Corner Trap Nonsense

[Tim Link]

There's more going on with sound in a room than just modes. Frequencies that aren't associated with a mode still bounce around. Measuring the clarity of the upper bass and lower midrange before and after treating the corners can explain a lot more than just looking at the frequency response, which isn't always greatly changed by treatment.
Amroc room mode calculator is a cool tool, but real rooms do structural things as well, so the actual results are not always what is predicted by models that don't include that factor.
The cornerless room thought experiment is interesting, and if you model waves bouncing around in a room it becomes obvious that just treating the corners alone is not optimal. It's generally a good starting point. When setting up my current room I did some clap tests while it was empty, and got a strong, buzzy flutter echo. When I started to bring things in I first just assembled one of the floor to ceiling speaker grill frames and set it in the corner. This is just wood but it broke up the shape of that corner. I tried the clap test again and could not hear the flutter echo after just disrupting that one corner. I had imagined the echo as bouncing back and forth between the parallel walls, but apparently it was a longer delay than that, involving sound traversing forward, until getting to the front corners of the room, and then traversing back toward me, bouncing back and forth multiple times before reaching my ears again.

 

[basscleaner]

I would like to say some words and sorry in advance, if these words were discussed before me. First of all, it is not correct to analyze LF behavior by modes only, because there are crossfiltering effects too by simple reflecting from walls. The second, a FR behavior or SPL distribution depends on effective LF center position of source(s) too and we always have a deal with mixing of p.1 and p.2. Third, corners are places, where we have mixing of peaks and dips and that's why there are sometimes more sensitive places for tube traps setting and no more. It's related to the room dimensions set as a rule. The fourthly, it relates to hard room boundaries, because acoustical finishing can shift modes for a while. You need always remember, that any room is the resonator for any sound source, even noise.

 

[Bjorn]

Room modes terminate in the corner unless there are windows or light wall. Meaning the resonances are strongest in corners. But a corner within acoustics is also where two surface meet each other.

So placing bass traps in corners makes sense. However, one needs to measure to be certain and small bass traps will not be efficient for low room modes due to large wavelengths.

P.S. I have followed Ron Sauro's research (basically what John Brandt shares) with interests for years. But I would take their comments with a grain of salt. I have seen other results on a consistent basis, thus some Ron's conclusions might very well be wrong.

 

[basscleaner]

Room modes terminate in the corner unless there are windows or light wall. Meaning the resonances are strongest in corners. But a corner within acoustics is also where two surface meet each other.

So placing bass traps in corners makes sense. However, one needs to measure to be certain and small bass traps will not be efficient for low room modes due to large wavelengths.

P.S. I have followed Ron Sauro's research (basically what John Brandt shares) with interests for years. But I would take their comments with a grain of salt. I have seen other results on a consistent basis, thus some Ron's conclusions might very well be wrong.

Allow to myself replay, that you may to construct and use your own "bass-trap" for LF resonances even for lower frequencies. Because they could be placed along your longitude wall briefly under your ceiling. There is not at all necessary to place it at corners. For instance, if you have resonant frequency at 34 Hz, than it will be possible to place tube trap 5 m along the 6 m wall under the ceiling. And it may be the only decision, although not so beautiful.

 

[Bjorn]

A tube trap is a silly design since it doesn't cover the corner properly and has a small diameter. It's basically a fancy commercial product for eye catching.

 

[Geert]

Because they could be placed along your longitude wall briefly under your ceiling.

Which is also a corner ("a place or angle where two sides or edges meet"), no?

For instance, if you have resonant frequency at 34 Hz, than it will be possible to place tube trap 5 m along the 6 m wall under the ceiling. And it may be the only decision, although not so beautiful.

A single tube trap for a 34Hz room mode. What size of tube trap do you have in mind, and do you have any measurements to show us how effective it is at these frequencies?

 

[Randyman...]

