# Why don't traps work?



## hrpschrd (Sep 16, 2008)

Okay, a little exaggerated title, but the question is real.

I have a room with dips at the 2nd and 3rd axial modes created by a sub on the long wall. The dips at 54Hz and 80Hz were completely unaffected by placing 2x2 RealTrap minis in the two floor tri-corners along that wall (see graph NOT changing by having or not having traps at either end or both). Actually the trace that is 3dB less at 54Hz is actually with no traps at either end.

I know these are good traps because I can listen to bass concentrated in another corner; put in trap and bass boom gone.

Why doesnt' this show up on the REW graphs in this case? I am missing something basic here.


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## Diskohouse (Jun 2, 2011)

Have you tried different sub-woofer locations? Have you try to adjust your crossover setting on your main speakers and see if it helps with the dip? Maybe, if you change your crossover point on the main speakers to let say 40 hz, it will help with the dip located at around 55hz. I know that others have mentioned this approach before. It does not hurt to try.....

I tried what I am suggesting out before........and it helped to clear up my dip. With that said, I was not really happy with the way it sounded with LFE, so I changed it back.


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## hrpschrd (Sep 16, 2008)

Actually, I have a plan to reduce those dips but my question is why did the traps not have any effect on the room modes?


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## JohnM (Apr 11, 2006)

hrpschrd said:


> I have a room with dips at the 2nd and 3rd axial modes created by a sub on the long wall.


You are making a bit of an assumption in attributing the dips to specific modes, and another in assuming that placing quite small treatments would have a significant effect at low frequencies. You would need to look at the response to higher frequencies and at the waterfalls to see what effect the treatments had.


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## hrpschrd (Sep 16, 2008)

As I suspected: I don't understand the basics properly.

I followed your reference to Paul Spencer's articles and the Harman Calculator. Putting in my measurements, it seems I should have the dips I do have. Is that incorrect? Why are higher frequencies more important? Those are higher modes and therefore smaller effects, right?

RealTraps, to mention just one, suggests as many traps as possible but if the effect of traps is additive I would expect to see SOME effect of two traps in the tri-corners (best spots). You seem to be saying that two traps show < 1dB improvement so I (and everyone else) needs dozens of traps?! (I ask as information not as criticism.)

I also want to check a basic assumption: I am using the mode calculator to find the best compromise in placing the listening position, sub, and speakers. I am avoiding dips and peaks in modes. Is that correct?

So many questions! Just trying to learn.


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## JohnM (Apr 11, 2006)

It is not that higher frequencies are more important, but rather that small objects such as the traps you mentioned can have a measurable effect at higher frequencies but are all but invisible for very low frequencies. Mode calculators are of limited use as few of us live in perfectly rectangular, empty rooms - a measurement will show you what the actual response is, and measuring at various places and with your speakers in various positions will show you how the response changes with position (tip: play back the REW Pink PN test signal and use the REW RTA with Rectangular window and no averaging to get a live view of the frequency response as you move the mic around).


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## SAC (Dec 3, 2009)

The problem lies in the fact that reality does not follow your very simplistic (and erroneous) assumptions.

May I suggest obtaining a copy of Acoustic Absorbers and Diffusers by D'Antnonio & Cox via an interlibrary loan.

Porous absorbers for use in low frequency mitigation have significant shortcomings. Not to mention that corners are *not* necessarily the the optimal place to damp a mode despite what you commonly here. The fact that many converge there does not insure that this is a point of maximum velocity (aside from the limitations of velocity based absorption being placed next to a boundary anyway...).

Also, it might help to play with the modal calculator here.

It is no more accurate than any other due to fundamental simplifications in the modeling calculations, but it does give you a MUCH more insightful idea of how modes actual are distributed in a 3space environment - as opposed to the 2D plot commonly displayed that corresponds to nothing in reality.


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## hrpschrd (Sep 16, 2008)

It's frustrating to get commentaries that are so conflicting. I recognize some people's comments as useless and others seem to be helpful.

I am a scientist. I experiment. I have learned by experiment that even small bass traps can absorb mid and low bass quite significantly. It could be coincidence but when I calculate mode dips and they show up predictably, I tend to think the calculator is not completely wrong, even though simplistic. I move the sub and the listening position and the results are what I predicted based on simplistic calculations.

