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Velocity based vs. pressure based absorbers Bass Traps
Old 31st October 2010
  #1
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Velocity based vs. pressure based absorbers

I would like to see a discussion on this and to see more clearly the benefits and drawbacks of different types of treatment. I know it's been discussed before in other threads, but it would be nice to have it gathered at one place.
Old 1st November 2010
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Old 1st November 2010
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Old 3rd November 2010
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There are many variables, and I definitely recommend more detailed discussions linked above.

However a quick rule of thumb: velocity absorbers absorb over a wider bandwidth, whereas pressure absorbers have a narrower bandwidth but absorb more within that bandwidth. To use an EQ analogy, pressure absorbers have a higher "Q" than velocity absorbers do.

Note that this is greatly oversimplified and there are a lot of factors, but it is useful to think of them in this way.
Old 3rd November 2010
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I'm aware it's been discussed before, but often with a different title and initially another intention of the threat. It's not easy to find that information by searching and I think it would be beneficial for many to have one all gather thread. And perhaps also discuss this more in detail then what has been the case so fare.

A few questions I have:

1. If I use pressure based absorbers rather then velocity absorbers in tri-corners to treat low frequencies. Does that imply that I will not be able to treat midbass and upper bass in that corner? And what will then the overall effect be compared to using porous materal?

2. What is the best way to combine the two? I understand that it depends on the room and speakers, but are there are general knowledge one can have as a starting point?

3. I see some are treating the room with only Helmholtz resonators (under 100 Hz) and diffusors (working from around 500 Hz). This should imply that the frequencies between approximately 100 Hz and 500 Hz are not dealt with. Isn't this a problem?

4. How vital is it to reduce the reverb time in all bass frequencies?
Old 3rd November 2010
  #6
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Family Guy

When I see that same question I see Cleveland going no nO NO NO NO......

1. If I use pressure based absorbers rather then velocity absorbers in tri-corners to treat low frequencies. Does that imply that I will not be able to treat midbass and upper bass in that corner? And what will then the overall effect be compared to using porous materal?

Pressure Absorbers are broadly or tightly tuned to one frequency only. But tuned is tuned. i.e. One centre frequency.

2. What is the best way to combine the two? I understand that it depends on the room and speakers, but are there are general knowledge one can have as a starting point?

Take a look a the design of the Primacoustic MaxTrap, it kinda ticks a couple of boxes. Ditto Corner Traps with Membranes on them, MiniTraps the MegaTraps, again broadly ticking various boxes.

3. I see some are treating the room with only Helmholtz resonators (under 100 Hz) and diffusors (working from around 500 Hz). This should imply that the frequencies between approximately 100 Hz and 500 Hz are not dealt with. Isn't this a problem?

Really? There are some many problems to pick from here. Simple answer, yes.

Back to 1 and 2, maybe you can have your cake and eat it. Broadband porous in Corners, floor to ceiling. Pressure in the middle of the walls tuned to that axial.

4. How vital is it to reduce the reverb time in all bass frequencies?

The BBC had a rule of thumb for an acceptable control room.
No third octave decay time should differ from it's neighbours by more than 10% over a spectrum of probably 100-4K. That would be a nice room IMHO.

DD
Old 3rd November 2010
  #7
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Quote:
Originally Posted by DanDan View Post
Pressure Absorbers are broadly or tightly tuned to one frequency only. But tuned is tuned. i.e. One centre frequency.
Velocity based vs. pressure based absorbers-pp.gif
Old 4th November 2010
  #8
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Wow

That is amazing Jens, and quite different to what we normally see.
What is that graph representing? Is it a three layered device or what?
Is it hypothetical or have you made one of these?
If real do you have test results?

By way of contrast here are some actual test results from the Master Handbook of Acoustics and a commercial product.
Velocity based vs. pressure based absorbers-picture-2.png
Velocity based vs. pressure based absorbers-picture-1.png


DD
Old 4th November 2010
  #9
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I would say that's definetly quite broadband. thumbsup
Jens:
Are you able to make something similar for frequencies below 100 Hz?

Also wondering what advantages and disadvantages a membrane panel has vs. a perforated panel.
The RPG Modex Plate is a membrane, right?
Old 4th November 2010
  #10
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Quote:
Back to 1 and 2, maybe you can have your cake and eat it. Broadband porous in Corners, floor to ceiling. Pressure in the middle of the walls tuned to that axial.
thumbsupthumbsup
Old 4th November 2010
  #11
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Quote:
Originally Posted by bwo View Post
I would say that's definetly quite broadband. thumbsup
Jens:
Are you able to make something similar for frequencies below 100 Hz?