Isn't the thought that walls/corners/boundaries have high pressure and low velocity - and most fiberglass/fluffy absorbers work best with high velocity/low pressure? This is one reason spacing them off the wall allows far better effectiveness (a 4" fluffy absorber spaced 4" off the wall is not far from a full 8" thick absorber of the same density/flow resistance).

IIRC, a membrane or hemholz absorber/resonator would be most effective at a high-pressure/low velocity situation such as walls/corners - but their design and construction is far less forgiving than fairly idiot-proof fluffy broadband absorption.

That said - I did 24" OC703 super chunks floor-to-ceiling, various 4" wall panels spaced 4" off the walls, and 12" thick 4' x 8' hard-topped clouds in my studio. Listening Room has ~ 14"x14" OC703 soffits along most of the wall-to-ceiling perimeter (act as bass traps - OC703 is probably too dense for this thickness, but they work) and 4" panels on the walls and ceiling, all since wall/corner/soffit/ceiling placement just makes practical sense. But fluffy fiberglass would be more effective in a high-velocity area of the room - but having fluffy absorptive bass traps in the middle of your room (away from boundaries) is a non-starter for most...

 

[Bjorn]

Isn't the thought that walls/corners/boundaries have high pressure and low velocity - and most fiberglass/fluffy absorbers work best with high velocity/low pressure?

That is correct and is why porous marerial needs to be very thick. Pressure based traps don't need to be very thick to be effective.

This is one reason spacing them off the wall allows far better effectiveness (a 4" fluffy absorber spaced 4" off the wall is not far from a full 8" thick absorber of the same density/flow resistance).

But this isn't correct and is a myth. The reason is that spacing the material out this much leads to an outflow of energy, thus the absorption is lost.

IIRC, a membrane or hemholz absorber/resonator would be most effective at a high-pressure/low velocity situation such as walls/corners - but their design and construction is far less forgiving than fairly idiot-proof fluffy broadband absorption.

That said - I did 24" OC703 super chunks floor-to-ceiling, various 4" wall panels spaced 4" off the walls, and 12" thick 4' x 8' hard-topped clouds in my studio. Listening Room has ~ 14"x14" OC703 soffits along most of the wall-to-ceiling perimeter (act as bass traps - OC703 is probably too dense for this thickness, but they work) and 4" panels on the walls and ceiling, all since wall/corner/soffit/ceiling placement just makes practical sense. But fluffy fiberglass would be more effective in a high-velocity area of the room - but having fluffy absorptive bass traps in the middle of your room (away from boundaries) is a non-starter for most...

Placing the fluffy material in the middle of the room wouldn't work well.

 

[Esprit]

How to treat corners according to Genelec:
[…] A
Cut the room front corners at 30 degree angle using high-mass materials (concrete, bricks, multi layered gypsum board, etc). In case building materials have medium mass, be sure to fill the empty space behind these walls with mineral wool. […]

Calibration & Acoustics - Genelec.com

The acoustic environment of the listening space has a major influence on the sound quality. Walls, ceiling, and floor as well as large objects like mixing consoles, tables, equipment racks, and furniture cause reflections.

www.genelec.com

 

[Randyman...]

That is correct and is why porous marerial needs to be very thick. Pressure based traps don't need to be very thick to be effective.

But this isn't correct and is a myth. The reason is that spacing the material out this much leads to an outflow of energy, thus the absorption is lost.

But if pressure is high and velocity is low at the wall/corner - then simply moving it out where the velocity is higher and pressure is lower would naturally make it more effective, correct?

Can you shed light on the bolded part? "Outflow of energy". Isn't that what velocity absorbers do by converting the motion into heat? If the absorber is at a high-velocity location in the room - it would naturally turn more particle motion into heat.

Placing the fluffy material in the middle of the room wouldn't work well.

Agree - I should have said "area with highest particle velocity" - which is not along boundaries - and inevitably means eating up floor space.

 

[olieb]

But this isn't correct and is a myth. The reason is that spacing the material out this much leads to an outflow of energy, thus the absorption is lost.

