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DanDan
#1
29th July 2010
29th July 2010
#1
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Q 4 Avare

Hi Andre, I thought it best to ask this in public. I hope you don't mind or even better appreciate why I chose this.

Could you please give my logic a health check here and finally answer the punchline question.

Regular medium density semi rigid batt. Say 703, no FRK.

The LF performance changes when distance from the boundary is introduced.
There is a slight peak of absorption, accompanied by a slight lack of overall linearity further up in frequency. The frequency of this peak of course changes with the distance from the boundary.

Some have said that an airgap equal to the panel thickness is optimum.

This suggests that any increase of gap beyond panel equality starts to diminish the LF improvement/peak.

Does it?

DD
#2
29th July 2010
29th July 2010
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Porous Absorber Calculator

DanDan
#3
29th July 2010
29th July 2010
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Joined: Aug 2003
Location: Cork Ireland
Posts: 11,094

Mac

Thanks John. You are more than welcome, see further questions below.
I directed to Andre to remind him that his Gas Flow Resistivity piece will be needed shortly for my website.....

Unfortunately those Calcs don't seem to work on my Excel on Mac OSX. Most things do. I will give it another spin tomorrow.

I have a hunch that gaps greater than the panel thickness simply lower the frequency of the absorption peak. Meaning that equal gap thing is a myth or is it?
I guess there must be a limit to this phenomenon, i.e when the gap becomes very large. But what is very large? A suspended ceiling, thin panel, huge gap, large area, seems to work well.

Does that calculator factor in when the phenomenon begins to fade for whatever reason? e.g. When the distance from the boundary becomes say larger then the other panel dimensions.

DD

Last edited by DanDan; 29th July 2010 at 02:48 AM.. Reason: Tysop ect
#4
29th July 2010
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Equal gap is for best linearity. Like an electronics filter, the higher the Q, the more 'ringing' you get. So, the larger the space behind the absorbent, the higher the Q. It drops the frequency of resonance but introduces ringing which translates to peaks and dips in the absorption response.

- John
#5
29th July 2010
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Thanks Dan. You hit the nail on the head, or rather identified exactly the point that has been holding me back on the article about gas flow resistivity. Specifically the effect of gaps between porous absorbers and reflective surfaces. I have not been able to determine the best manner in which to explain the effect and that gaps greater than the thickness of the porous material provide flat absorption.

There. I wrote the important thing. Now I just thave to expain it in an effective manner.
In the following explanation I will try to work from the basics in a way that other readers can hopefully understand the priniciples involved also.

The classic way that the effect of gaps is explained is by use of graphs of thin absorbent material spaced away from a wall. The graphs show high values of α where the distance from the wall is 1/4 wavelength and 0 when the wavelength is 1/2. In other words the porous material is effective only where the particle velocity is high. These graphs are appropriate for thin material. The porous materials that we consider when discussing the use of gaps are not thin at the wavelengths significant to us. Therefore the graph is not accurate for our use of gapping!

Gapping is used to lower the effective frequencies of the sound absorption. It is usefull to start with the effect of thickness of homogenous porus material against a solid surface. This the mounting of material used in the reporting of the absorption of materials as used traditionally in studios. With typical porous material, using 703 for the example, at 4" thickness, α is 1 down to ~250 Hz and usually considered effective down to ~125 Hz. At 250 Hz the wavelength is 4.52 feet. Dividing the thickness of the material by the wavelength (.333/4.52)gives us a ratio of .0737, or ~7%. So the thickness of a porus absorber has to be at least 7% of the wavelength for flat absorption. If we consider 703 material absorption at 125HZ to be practically 1, this gives a ratio of 3.5%.

Remember, the previous paragraph deals porous material against a solid surface. This is the area where the particle velocity is lowest in the sound wave. The efectiveness of a non thin absorber is not significantly reduced when located in the relatively low region of particle velocity.

Having shown what the depth of a porous absorber has to be in order to be effective, this leaves the question of the required material depth to gap ratio for effective absorption. This is not as clear as the overall depth calculation due the physics involved and some other non-intuitive factors. The usual belief is that the path of sound sound through an absorber is straight through the material. This is also named the normal incidence. However when sound impinges on an absorber at a non normal angle, the path is greater. The significance of this is a reduction, up to a complete removal, of the point on the thin absorber graph where no absorption occurrs.