Also wondering what advantages and disadvantages a membrane panel has vs. a perforated panel.
The normal use of perforated panel is broadband. It’s when you try to get to the lowest frequencies the devise gets a high Q (unless very deep). The pic above illustrates a standard single layer with 120mm wool and 120mm gap. To achieve a lower Q a low frequencies you need more depth but usually the lowest problem frequencies are very area specific so tuning to individual frequencies is usually not a problem. The problem with membrane absorbers is the difficulties predicting the resonant frequency.

Velocity based vs. pressure based absorbers-pp2.gif

/Jens
Old 4th November 2010
  #12
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Quote:
Originally Posted by bwo View Post
The RPG Modex Plate is a membrane, right?
No it's a compound baffle absorber heh

It tells you how they work on here:

RPG Diffusor Systems

The main difference between this type of absorber and a traditional panel/membrane absorber is the mounting of the plate.
RPG's states the plate behaves pistonically, just like a mass bouncing on a spring or a loudspeaker. Therefore it will have a resonant frequency, but a plate itself will have resonant frequencies (modes, 2D versions of room modes). The foam is therefore doing 3 things

1. Acting as a spring/damper that the plate bounces up and down on
2. Acting to damp plate resonances (modes)
3. Absorbing sound at the sides that diffract round the plate
Old 4th November 2010
  #13
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Thanks.
Is it possible to say anything about the performance of a Modex Plate compared to a perforated panel?

Do you have any pictures of your perforated panels Jens?
Old 4th November 2010
  #14
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Quote:
Originally Posted by bwo View Post
Is it possible to say anything about the performance of a Modex Plate compared to a perforated panel?
The Modex plate will be generally limited to low frequency, Jens has shown you can design a perforated panel that goes relatively high in frequency.

You could more easily DIY a perforated panel absorber than a Modex Plate type.
Old 4th November 2010
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Quote:
Originally Posted by bwo View Post
Thanks.
Is it possible to say anything about the performance of a Modex Plate compared to a perforated panel?

Do you have any pictures of your perforated panels Jens?
They are custom built for every project and the look is subject to tremendous variation determined by the clients need (or lack of) cosmetic performance. It can be anything from an unpainted or otherwise treated particleboard to a painted/coated MFD or commercially available panels. The problem (in Sweden at least) is to find ready made panels with low perforation percentage.


/Jens
Old 5th November 2010
  #16
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Quote:
Originally Posted by Jens Eklund View Post
They are custom built for every project and the look is subject to tremendous variation determined by the clients need (or lack of) cosmetic performance. It can be anything from an unpainted or otherwise treated particleboard to a painted/coated MFD or commercially available panels. The problem (in Sweden at least) is to find ready made panels with low perforation percentage.


/Jens
Next time I am over at the UK office I should hope a plane over to your city. We need to talk some day. heh
Old 5th November 2010
  #17
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Quote:
Originally Posted by Glenn Kuras View Post
Next time I am over at the UK office I should hope a plane over to your city. We need to talk some day. heh
Absolutely!
Old 5th November 2010
  #18
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DanDan's Avatar
Parade

I find Jens advocacy of perforation a valuable addition to the information around here. His treated room with actual test results was spectacular.
It was an incredible amount of treatment, but in fairness, an incredible result.
On balance I am quite sure that any of several other treatment methods would have achieved a similarly incredible result. I am sure the job could have been done with say RPG products alone. Or blue products. Many approaches are valid.

I believe we need to identify where and when to use each type of treatment, and how to actually do it, rather than one camp saying Broadband the other Porous, the other Membrane, then other saying ETC heh

Clearly pressure traps are useful when we are faced with an available surface suffering from a strong single frequency mode.

Perhaps these large back wall traps should be a combination. We should be prepared to lose a foot of back wall.

We have seen very confused efforts at helmholtz with little or poor result.
It would be great if Jens would write on how to make them function, with perhaps some more examples of actual successes. As is Jen's, G.E.'s work has been inspirational.

In the meantime however, presenting graphs of apples vs oranges, however impressive is illusory.

The spectacular graph above appears to be predicted figures for a very large device. Nearly 10 inches deep. It is not real but a prediction. It is based on just one Helm calculator. Try a google, see the plethora of 'is this wrong' Real test results are much more impressive IMHO.
But if we were to compare apples to oranges, using the same weighing scales....