The REW software is relatively easy to use and is reliable. I don't know anyone with a simple rectangular room but you have to start somewhere. Thanks for the tip JohnM on the REW RTA. Rather than theory I will go back to experimentation. 

Perhaps this site is no more useful than other audio sites that depend on golden ears.


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## localhost127 (Jan 2, 2011)

hrpschrd said:


> I am a scientist. I experiment. I have learned by experiment that even small bass traps can absorb mid and low bass quite significantly.


do you understand what and how "bass traps", in this case, LF porous (velocity-based) absorbers even work - and where they must be placed relative to rigid boundary in order to be effective at a given frequency or given bandwidth?

it may help to read the book referenced in post #7 of this thread. 



hrpschrd said:


> Rather than theory I will go back to experimentation.


blind experimentation without understanding the fundamental issues NOR the fundamental characteristics of porous absorbers and their proper design and placement in order to be effective in the modal region?


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## SAC (Dec 3, 2009)

Here is some information presented by P. D'Antonio, with a PhD in diffraction physics - he knows a 'bit' about the science of absorption and diffusion. I think that means he does not depend solely on reading forums or searching online for opinions regarding the understanding of absorption and bass traps in particular. He also owns a small company called RPG, Inc.

From Acoustic Absorbers & Diffusers by D'Antonio & Cox

1.3 Modal control in critical listening spaces

..._Particularly prominent modes are usually treated with bass absorption, often referred to as bass traps or bins. (It is not usually possible to treat this problem with diffusion because the sizes of the diffusers become prohibitively large, although Section 2.2.3 discusses a case where this has been done.) Porous absorbers are not usually used, as they would have to be extremely thick to provide significant bass absorption. Porous absorption is most effective when it is placed at a quarter wavelength from a room
boundary, where the particle velocity is maximum. For a 100 Hz tone, this would be roughly 1 m from the boundary.(At the 1/4 wavelength of the lowest frequency to be absorbed...) 

Placing porous absorbers directly on a room boundary, while the most practical, is not efficient, because the particle velocity at a boundary is zero. Too often, many people place porous absorption in corners of rooms thinking this will absorb sound, since all the modes have a ‘contribution’ in the corners. However,
while the modes have a maximum pressure in the corners, the particle velocity is very low and so the absorption is ineffective. For these reasons, resonant absorbers are preferred for treating low frequencies.

Resonant absorbers are mass spring systems with damping to provide absorption at the resonant frequency of the system. The mass might come in the form of a membrane made of plywood or mass-loaded vinyl. Alternatively, the vibrating air in the neck of a hole might form the mass, as is the case for a Helmholtz resonant absorber. The spring usually comes from an air cavity. Damping is most often provided by sound being forced through a porous resistive material: mineral wool, fibreglass or acoustic foam.

The problem with resonant absorbers is that they usually only provide a narrow bandwidth of absorption. To cover a wide bandwidth, a series of absorbers are required, each tuned to a different frequency range. Alternatively, double-layered absorbers can be used, but are expensive to construct. In recent years, a new resonant absorber has been constructed where the vibrating mass is a metal plate and the spring is formed from foam or polyester, and this provides absorption over a broader bandwidth._

_1.11 Summary
This chapter has outlined some absorber applications, and touched on some of the issues that will be important in future chapters. It has also introduced some necessary mathematical principles. The remaining chapters concerning absorption are as follows:
• Chapter 3 discusses measuring absorber properties from the microscopic to the macroscopic.
• Chapter 5 discusses the application, design and theoretical modelling of porous absorbers.
• Chapter 6 discusses the application, design and theoretical modelling of resonant absorption, especially Helmholtz and membrane devices.
• Chapter 7 sets out some miscellaneous absorbers, which did not obviously fit into Chapters 5 and 6. Seating in auditoria, turning Schroeder diffusers into absorbers, sonic crystals, trees and vegetation are considered.
• Chapter 11 discusses hybrid diffusers, and as these cause absorption, they are also an interesting absorber technology.
• Chapter 12 discusses how to use predictions and laboratory measurements of single absorbent properties (mainly absorption coefficients), in room predictions, including the role of absorption coefficients in geometric room acoustic models.
• Chapter 13 rounds off the work on absorbers by looking at active impedance technologies.
_

Its your choice.
We can give you general guidelines for bass traps if you like. Or you can simply serendipitously copy all of the advice on the web and 'experiment'. Of course part of the 'fun' of doing that is discovering how much of what so many says is correct, is actually wrong. But hey - I certainly wouldn't want to be accused of stifling anyone's 'creative' intuition.