When sound travels in air, it doing so in an isothermal manner. That is that at the points where the presuure increases, and the temperature (the combined gas law), the additional heat remains that area. That is, there is movement of the energy in the sound wave. In porous material, the material conducts the heat away from the ares of high temperature to areas of lower temperature.

This is called adiabatic. The ratio of the square root of specific heats of air for constant volume vs constant pressure is the same as the ratio of the speeds of sound in air when traveling isothermally vs. adiabatically. The effect is that that porous absorbers behave with effective thicknesses ~120% greater than the physical depth. The practical result is that for acoustic matching to the peak velocity of a sound wave and covering the full 1/4 cycle, the depth of the gap is 1.2 times the depth of the depth of material.

There is of course the variable sound path length from the various angles of incidence also. So the true effective depth for a gapped porous absorber is more than 1.2 times the material thickness. This leads to the question of how much more? In acoustics, we have the ultimate arbiter of test data. Gapped porous absorbers are used in thousands of spaces with gap to depth ratios up to 20:1. These absorber systems are called acoustic ceilings. This sort of mounting is calld E-405. It consists of absorbent material suspended 405 mm (16") away from a solid surface. The acoustic tiles are as thin as ~20 mm(3/4"). Studying test data on such mounted materials does not show any dip at the 1/2 wavelength frequency implied by the thin absorber graph.

An example of a purpose built absorber using a 2:1 gapping ratio is in the Heinieken Music Hall, construction details in fig 5 in Acoustics for Large Scale Indoor Pop Events. There is an internationally recognized studio designer, who in the process of designing a wolrd class facility did testing on gaps and used gap to material ratios of up 2.2:1. He is a rather quiet person regarding his work and out of respect for him, I am not disclosing his name or the studio.

The point to answer your question, is no, 1:1 is not a limiting ratio for gap to absorbent material ratios. Ratios up to 20:1 are used regularly in non-critical acoustic spaces, and ~2:1 for critical acoustic spaces, including studios.

I hope this helps with the ostensible question. Reaction to this post and thread and this post will help me compose the major piece on soound absorption and porous absorbers. You got me started in getting past the stumbling block Dan.

Well spaced,
Andre
#6
30th July 2010
30th July 2010
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(thousands of people chanting)

Book, book, book, book!
thumbsup
DanDan
#7
30th July 2010
30th July 2010
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Much Thanks

Andre, thank you very much. John too, both are useful views. I will reinstall Office to see if I can get that Excel thing going.

Book indeed. Maybe this post should be a Sticky? However in the meantime, I have offered to host such on my coming shortly irishacoustics.com
I plan to have a place for these in-depth views. They may be too rich for the context here, but I believe it would be really useful to be able to quote a link.
Lupo is writing a piece on the practical use of ETC also. Aren't you Andreas...?
Pressure .
DD
#8
30th July 2010
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Quote:
Originally Posted by jhbrandt
Equal gap is for best linearity. Like an electronics filter, the higher the Q, the more 'ringing' you get. So, the larger the space behind the absorbent, the higher the Q. It drops the frequency of resonance but introduces ringing which translates to peaks and dips in the absorption response
It is getting into the physics even more . The appropriate analogy is to a transmission line and its termination. The total depth of the absorber actiing as the controling factor of the knee frequency of a high pass filter.

For those somewhat confused with basic acoustics, the above should remove any doubt the possibility of confusion.

Well obfuscating,
Andre
#9
30th July 2010
30th July 2010
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Quote:
Originally Posted by DanDan
Andre, thank you very much. John too, both are useful views.
Views? tutt
Are we also looking for a consensus on the sum of 2 and 2?
It is physics, not opinion.
Seriously, you got me over the hump, and the piece on gas flow resisitivity, gapping, and the meaning of the universe is iminent. thumbsup

Iconically ,
Andre
DanDan
#10
30th July 2010
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Brill

Looking forward to that.
Should have said both posts are useful. The Calculator that John linked to looks very comprehensive, and useful. Sometimes things get established by repetition. e.g. this equal gap thing, or the drive both speakers when measuring etc. etc. Life is rarely that simple!
So, again, thanks.