My Porous calculator is not working. Perhaps someone would so us the favour of calculating the absorption graph of a porous absorber 120mm thick with a 120 mm airgap behind it.

Previous BBC figures for a 12 inch device, have shown 100% absorption from 49 Hz upwards.

Let's guess that our one would do 100% from 80Hz up.

If one were to lose 10 inches of space, why would one chose the perforated version?

I am afraid I cannot see a reason to cover up this high performer. If I did want some HF return, scattering, even diffusion, I would put slats on it. They look good too.

DD
Old 5th November 2010
  #19
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Quote:
Originally Posted by DanDan View Post
I find Jens advocacy of perforation a valuable addition to the information around here. His treated room with actual test results was spectacular.
It was an incredible amount of treatment, but in fairness, an incredible result.
On balance I am quite sure that any of several other treatment methods would have achieved a similarly incredible result. I am sure the job could have been done with say RPG products alone. Or blue products. Many approaches are valid.

I believe we need to identify where and when to use each type of treatment, and how to actually do it, rather than one camp saying Broadband the other Porous, the other Membrane, then other saying ETC heh

Clearly pressure traps are useful when we are faced with an available surface suffering from a strong single frequency mode.

Perhaps these large back wall traps should be a combination. We should be prepared to lose a foot of back wall.

We have seen very confused efforts at helmholtz with little or poor result.
It would be great if Jens would write on how to make them function, with perhaps some more examples of actual successes. As is Jen's, G.E.'s work has been inspirational.

In the meantime however, presenting graphs of apples vs oranges, however impressive is illusory.

The spectacular graph above appears to be predicted figures for a very large device. Nearly 10 inches deep. It is not real but a prediction. It is based on just one Helm calculator. Try a google, see the plethora of 'is this wrong' Real test results are much more impressive IMHO.
But if we were to compare apples to oranges, using the same weighing scales....

My Porous calculator is not working. Perhaps someone would so us the favour of calculating the absorption graph of a porous absorber 120mm thick with a 120 mm airgap behind it.

Previous BBC figures for a 12 inch device, have shown 100% absorption from 49 Hz upwards.

Let's guess that our one would do 100% from 80Hz up.

If one were to lose 10 inches of space, why would one chose the perforated version?

I am afraid I cannot see a reason to cover up this high performer. If I did want some HF return, scattering, even diffusion, I would put slats on it. They look good too.

DD
First of all, even if perforated (or slotted) panel didn’t offer any extension in the bass region compared to only porous, you still usually want to avoid excessive absorption in the higher range or it would otherwise be close to impossible to achieve a Haas kicker whatsoever and the decay times in the mid-high would be too low compared to the low mid - lows. Naturally the real bonus of perforation comes with the increase of performance in lower frequencies compared to porous only, if the open area is less than approx. 10%.

When you refer to “100% absorption from 49 Hz upwards”, which measuring technique was used? I once again want to stress the difference between reverberation chambers and impedance tube testing:

https://www.gearslutz.com/board/5964136-post20.html

 
Sincerely Jens Eklund
Old 5th November 2010
  #20
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Haas

The Haas Kicker is not popular with everyone. Jay did an interesting post on that, a search should easily find it.
I could answer all the rest with see Q4Avare. It's all there to be accepted or challenged.

However.

The 12 inch full range from 50Hz thing I gleaned from chat with Andre. He has referred to it many times. It is from the BBC. I have no reason to doubt it. I am sure the BBC are well aware of incidence issues. They used these devices on boundaries lets face it ;-)
Speaking of which, the difference between normal and random incidence results can easily be overstated. Most acoustic trap manufacturers use the decay based test methods for good reason. Many of the know what they are doing I have found. Particularly when one takes in long bass wavelengths, tangentials and obliques. I have no doubt whatsoever that a tuned pressure device is really the only way to deal with those long strong primary axials. If that is your prime or only concern. I would be tempted, as they are normally the biggest things on the graph by far.
However I do believe that corner absorbers make a lot of sense as a goto. They have unarguable benefits unique to them. Each trap covers two axials for a start. Furthermore they cover the harmonics of those axials. They are also guaranteed to work irrespective of construction skills. We have recently seen struggles and failure to gain any result from a perforated panel construction attempt. They are not easy to get right. Normal tuned devices are not at all broadband, tuned DOH!. Your exception is very interesting, but the BBC tests are very real, as is the Modex Corner graph beneath.

