial modes.

All these modes determine which frequencies are supported (resonate) more in a given room.

With any lower frequency (the main problem area in any room) it acts like a wave. This means that at any boundary (wall) since it can't go any further, it has to have a VELOCITY of ZERO, and therefore MAXIMUM Pressure, at the wall. At some point AWAY from the wall, depending on its frequency, the sound wave will have just the opposite - MINIMUM Pressure, but MAXIMUM Velocity.

If we place an acoustic RESISTANCE (Fiberglas insulation, for example) at the point where the wave VELOCITY is at MAXIMUM, and the sound can find no path AROUND this resistance, then the high velocity air will generate HEAT in its effort to pass through the insulation. Since (theoretically) energy cannot be destroyed, only CHANGED, what happens is that a portion of the bass energy is converted from SOUND energy into HEAT energy.

In order for this to be as efficient as possible, it is necessary for the Acoustic Resistance to be at the point where the velocity is greatest for the particular frequency we want to attenuate. An ENTIRE cycle of this wave will have maximum PRESSURE at the two ends (walls) and at the center of the room (assuming it's the first harmonic, or lowest resonant frequency) and it will have its maximum VELOCITY halfway between those three points. This means that max velocity will occur at 1/4 wavelength of the frequency we're trying to attenuate.

So, in John's example (SAE, absorption pages) of 50 hZ, where the wavelength is at nearly 22 feet, if you build a simple bass trap for that frequency the absorbtion element would need to be about 5.5 FEET from the wall! (1/4 of 22 feet)

This means that unless you use more sophisticated means, such as hangers, etc, you would need to start with a room that is about 12 feet longer in each dimension than the space you need, which would get kind of expensive.

If you want deeper explanations or more info, you should consider either buying a copy of F. Alton Everest's book, Master Handbook of Acoustics, (about $25 @ Amazon.com) or checking out these other links I recommend BOTH...

http://www.ethanwiner.com/basstrap.html

http://www.customaudio.freeserve.co.uk/techfrmset.htm

http://www.usg.com/Design_Solutions/2_3_8_tableofcontent.asp

http://www.geocities.com/jonrisch/a4.htm

Those links require cut and paste...

Hope I covered some of your questions, if John begs to differ with any of this, believe him, not me... Steve

Oh, almost forgot - the reason traps are normally used in corners: Bass buildup occurs at boundaries (walls) - the more boundaries, the more buildup. Junctions of wall and wall, or wall and ceiling, or wall and floor, have TWO boundaries - corners have three. Guess where the most buildup occurs? Now, guess where the best place for a trap would be? Now, if you guessed wrong, go stand in the corner - There, that's better... :=)
 
Steve,

Everytime you write an explaination I believe it becomes more and more clear in your head, and so you are better able to pass it on to all of us.

The teacher will always be the greatest learner.

Thanks

Eric
 
Great explanation, let me prod a little more ... if I may ?

knightfly said:
1. they need to be sealed. See explanation on how bass traps work, later this post.
Click to expand...


OK -- I was struggling with the idea of "sealed" and "ported" in the same concept :) Having read the comments on ported, I'll avoid that one!

2. Either way will work. Sealing - yes. Floor to ceiling - optional, depends on how much bass you need to cut.
Click to expand...

More bass by volume, as in more db of cut, not more Hz of range?

5. The triangular shape isn't mandatory - what it does, is widen the frequency range the trap works at. See explanation at end of this post.
Click to expand...

I hadn't thought of a bass trap in those terms, the changing depth between the wall and the surface acts on different lengths of wave. (Like a slot resonator). A light bulb of understanding just came on.

7. Tuning - check the SAE site under absorbers, midrange - There is a formula, AND an Excel spreadsheet for calculating slat absorbers.
Click to expand...

Which I thought didn't apply here, because this isn't a slat absorber ... I'll go and look again :

For most of your questions, you need to go back to the SAE site
The only thing I didn't see there was a basic explanation of HOW a trap works -
Click to expand...

I don't dispute that the information's there, it's just getting my head round bits of it that wasn't working.

Thanks,

Mike.
 
This is how I KILLED the corners in the front of my small 9x12 room. (front being the 12 ft.) I took 2'x4' ceiling tiles and wrapped them in fabric and stood them up diagonally across the corners and attached them with long course thread sheetrock screws. Since my ceiling is at an angle, I left a gap of about 2" at the top so they are not sealed whatsoever but my ears are never at ceiling level so who cares. I know this is not scientific or anything, I just had this material lying around so I thought I'd try it. I have no any bass build-up in the front corners but YMMV.
 
mjbee-

quote:
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7. Tuning - check the SAE site under absorbers, midrange - There is a formula, AND an Excel spreadsheet for calculating slat absorbers.