DD
#11
30th July 2010
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Quote:
Originally Posted by DanDan
Looking forward to that.
Should have said both posts are useful. The Calculator that John linked to looks very comprehensive, and useful. Sometimes things get established by repetition. e.g. this equal gap thing, or the drive both speakers when measuring etc. etc. Life is rarely that simple!
So, again, thanks.
You are welcome.

I am suprised at the lack of posts. I thought there would be many, considering that it is against common folklore. Perhaps epople are too busy with soundproofing foam.

Andre
#12
30th July 2010
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Quote:
Originally Posted by avare
I am suprised at the lack of posts. I thought there would be many, considering that it is against common folklore.
Andre
I'll bet it's because people have heard so many conflicting and unreliable statements about this over the years that there isn't even a stable folklore.

So the main idea is that
Quote:
gaps greater than the thickness of the porous material provide flat absorption.
Paraphrasing you, here is a cluster of statements that raises questions:
Quote:
The porous materials that we consider when discussing the use of gaps are not thin at the wavelengths significant to us...
If we consider 703 material absorption at 125HZ to be practically 1, this gives a ratio [of absorber thickness to wavelength] of 3.5%...
The effectiveness of a non thin absorber is not significantly reduced when located in the relatively low region of particle velocity.
I would say that a panel that is 3.5% the thickness of a wavelength seems at first glance to be thin compared to that wavelength. So I take it the point of the measurements you laid out is that a panel that is 3.5% the thickness of a wavelength is acoustically non-thin compared to that wavelength?

Then, for various reasons you explained, a panel spaced away from a hard surface will absorb essentially flatly down to its nominal minimum frequency. If a 4" panel absorbs flatly down to, say, 125Hz, then is that to say that a 4" panel spaced 4.8" from the surface will absorb flatly down to ~54Hz?

Quote:
There is an internationally recognized studio designer who…did testing on gaps and used gap to material ratios of up 2.2:1.
There is clearly a sonic benefit to using a ratio of 2:1 vs a ratio of 20:1. If the "expected" half-wavelength dip in absorption is absent at 20:1, then what are the detriments to absorption that do occur at that ratio? Would that be something like the kind of "high-Q ringing" we see in Chris Whealy's Calculator?

Thanks for taking the effort to lay this all out for all to see, and I am looking forward to more on the "physics involved and some other non-intuitive factors!"

DanDan
#13
3rd August 2010
3rd August 2010
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Sticky?

I am just bumping this back up. Perhaps it should be a sticky.

I am thinking about it at length and may have questions, but persistent re-reading of the post kinda deals with most of it.

I do hope Andre comes back to this and teases out the 'thin' issue as raised by Brain.

For now, it might be worth considering some stark single line extracts.

Quote:
gaps greater than the thickness of the porous material provide flat absorption.
Quote:
Gapping is used to lower the effective frequencies of the sound absorption.
Quote:
The efectiveness of a non thin absorber is not significantly reduced when located in the relatively low region of particle velocity
DD
#14
3rd August 2010
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Quote:
Originally Posted by Brainchild
Paraphrasing you, here is a cluster of statements that raises questions:

I would say that a panel that is 3.5% the thickness of a wavelength seems at first glance to be thin compared to that wavelength. So I take it the point of the measurements you laid out is that a panel that is 3.5% the thickness of a wavelength is acoustically non-thin compared to that wavelength?
You summed up the first point perfectly in the bolded (by me) phrase. Something that has been present in absorption test data for decades, but ignored becasue it is non-intuitive.

So what else is new? This is acoustics we are discussing.

Quote:
Then, for various reasons you explained, a panel spaced away from a hard surface will absorb essentially flatly down to its nominal minimum frequency. If a 4" panel absorbs flatly down to, say, 125Hz, then is that to say that a 4" panel spaced 4.8" from the surface will absorb flatly down to ~54Hz?
Correct. The qualifier being what do we consider to be "flat." In our field of studio design .84 is taken as acceptably high to be considered flat.

Quote:
There is clearly a sonic benefit to using a ratio of 2:1 vs a ratio of 20:1. If the "expected" half-wavelength dip in absorption is absent at 20:1, then what are the detriments to absorption that do occur at that ratio? Would that be something like the kind of "high-Q ringing" we see in Chris Whealy's Calculator?
Thanks for bringing up Chris' great calculator. It is a fantastic tool for understanding what if going on.

Partially cost. The gap has to maintained meaning additional physical support. If the absobent material is the same cost per pound, then the gapping is more expensive. If normal incidence absorption is significant, then definitely the dip you are calling ringing.

