Gap behind trap vs. thicker trap?

[travelboy]

I've heard many times that a given bass trap is more effective when placing it not directly on the wall but leaving a gap behind it. That goes, however, for traps where you cannot change the thickness.

But what if you're building your own and you can choose whether to leave a gap or fill the gap with ADDITIONAL absorbant (resulting in a thicker trap)?

I basically have two options: 25cm trap + 10cm gap or 35cm trap + no gap. What will be more effective for absorbing low frequencies?

[mobius.media]

Thicker wins, no doubt, from the specs I've seen.

Gaps are just good for making the most of what you have.

[XAXAU]

was just about to ask the same thing.

does this apply for a corner trap too? instead of leaving a gap behind it you fill it all the way into the corner?

cheers...

[mobius.media]

Originally Posted by XAXAU

was just about to ask the same thing.

does this apply for a corner trap too? instead of leaving a gap behind it you fill it all the way into the corner?

cheers...

Absolutely.

[Ethan Winer]

Originally Posted by travelboy

I've heard many times that a given bass trap is more effective when placing it not directly on the wall but leaving a gap behind it.

What matters is the total thickness from the room surface to the front edge of the panel. If you're willing to give up four inches of space you could use a 4-inch panel with no gap, or a 2-inch panel spaced off 2 inches. That gives similar results, though the 4-inch panel absorbs a tiny more.

Originally Posted by XAXAU

does this apply for a corner trap too? instead of leaving a gap behind it you fill it all the way into the corner?

Panels straddling corners should be tight into the corner. There's already a built-in gap behind the panel. Or you can fill the corner solid for slightly better results.

--Ethan

[travelboy]

Great, thanks for all these helpful insights! I'll go for the thicker one My small room is nastier than I expected...

[foxfyr]

What they said above.

Put very simply...

Traps dissipate acoustical energy as heat.

Thus, the more velocity the wave exhibits, the greater the dissipation of energy.

The velocity of the wave at the incident wall surface is zero (think of tossing a ball straight up into the air, and at the moment of its maximum altitude, the upward velocity goes to zero, before reversing direction and falling with increased downward velocity).

Thus, the region closest to the wall is not as effective in dissipating the energy and velocity as the region further from the wall.

So, while the added mass will certainly increase the total dissipation of energy if filled in closest to the wall, the % increase in the results diminish.

Stated another way... you get the maximum return on investment with a spaced bass trap, although the total space filled with the trap would perform incrementally better, but at higher material costs.

[matt thomas]

you probably have your reasons, but you should consider the thicker trap WITH gap as well

narco

[travelboy]

I do have proper polyurethane foam bass traps in the wall corners already. It helps a lot, but I feel I need even more for the low end. Thus I am building my own bass trap that will cover the ceiling. The room is high enough to make it 35cm (1ft) and the rock wool does not make a significant difference in cost. So, if filling the whole thing may get me even slightly better results than leaving a gap, I have no reason NOT to fill it.

[bpape]

Before you concentrate all that material on the ceiling, I'd take a very close look at any specific issues you're having and address those appropriately.

Most height related frequency response issues are high enough in frequency that 4" with a couple inch gap is sufficient to deal with them. Decay time is a different matter. Just trying to address the most things in the room for the money. Almost always, you're better off spreading things around a bit for best efficiency rather than concentrating them in one place.

Bryan

[Weasel9992]

Originally Posted by foxfyr

What they said above.

Put very simply...

Traps dissipate acoustical energy as heat.

Thus, the more velocity the wave exhibits, the greater the dissipation of energy.

The velocity of the wave at the incident wall surface is zero (think of tossing a ball straight up into the air, and at the moment of its maximum altitude, the upward velocity goes to zero, before reversing direction and falling with increased downward velocity).

Thus, the region closest to the wall is not as effective in dissipating the energy and velocity as the region further from the wall.

So, while the added mass will certainly increase the total dissipation of energy if filled in closest to the wall, the % increase in the results diminish.

Stated another way... you get the maximum return on investment with a spaced bass trap, although the total space filled with the trap would perform incrementally better, but at higher material costs.

That was a great post. Very well explained!

Frank

[XAXAU]

Thanks for the help guys... Appreciated...

[jwl]

Another thing to point out with regard to trap thickness....

Coverage area matters a lot, too. If you have a given amount of material to build with, then I'd recommend a combination of 2" panels spaced 2" from the wall at reflection points, and 4" panels across as many corners as possible.

The rear wall should also get some attention, with 4" if not 6" panels.