Hm, what kind of energy is "flowing out" and where exactly is it going?
Absorption is the change of sound energy to heat.
In porous absorbers that happens when the air is moving inside the material. And that happens most, where the sound moves the air most.
EDIT:

Agree - I should have said "area with highest particle velocity" - which is not along boundaries - and inevitably means eating up floor space.

Yes, quarter of wavelength is enough "moving out", but in bass that can be the middle of the room. For high frequencies centimeters might be enough. That is the reason that porous stuff on the walls tends to absorb better in high frequency range.

 

[Geert]

Sound bounces back from the wall behind the absorber. If it needs to go through the absorber again without another path to escape, the absorber becomes twice as effective.

 

[Randyman...]

Sound bounces back from the wall behind the absorber. If it needs to go through the absorber again without another path to escape, the absorber becomes twice as effective.

It also passes through the absorber twice when flat against a wall

If spaced - it passes through twice, while also being in a higher-velocity portion of the wave. Ergo - more motion/velocity transferred to heat.

 

[Geert]

It also passes through the absorber twice when flat against a wall

That's my point, while you said "This is one reason spacing them off the wall allows far better effectiveness".

 

[Randyman...]

That's my point, while you said "This is one reason spacing them off the wall allows far better effectiveness".

Sound bounces back from the wall behind the absorber. If it needs to go through the absorber again without another path to escape, the absorber becomes twice as effective.

It's not that the long waveform does or doesn't have "a path to escape", it's that it is interfacing with the (long/bass) waveform at an area of higher velocity, where it is more effective at converting that energy into heat and therefore absorbing the acoustic energy.

 

[Bjorn]

But if pressure is high and velocity is low at the wall/corner - then simply moving it out where the velocity is higher and pressure is lower would naturally make it more effective, correct?

It doesn't work that way. First of all, a big airgap leads to a loss in absorption in parts of the frequency area. So using an airgap is really never the best option. Secondly, when the sound energy is not contained it will flow and not be absorbed. We know this now based on newer studies.

The best way to use porous material for bass trapping is using the correct material vs the thickness and flushed against the wall. Something done below in a room I designed where red graph is before and blue after bass trapping applied. As can be seen, porous can work really low when done correctly.

 

[Randyman...]

Agree flush with the wall is best if total thickness from the wall is identical. You still get additional absorption from filling in the "empty" area, but it's not as impactful due to lower velocity in that area. And yes - flow resistance and total thickness work as a system. The OC703 is technically too dense for the 14"x14" soffit traps I built to be ideally effective - but I didn't want to frame out for something less rigid/less dense. Compromise as we do...

 

[Wolf11Man]

There is simply too much money involved, to find an objective explanations why they don't work.
In my country people that offer them, only sell them as "made to order". This is a way to get around the mail order return and customer protection laws.
So no return, no refund. Also, they do not guarantee them to work, if you read the fine print. "Performance depends on individual room conditions."

While the idea seem logical to the simple mind that has no clue of the involved physics, the reality is different. The unwanted bass frequency does not magically enter the opening of such a trap and disappear inside. Once released by the speaker, the energy is in the whole room, not "concentrated" in a corner.

Also, if they would work, they would suck out not only some unwanted, single frequency, but a broad band of low frequency, some of it badly wanted in music reproduction, reducing your sub woofers output.
One more thing. Porous absorber like Rockwool do not work at low frequency, their main effect is in the upper range. So filling your corners with a heap of mineral wool will not reduce modes or standing waves.

Something completely different are tuned absorbers inside a loudspeaker. They reduce unwanted resonances, BEFORE these exit the speaker.

Snake oil sellers are very good in mixing facts with fiction, that is why so many of them moved into politics, giving simple, wrong answers to complicated questions.

 

[Geert]

A professional acoustician just showed evidence porous absorbers can work 2 posts earlier... And here's the effect of large corner absorbers in a studio I did:

'Large', that's the keyword.

 

[Mschn]

How large?

/Martin