The biggest reason for the large gap is that it provides space for HVAC ducts, (recessed) lighting, open space for cable runs, etc.

Quote:
Thanks for taking the effort to lay this all out for all to see, and I am looking forward to more on the "physics involved and some other non-intuitive factors!"
You are welcome. If just the flush against the partition test data is studied, the rest sort of falls into place.

Andre
#15
3rd August 2010
3rd August 2010
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Quote:
Originally Posted by DanDan
I am just bumping this back up. Perhaps it should be a sticky.
Stickies have been tried here in the past, but even when the title obviously is dealing with exactly what someone wants, people still blandly post and ask...

Quote:
I am thinking about it at length and may have questions, but persistent re-reading of the post kinda deals with most of it.
The points ae all there, but have to be developed in a more explicit manner. That is for my longer post/article.

Quote:
I do hope Andre comes back to this and teases out the 'thin' issue as raised by Brain.
I hope I addressed it in my previous post.

Quote:
For now, it might be worth considering some stark single line extracts.
You summed it up pretty well with the lines you quoted. They are the terse version. Now I have to expound them to be (relatively) easily understood on inital reading

Andre
#16
3rd August 2010
3rd August 2010
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Quote:
Originally Posted by avare
Thanks Dan. You hit the nail on the head, or rather identified exactly the point that has been holding me back on the article about gas flow resistivity. Specifically the effect of gaps between porous absorbers and reflective surfaces. I have not been able to determine the best manner in which to explain the effect and that gaps greater than the thickness of the porous material provide flat absorption.

There. I wrote the important thing. Now I just thave to expain it in an effective manner.
In the following explanation I will try to work from the basics in a way that other readers can hopefully understand the priniciples involved also.

The classic way that the effect of gaps is explained is by use of graphs of thin absorbent material spaced away from a wall. The graphs show high values of α where the distance from the wall is 1/4 wavelength and 0 when the wavelength is 1/2. In other words the porous material is effective only where the particle velocity is high. These graphs are appropriate for thin material. The porous materials that we consider when discussing the use of gaps are not thin at the wavelengths significant to us. Therefore the graph is not accurate for our use of gapping!

Gapping is used to lower the effective frequencies of the sound absorption. It is usefull to start with the effect of thickness of homogenous porus material against a solid surface. This the mounting of material used in the reporting of the absorption of materials as used traditionally in studios. With typical porous material, using 703 for the example, at 4" thickness, α is 1 down to ~250 Hz and usually considered effective down to ~125 Hz. At 250 Hz the wavelength is 4.52 feet. Dividing the thickness of the material by the wavelength (.333/4.52)gives us a ratio of .0737, or ~7%. So the thickness of a porus absorber has to be at least 7% of the wavelength for flat absorption. If we consider 703 material absorption at 125HZ to be practically 1, this gives a ratio of 3.5%.

Remember, the previous paragraph deals porous material against a solid surface. This is the area where the particle velocity is lowest in the sound wave. The efectiveness of a non thin absorber is not significantly reduced when located in the relatively low region of particle velocity.

Having shown what the depth of a porous absorber has to be in order to be effective, this leaves the question of the required material depth to gap ratio for effective absorption. This is not as clear as the overall depth calculation due the physics involved and some other non-intuitive factors. The usual belief is that the path of sound sound through an absorber is straight through the material. This is also named the normal incidence. However when sound impinges on an absorber at a non normal angle, the path is greater. The significance of this is a reduction, up to a complete removal, of the point on the thin absorber graph where no absorption occurrs.

When sound travels in air, it doing so in an isothermal manner. That is that at the points where the presuure increases, and the temperature (the combined gas law), the additional heat remains that area. That is, there is movement of the energy in the sound wave. In porous material, the material conducts the heat away from the ares of high temperature to areas of lower temperature.

This is called adiabatic. The ratio of the square root of specific heats of air for constant volume vs constant pressure is the same as the ratio of the speeds of sound in air when traveling isothermally vs. adiabatically. The effect is that that porous absorbers behave with effective thicknesses ~120% greater than the physical depth. The practical result is that for acoustic matching to the peak velocity of a sound wave and covering the full 1/4 cycle, the depth of the gap is 1.2 times the depth of the depth of material.