If you do all this and still have additional material, then you can think about making the corner traps thicker.

Even though a fully-filled corner will outperform a thinner panel straddling the corner, the same amount of material spread out to a greater coverage area in the room often yields better performance still.

[spm_gl]

Simply increasing the distance to the wall is not the same as using a thicker trap. If it were, a curtain spaced away from the wall would be a bass trap. The thinner the material, the more frequency-selective it is, the greater the depth, the lower the effective frequency response goes. Basic physics, actually.

[Glenn Kuras]

Originally Posted by spm_gl

Simply increasing the distance to the wall is not the same as using a thicker trap. If it were, a curtain spaced away from the wall would be a bass trap. The thinner the material, the more frequency-selective it is, the greater the depth, the lower the effective frequency response goes. Basic physics, actually.

Agreed, there is a curtain thickness you want to start with.

Another thing to point out with regard to trap thickness....

Coverage area matters a lot, too. If you have a given amount of material to build with, then I'd recommend a combination of 2" panels spaced 2" from the wall at reflection points, and 4" panels across as many corners as possible.

The rear wall should also get some attention, with 4" if not 6" panels.

If you do all this and still have additional material, then you can think about making the corner traps thicker.

Even though a fully-filled corner will outperform a thinner panel straddling the corner, the same amount of material spread out to a greater coverage area in the room often yields better performance still.

This is a great point when trying to determine budget. I for one think that acoustics are about the lowest cost "tool" in the studio which will yield the highest impact on getting a professional sound, so I recommend not spending $2000 on monitors and $200 on acoustics.

[DanDan]

I am of the opinion that more is generally better. Therefore thicker is better. Unfortunately I have never performed comparitive tests. I will when a suitable opportunity presents itself.
For now I can only say:-
1 My intuition says more has to be better, in the case of depth of Bass Trapping.
(within reasonable Gas Flow limits)
2 I have achieved remarkable results at 34Hz using 34 inch wide Studiotips Superchunks.
3 This may be heresy but I am going there for the sake of teasing this out.
Fox- I am not at all convinced that consideration of wave velocity alone gives a full picture. By the way are we talking about wave velocity here, or particle velocity, or particle displacement?
Let's look at this from an observational, intuitive point of view. For example, generate a sine wave at a room mode frequency. As one moves towards a boundary the sound level increases dramatically. Surely material placed closer and closer to that boundary will affect the sound increasingly. It is hard for me to imagine that it wouldn't.
Another way of looking at it- Let's say the first axial mode, lenghtwise, of a typical room. Say 40Hz. Generate a sine wave at this frequency. At the front and back wall the sound level will be extremely high, with velocity at minimum. Conversely, at the room centre there will be virtually no sound, but velocity must be at it's peak.
If it were otherwise possible, would one put a Bass Trap there? Again, hard for me to imagine that it would work best there.

DD

[nycwil]

sorry to ask here, but how do i start a post? where do i click, etc.? thanks

[spm_gl]

We are talking particle velocity, because that is the energy we can actually absorb.
At 1/4 wavelength, the particle velocity is highest, at 1/2 wavelength it is zero again. So a porous absorber is most effective at depth = 1/4 wavelength.
Take 100Hz, WL=3,4m, most energy at 0.85m. But for 200Hz, 0.85 is half wavelength, so no particle displacement, no energy to dissipate, no absorption.
So if our absorber is too thin, placing it away from the wall makes a comb-filter.
Edit: This is, of course, for 90° incident angle.

[DanDan]

Spm, I am well aware of the classic understanding of such matters. However, to pursue my heresy, 'no particle displacement, no energy'- I just can't accept that. Energy does not go away, it goes to the other vector. Would you put your Bass trap at the room centre, hypothetically of course?
DD

[spm_gl]

Originally Posted by DanDan

'no particle displacement, no energy'
DD

That's what room modes are about. Excite an empty room at a modal frequency, and you'll find places that have no energy. Of course, this is very simplified, because of reflections, air movement (thermal and otherwise), etc. And we are talking about a tiny piece of the sound wave, not the wave as a whole.
So you want to place your bass trap as close to the max as possible. But that won't really work below 100Hz, that's why I use membrane resonators for low bass absorption.
Of course, you could put your trap in the middle of the room, but then it kind of gets in the way of other wavelengths, not to mention clients.