And seeing as you are already complaining about that, it seemed prudent to refer you to THE definitive source on the subject. 

The current 'best of breed' porous corner trap is to build a 2foot by 2foot (or 2.5 foot) by ~34inch (or 38 inch) face covered with a minimum of 6mil thick plastic. The interior is divided into several sections with plastic orchard netting to minimize compression of fluffed 'pink fluffy' Fiberglass fill and the front netting stapled closed. This is then covered with >=6mil plastic and an aesthetic cloth face cover. These are typically needed in a _minimum _of all four vertical corners...

But hey, its your choice.

And don't ignore resonant absorbers...

Good luck.


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## hrpschrd (Sep 16, 2008)

Obviously I wouldn't have started this thread if my experiments were making perfect sense. In some cases the RealTraps I bought absorbed well and some they didn't. I thought there was a simple explanation. I guess I learned that lesson, didn't I? I beg your pardon for my naivete'.

Like most people I have an irregular room with no serious problems. I'll just tinker enough to eliminate large peaks and dips and call it a day. I will certainly no longer recommend room treatment to friends. They might come here.


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## SAC (Dec 3, 2009)

hrpschrd said:


> Obviously I wouldn't have started this thread if my experiments were making perfect sense. In some cases the RealTraps I bought absorbed well and some they didn't. I thought there was a simple explanation. I guess I learned that lesson, didn't I? I beg your pardon for my naivete'.
> 
> Like most people I have an irregular room with no serious problems. I'll just tinker enough to eliminate large peaks and dips and call it a day. I will certainly no longer recommend room treatment to friends. They might come here.


Porous velocity based traps can be useful to help mitigate modal peaks provided they are placed in regions of high velocity - which means both according to the modal spatial distribution in the room as well as away from boundaries where the velocity of the wave goes to zero.

Add to that the fact that they must be physically large - as wavelengths larger than their size merely diffract around them.

They are not going to mitigate modal nulls where by definition there is little if any spatial distribution or velocity.

Think about it for a while as you do nothing but offer insults and complaints and expect others to magically solve and compensate for that which you do not know.

I hope someone else can think of an answer that will satisfy all of your expectations.


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## jbjb (Jun 21, 2009)

hrpschrd said:


> Okay, a little exaggerated title, but the question is real.
> 
> I have a room with dips at the 2nd and 3rd axial modes created by a sub on the long wall. The dips at 54Hz and 80Hz were completely unaffected by placing 2x2 RealTrap minis in the two floor tri-corners along that wall (see graph NOT changing by having or not having traps at either end or both). Actually the trace that is 3dB less at 54Hz is actually with no traps at either end.
> 
> ...


I had the same experience as yours with my own DIY superchunk corner bass traps (a pair shoved into the corners). Using REW to graph the before and after FR revealed only marginal difference in the 80Hz region and up.

The point that you are missing (and I too at the time) is that low bass frequencies are very hard to trap. To do so effectively requires massive trapping. And the lower the freq the more massive traps are required (hence you often see the suggestion to install many traps). If you look at most bass traps I think they only publish the lab data down to 125Hz. Presumably the effect below that is negligible hence no data is published.

To deal with 80Hz or even your lower dip at 54Hz requires a trap many inches deep (and I reckon beyond the capabilities of your 2x2 RealTrap minis). I recall there was some calculator out there. In any case, generally not practical because it would eat up too much room/floor space. So it's a matter of adjusting expectations and knowing the limits of your traps.


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## hrpschrd (Sep 16, 2008)

Yes, jbjb, I have gleaned that from some of the threads. Sometimes the obvious is not immediately obvious until it is stated simply and that is exactly the answer to my original question. (Of course I can't explain the measurements on the RealTraps website which indicate good absorption of a small trap to 80Hz and below.)

Also obvious (from positive and negative comments) is that the answer to the bigger question of how one could correlate modes in a room with frequency spectrum is not a simple calculation from room dimensions. I like JohnM's suggestion of real-time "walking-about" with REW and a microphone. That should be fun and educational.


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