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Which I thought didn't apply here, because this isn't a slat absorber ... I'll go and look again :

--------------------------------------------------------------------------------

I'll take this one.

The way the trap works is that the surface (membrane) resonates and this movement absorbs the bass. Different masses (ie thicknesses) will resonate with different frequencies. If you use a more massive material (thicker or denser) it will absorb lower frequencies. The combination of depth of the cavity and mass of the surface (membrane) will determine the frequencies absorbed. I think a peice of 1/4 plywood sealed across the corners of your room with some fiberglass or rock wool (not touching the surface) should do it.

There is a formula for this in Everest's book. I don't have mine here at work, if anyone could share it with us I'd appreciate it.
 
The way the trap works is that the surface (membrane) resonates and this movement absorbs the bass. Different masses (ie thicknesses) will resonate with different frequencies.
Click to expand...

Eric - Steve - that is how I understand they work. I haven't been able to find any articles on how these traps work. I remember an article on it in a Recording Engineer Producer magazine back in the 70's but I haven't seen any articles since. The resonate frequency of the sheet of fibreboard (membrane) varied with the size of the sheet. So low frequencies required a large sheet whereas smaller sheets resonated at higher frequencies.

All the low frequency absorption in Hidley studios is achieved through the use of these hangers. You don't see them as they are behind the internal walls. He has cavities 4' - 6' deep full of hangers for this purpose.

As another explanation.....I live near a river where oyster growers farm. They put out the young oysters in hanging frames in rows similar to the hangers in a studio. I was watching the effect they had on waves as a boat went past. When the boat wash hit the hangers they moved back and forth and absorbed the energy of the wave in a similar fashion so the wave that hit the river bank was considerably reduced from the original wave from the boat.

cheers
john
 
John, that's the way I understood hangers to work, from reading Everest's book - the actual DEPTH of the bass trap determines the frequency, the hangers determine the amount of absorption - more hangers, more absorption. Makes sense, when you consider that in physics, air IS considered a liquid. The phenomenon you observed with the oyster hangers in water would be EXACTLY how the hangers in a bass trap would work.

On the PANEL type absorbers, Eric's right on the money - the resonant frequency of the panel determines the frequency that is cut - other than that, the loss is from essentially the same reasons - it takes energy at the resonant frequency to cause the panel to move, so THAT frequency of energy is what is DISSIPATED in the form of heat, as well as being trapped partially inside the panel absorber if it gets through.

It is good to remember, that if you want to get rid of a particular band of frequencies, you either have to let them ESCAPE thru a window (tuned or otherwise) or CONVERT them to a different form of (quieter) energy. (such as heat)

Another thing I try to keep in mind, is that although formulas are very useful for PREDICTING what a room will do (at least, a rectangular one), there is no substitute for HEARING what the room actually DOES. Things like the rigidity of framing, the flexibility of whatever panels are attached to that frame, the distance between framing members, all add up to various "traps" of their own -

In addition, there are variations in wall coverings/paints/texturings, etc, that will change the absorption at each and every frequency you care about - charts of absorption are fine, but as Everest points out, the exact same amount of absorptive material placed in various locations can have up to 20 dB difference in effect depending on that location - sooo, in the end you still have to use your ears, and preferably measuring tools, to VERIFY the response of the room.

In the end, it's all FM anyway, and sometimes you just need to turn it up to 11 and wail... Steve
 
bass trapps

I did something a little different than most. When i built by studio, I ran across some slabs of foam. (90 lb comm., 2" thick, 90" by 80". So this almost covers the whole wall from top to bottom. The slabs rounded the corners perfetlly, so no need for bass traps. great sound. I have about 7 or 8 left. The cost about 100.00 each, but i got them for 50. If you :(or anyone) are close to NC and are interested let me know. I'll make a great deal.
 
Ok I need further expanation about needing to be sealed. I thought that 4 inch thick panels of 703 could be propped up in the corner and function just fine. When you say sealed, do you mean these need to be caulked ot the walls or something?

The only need for a seal i have heard of is with the "pressure" traps that are cylindrical pieces of pipe insulation that theoretically need to be sealed so that the low frequency waves want to tget inside do to pressure difference, so they are supposed to suick in the bass, so to speak.

Can you smarts types explain the need for a seal for me with standard 703 panels and also wiegh in with the validity of the :pressure" trap?

Thanks very much!
Daav
 
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