[DanDan]

I have a problem with that No Energy idea. Where did it go?
It think the Classic view of absorption in fibrous material is quite vague and undefined.
I reckon a nuanced understanding would be more to the point.
DD

[mikahanau]

I have a problem with that No Energy idea. Where did it go?

It`s very simple

Let a man pull at a row with 100 newton, and another aan at the other side of the rope also with 100N in the other direction. Will the rope move? No because the energy is canceled, There is maximum pressure, but no flow

It`s the same with soundwaves.
cheers
mika

[spm_gl]

At a null point of a standing wave, there never was any energy. At the maximum, there is lots of energy. We are talking particle movement here, kinetic energy of particles. At the null point, there is no movement, ergo no energy, E = 1/2 mv²
But this kinetic energy is what we convert to heat in a porous absorber.
We can get a lot deeper into this discussion (I'm having fun, hope you are too) once I get back to my office tomorrow, where my acoustics and physics books reside.

[Nordenstam]

The energy in sound waves manifests as both velocity and pressure. Those energy quantities are tradeable as long as the energy total is the same (given a lossless medium). The energy doesn't go away. It goes from kinetic to potential energy and vice versa.

Ears are pressure transducers. Microphones too. We only hear the pressure side of sound, not the velocity. Like we hear the effect of a certain voltage level, with the amperage being a product of the practical implementation.

Porous absorbers needs velocity in the air moving through it to do any work. They don't oppose pressure. That's why there is membrane absorbers!

Originally Posted by DanDan

It think the Classic view of absorption in fibrous material is quite vague and undefined.
I reckon a nuanced understanding would be more to the point.

This is it: Acoustic Absorbers and Diffusers: Theory, Design and Application by Trevor J. Cox and Peter D'Antonio (Spon Press 2004).

[spm_gl]

Well explained, thank you.

Originally Posted by Lupo

Porous absorbers needs velocity in the air moving through it to do any work. They don't oppose pressure. That's why there is membrane absorbers!

Just wondering, what about Helmholtz resonators? Do they react to pressure or velocity?

[Nordenstam]

Hi!

Good question! There's probably some subtle caveats to this, but generally speaking, it's the absorber inside the resonator that dampens sound and this works on velocity.

[spm_gl]

True, but does the resonator itself work on pressure or velocity? Damn, I should know this, did an experiment at varsity with helmholtz resonance. Let me think, it should actually be the pressure gradient, which causes the air in the neck to resonate.

[spm_gl]

Originally Posted by DanDan

It think the Classic view of absorption in fibrous material is quite vague and undefined.
I reckon a nuanced understanding would be more to the point.
DD

I have Mechel's "Formulas of Acoustics" here, if you want a very mathematic view on the subject. Useless for practical application though, unless you need an absorber that covers the 1123.64 to 3345.32 Hz range.

[Nordenstam]

Originally Posted by spm_gl

True, but does the resonator itself work on pressure or velocity? Damn, I should know this, did an experiment at varsity with helmholtz resonance. Let me think, it should actually be the pressure gradient, which causes the air in the neck to resonate.

The air rushes into the chamber with a high velocity. As the air reaches the boundaries in the chamber and have no where to go, the velocity(kinetic) energy is traded for pressure(potential) energy. This stored energy pushes the air back out of the chamber, trading back the potential energy for more kinetic energy. Inertia will make the resonator push "too much" air out of the cavity, creating an underpressure. This sucks air back into the chamber and the cycle repeats.



PS, for DanDan and others: this java applet shows how the pressure sones works with various resonant modes in a 3D box: Box Modes Simulation (from the ever so amazingly useful Math, Physics, and Engineering Applets page)

[DanDan]

There are a lot of absolute truths cited here. I really don't think the world is that simply absolute. I believe Porous absorbers do mess with and diminish pressure variations at audible frequencies, especially when pressure it is at its maximum, near the boundary. Velocity and pressure are interconnected. You cannot have a pressure variation without particle movement. I think it is illusory to talk about one without the other, e.g. 'no energy' etc.
If there is an air gap behind an absorbent panel, resonances can build up between the boundaries of that void. The BBC used cardboard dividers in the gap of their absorber modules to eliminate this. The fibre batts used in stud partitions can add greatly to the performance of the wall, again by preventing resonances from building up.
I think few will disagree that a full deep trap is likely to perform better than a panel plus gap. What is at issue here is how much better is the full trap and at what cost? Perhaps also how light can we go with the density in order to bring down cost? I am guessing that a 4 or 6 inch dense batt straddling the corner, with the gap filled with light cheap attic insulation is the best balance of cost and performance.
DD