Corner trap depth vs efficiency

[atledreier]

Hi!

I sat playing with the distance/frequency quarterwave calculator on realtraps.com. Thought I'd see what the effective frequency of my DIY cornertraps are.

I'm thinking like this: My traps are 30cm deep from the surface to the wall at the deepest. The result is a 1/4 frequency of 287Hz. Does that mean my traps are only effective down to that? or should I input 30cm *4 to get the real lowest effective frequency? Doing that would make it 71Hz, which seems more reasonable to me.

[bpape]

First of all, you really can't ust that kind of calculation on corner traps. Corners are a completely different animal.

Plus, you don't divide by 4. You're only talking a few inches here. The frequency is just where 'optimal' absorbtion ends.

Bryan

[Lobotomy^]

I have heard that acoustic panel and airgap behind them combined have some effect of waves that have lenght up to 10 times as long. No significant effect on wavelengths longer than that. 30cm deep trap still have some effect on frequencys of 115Hz. This applies on panels that doesn't act like resonators.

In corners acoustic material acts different way. Sound energy in corners is in pressure and in the middle of room in form of moving air. As cornertrap prevents air movement, the pressure build up to surface of of trap while the middle of trap is still in underpressure. This pressure difference causes to air to move through trap surface and trap absorbs energy from this air movement. In a way cornertrap is some sort of resonator and effects on frequencys lower than 1/10 of wavelenght rule above suggests. This kind of behaviour however needs trap to be airtight everywhere else but not through acoustic material.

[atledreier]

So pulling my corner traps 6" out from the wall would make them effective at lower frequencies?

[Vaughan100]

Yes. Effectively what happens is that when you space the panel from the wall you are effectively slowing the sound wave down which then lowers the frequency of absorption.

The reason the wave cycles take longer to complete is because of frictional resistance that improves as the sound waves passes through the panel to the wall and then back through the panel.

--Regards,

[bpape]

Let's clarify a few things here.

First of all, a soft absorber like 703 or mineral wool is a velocity absorber, not a pressure absorber.

Second, the air behind it means zero - sorry. When you move it out, you're getting the leading edge of the panel farther from the hard surface where velocity is higher (so a % reduction is more significant) and you're getting closer to the optimal place to absorb a wave which is at it's quarter wavelength. THAT's what matters. You can actually take a very thin piece of material and hang it at the 1/4 wave of a specific frquency and filter that frequency pretty narrowly.

That said, the amount of material it must pass through also has an impact as that puts more of a specific wave inside the absorbtion at one time. The only 'pressure' component of what's happening here is the natural destructive wavefronts coming toward and away from the wall at the same time and colliding.

To get a true pressure absorber, you need a cavity that's sealed air tight. Yes, you can do tubes with caps on them and force a VERY VERY SMALL amount of additional pressure due to the resistance of air leaving the tube but it's really inconsequential as far as science can prove today.

Bryan

[Vaughan100]

Were you responding to me or someone else ?

--Regards,

[atledreier]

bpape wrote:

Second, the air behind it means zero - sorry. When you move it out, you're getting the leading edge of the panel farther from the hard surface where velocity is higher (so a % reduction is more significant) and you're getting closer to the optimal place to absorb a wave which is at it's quarter wavelength.

I think I understand that, to a degree. So what you are saying, as I understand it, is that moving it out from the wall will not make it more effective overall, meaning it will not absorb any more energy, but it will alter the BAND of absorbtion? So moving it out will make it absorb lower in frequency, but then less further up, so the toal absorbtion is constant?

Nasty sentence, but you get what I mean...

[bpape]

I was responding kind of to the whole thread - sorry. The idea that the air is doing something is just not correct. Seriously, 4" of 703 on the wall vs 2" with a 2" gap, the 4" will win all day long. But, the leading edge is in the same place. 4" with a 2" gap behind (leading edge at 6" from the wall) is even better.

Daniel,

It won't make it absorb more, it will make it absorb better, deeper down into the bass. However, there is a point of diminishing returns. General rule of thumb is that the max benefit you'll get and stay broadband instead of narrower filtering is for the gap behind to be the same as the thickness of the absorbtion.

Corners are a completely different animal. But the thoughts are the same. 4" straddling the corner is not as efficient as a solid chunk down in the lower frequencies. This is where the rub is. 4" straddling (assuming the same face width) will actually have it's leading edge farther from the corner. However, there is SO much more absorbtion behind it with a solid chunk that it ovecomes this limitation (not to mention that a solid 2' wide chunk face is approx 12" from the corner at 45 degrees and 17" from the corner along the wall edges).

Bryan

[atledreier]

Do you have ballpark figures on how much more effective a corner chunk would be if I were to extend the leading edge AND fill the gap behind it. Say I go from a 2' face to a 2'6" face. What would my potential lower absorbed frequency be?

[bpape]

It won't go a ton deeper but it will do some. It is also 25% more surface area - not just more depth. It's really more about getting more efficient absorbtion down low. So, instead of say a .5 at 50hz, you might go to a .65 or so (pure guessing here - just an example) rather than saying OK, now it's going to do 25hz instead of 30hz. Both will do something down that low, just a matter of how much.

The next logical step from a 17x17x24" solid triangle (8 pcs per 2x4 sheet) is a 24x24x34 triangle (4 pcs per sheet). This is almost 50% increase in facial surface area plus an additional 7" of depth - but at double the material cost and significantly more space out of the room.

Bryan