DD
Old 5th November 2010
  #21
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Quote:
Originally Posted by DanDan View Post
I believe we need to identify where and when to use each type of treatment, and how to actually do it, rather than one camp saying Broadband the other Porous, the other Membrane, then other saying ETC heh
+1. heh thumbsup
We have to achieve balance.

Andre
Old 6th November 2010
  #22
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What I’m saying is that you can extend the efficiency in the low frequency region a of porous absorber by adding a perforated (or slotted) surface to it. This effect is well documented:

Velocity based vs. pressure based absorbers-p211-aaad-se.gif
p211 in AAaD, se.

The advantage over porous only (100% open area) is obvious if you try to absorb low frequencies. If you are concerned with tuning it too low (high Q) then don’t, but even if relatively open (perforation percentage), you’ll still get the extended efficiency at lower frequencies and still absorb the low mids. Naturally you want to use porous only (or perhaps a membrane/perforated panel covered with porous) at geometric reflection points but at areas that doesn’t create specular reflections back to the sweet spot, you should be careful with porous if you want to archive an even decay through the entire range. Some people like dead rooms and in this case, a porous only corner trap could make sense if one can make them very deep.

Testing laboratories do know what they are doing and they usually state the limits and how people should interpret the results:

http://www.primacoustic.com/pdf/MaxT...0ExtFreq_E.pdf

Understanding the difference between different testing methods and the application of devices in non-diffuse field is important.
Old 6th November 2010
  #23
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LPF

Quote:
What I’m saying is that you can extend the efficiency in the low frequency region a of porous absorber by adding a perforated (or slotted) surface to it. This effect is well documented:
I am not sure that is a valid statement. One could regard membranes, slats, and other perfs as blockages. They bounce back the HF, leaving the LF absorption alone. A low pass filter, working by loss not enhancement.
There is also of course a slight resonant boost, but by the time this is damped to get a reasonably Q (if that is what is wanted) I really don't think 'add' is a fair description. I regard these devices part Low Pass Filter and part damped resonant soak. When allowed to be quite resonant and high Q, as in the Modex above, I can see great power, although it is all concentrated on the one frequency. Tuned. If that frequency is the big one, (and 60-80 very often is) the Modex should be superb, if you have enough of them.

It would be interesting to run the numbers I requested above.
120 fibre plus 120 gap. Then get an average absorption coefficient across the third octaves from say 100-4K for both devices. I think we will find a that partially blocked devices work by loss.

If I may highlight something which I think Jens has mentioned a couple of times. Deep broadbands will take out more as the frequency rises. This is fine for HF IMHO, I like the lack of comb filtering and the spectacular stereo imaging. However the loss of low mids bothers me.
Rooms trapped with the only simple broadband method sound very clear. I almost always turn the tweeters down. Coincidentally this achieves the classic listening curve espoused by B&K. A slanted line going from +3 at 100 sloping to -3 at 10K. I digress. I believe perfs and polys can help redress the balance, maintaining some warmth. Just an opinion, not tested by me yet.

DD
Old 6th November 2010
  #24
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Quote:
Originally Posted by DanDan View Post
The Haas Kicker is not popular with everyone. Jay did an interesting post on that, a search should easily find it.
First of all, the ETC is not a "treatment", which may indicate one 'problem'.

Secondly, Jay never addressed a problem or disagreement regarding a "Haas kicker" - or the termination of the ISD. The issue regarded an early integral discussion - which was indeed an active and fundamental aspect of the very research by those actually developing the applied models regarding the implementation of the concept itself - into whether a specular return, a diffuse return, or a specular return embedded within a diffuse return was optimal. This was an issue born largely as a result of early implementations of the concept prior to availability of high quality Schroeder based diffusion when polys were still the order of the day. The difference being a qualitative issue and not a substantive issue, as proper termination by any of the alternative means is superior to its absence; with a diffuse return qualitatively superior to a purely specular return.

(All of which is interesting as MOST have not, until very recently, even realized that an ISD or RFZ by definition included termination. In fact, only a small handful of persons here have even attempted to implement an ISD/RFZ, and the myriad tutorials make no reference to the integral concept despite the erroneous colloquial/slang use of the term RFZ. Considering most are not even aware of the need for termination, one doubts seriously that those unaware are preoccupied regarding variations in its early implementation - ALL of which are still superior to the lack of termination!)

Expressing a preference for a particular quality of return is trivial with respect to the overall concept - in addition to being but a footnote in the development of the applied concept as the 'issue' was effectively settled some 25 years ago (notwithstanding the fact that many still not familiar with the concept even now). But I guess we could similarly choose to debate the viability of the internal combustion engine or of electrical lighting based upon early issues of what material was optimal for the use in constructing a spark plug or incandescent light filament! A carbon filament indeed oxidizes a bit more quickly than a tungsten filament, but its still better than fumbling about in the dark.

The value of termination of the ISD itself has never been in dispute. The value of the termination has never been disputed since Leo Beranek first documented the ITD characteristics of large acoustical space concert halls. Nor since the initial research by Henry (and Fay-Hall, Haas, et.al.) into the psycho-acoustics of imaging, localization and intelligibility.

And the"Haas kicker" is simply a general term that can be used in context to refer to either the means by which energy is redirected back into the listening region or a term describing the termination energy itself that effectively terminates the ISD. Thus it is a misuse of the reference to take a very small qualitative reference to imply an objection with the overall concept. Disliking a particular color of, say, an automobile does not imply an objection to the concept of motorized transport. And in this case, because someone wants to react to a term, does not negate the larger concept to which it applies.

Thus far, the only unsubstantiated 'objection' to termination of the ISD come from the effectively anechoic dead room crowd who choose to selectively ignore the established psycho-acoustical value that comes with the termination of the ITD/ISD.
Old 6th November 2010
  #25
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Quote:
Originally Posted by DanDan View Post
I am not sure that is a valid statement. One could regard membranes, slats, and other perfs as blockages. They bounce back the HF, leaving the LF absorption alone. A low pass filter, working by loss not enhancement.
Don't think of it this way, it'll lead you the wrong way. I'll explain. You can see that some resonant component is operating when comparing the 100% open area to the 6% open area here:


If the perforation was merely blocking HF then the performance of 6% at 500Hz would be that of the 100% open area at 500Hz. The graph shows that it is much greater than that. This is due the the resonant component created by the perforated panel. This trend can be seen to be true of all levels of perforation below 100%.
Old 6th November 2010
  #26
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Joshing

The reference to ETC was of course a joke. Clue- little grinning Avatar...DOH!

Jay-
Quote:
One of the things that is less common these days is the traditional Haas kickers. I don't miss them.
I referred to that in order to gently suggest that Haas Kickers etc. were not exactly on topic. That would still be my view.

DD
Old 6th November 2010
  #27
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Quote:
Originally Posted by DanDan View Post
I am not sure that is a valid statement.
Was just going to type a reply to this but Dange has already explained.
Old 6th November 2010
  #28
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Good point

Well illustrated Dange. The application of the LPF has indeed shifted the action down in frequency. Point taken, a nice nuance, and a very useful behaviour for those low primary axials as I keep saying. However, let me restate my 'lossy' view using your diagram. Look at the area under the curve for 100% perf.
Compare that with the area 6% narrow peaked curve. Clearly the open device has vastly more absorption.
DD
Old 6th November 2010
  #29
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I have been trying to work out if it is just only the 'closed area' (i.e the solid bits between holes) that reflects the HF or if it's the reactance of the helmholtz/resonant hole itself too. I'll try and work out a way to validate it...possibly looking at hole size to compared to wavelength.

It boils down to, can a hole with an enclosed volume behind reflect a wavelength that would normal pass through it if the enclosed volume was not behind?
Old 7th November 2010
  #30
Quote:
Originally Posted by DanDan View Post
Jay-
Quote:
One of the things that is less common these days is the traditional Haas kickers. I don't miss them.
I referred to that in order to gently suggest that Haas Kickers etc. were not exactly on topic. That would still be my view.

DD
Haas kickers are a bit off topic, but I did indeed criticize the early type of Haas kicker, which I believe is what most envision when hearing the term, and what I think had been described in the post(s) I was responding to in that particular thread. It is also true that I was not suggesting the omission of termination in that model, but rather what kind of termination was preferred and how one may achieve that. But yes, I do not miss those early kickers.

I'll throw in a quick plug for the use of perforated panels as one element in a complete room plan. I have had great success with them. One excellent application is to use the fairly large, pre-existing cavity of a window when doing a residential retrofit. Put some ply inside the glass to create a back, half-fill the cavity, do a little easy math and drill some holes and put a perf-panel in front.

It's also worth noting that one can build compound treatments where there is some porous absorption in front (but not touching) a pressure-based trap. If you need a bit of high/mid control, the low frequency energy will go right through an inch of 703 and still interact with the pressure trap behind. There are some variables here regarding implementation, but these are possible.
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