There is of course the variable sound path length from the various angles of incidence also. So the true effective depth for a gapped porous absorber is more than 1.2 times the material thickness. This leads to the question of how much more? In acoustics, we have the ultimate arbiter of test data. Gapped porous absorbers are used in thousands of spaces with gap to depth ratios up to 20:1. These absorber systems are called acoustic ceilings. This sort of mounting is calld E-405. It consists of absorbent material suspended 405 mm (16") away from a solid surface. The acoustic tiles are as thin as ~20 mm(3/4"). Studying test data on such mounted materials does not show any dip at the 1/2 wavelength frequency implied by the thin absorber graph.

An example of a purpose built absorber using a 2:1 gapping ratio is in the Heinieken Music Hall, construction details in fig 5 in Acoustics for Large Scale Indoor Pop Events. There is an internationally recognized studio designer, who in the process of designing a wolrd class facility did testing on gaps and used gap to material ratios of up 2.2:1. He is a rather quiet person regarding his work and out of respect for him, I am not disclosing his name or the studio.

The point to answer your question, is no, 1:1 is not a limiting ratio for gap to absorbent material ratios. Ratios up to 20:1 are used regularly in non-critical acoustic spaces, and ~2:1 for critical acoustic spaces, including studios.

I hope this helps with the ostensible question. Reaction to this post and thread and this post will help me compose the major piece on soound absorption and porous absorbers. You got me started in getting past the stumbling block Dan.

Well spaced,
Andre
I have seen testing on this in a book, but for the life of me I can't remember where. Someone posted a picture at one time.
DanDan
#17
3rd August 2010
3rd August 2010
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Teasing it out

So 'thin' is done and dusted.

A couple more Q's please Andre.

At what extremes does the effect begin to fail?

A suspended ceiling is a full cover, what happens with just a few panels, spread out?

What happens with just one panel?

Put another way, when does the surface area or thickness of the trap begin to look small compared to the gap?

Is the quarter wavelength/high velocity zone thing overpowered by the flat gap effect? OR is there still value to be had by placing the panel in the high zone. e.g. an acoustic ceiling/cloud hanging at 1/4 wavelength of the height mode.

Best, DD

Last edited by DanDan; 3rd August 2010 at 05:33 PM.. Reason: Q and A
#18
3rd August 2010
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Quote:
Originally Posted by avare
Stickies have been tried here in the past, but even when the title obviously is dealing with exactly what someone wants, people still blandly post and ask...
I think those of us interested in topics beyond the FAQ's should start getting in the habit of rating threads with scientific content, as sometimes you can find some gems within the mundane "how would you treat this room" threads.

For what it's worth, I'm glad GS doesn't charge thread subscription fees.
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3rd August 2010
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Quote:
Originally Posted by dykstraster@gmai
I think those of us interested in topics beyond the FAQ's should start getting in the habit of rating threads with scientific content, as sometimes you can find some gems within the mundane "how would you treat this room" threads.

For what it's worth, I'm glad GS doesn't charge thread subscription fees.
Oooh.
I didn't know you could do that.
That's a good idea.
#20
3rd August 2010
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Quote:
Originally Posted by DanDan
At what extremes does the effect begin to fail?
and out of order:
Quote:
Is the quarter wavelength/high velocity zone thing overpowered by the flat gap effect? OR is there still value to be had by placing the panel in the high zone. e.g. an acoustic ceiling/cloud hanging at 1/4 wavelength of the height mode.
The testing that I am aware of, and trust the source, even though not having seen the results, is that someplace greater that 2.1:1 for critical acoustic applications. BBC has used gap ratios as great as ~ 5:1 in their A1,2,3, AND 9 absorbers. Rose's Guide To Acoustic Practice detailas the construction and testing of these absorbers.

Quote:
A suspended ceiling is a full cover, what happens with just a few panels, spread out?
I do not know.

Quote:
What happens with just one panel?
I do not know.
Quote:
Put another way, when does the surface area or thickness of the trap begin to look small compared to the gap?
Someplace between 2.1:1 and 5:1 for critical applications. Of course the other factor there is material used and its gas flow resistivity.

Andre
#21
3rd August 2010
3rd August 2010
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Dan,
Did you hear about our new GIK Skinny panel?? It is .25" thick and and only 2"x2". We recommend you hang it 4 feet from the ceiling and it will absorb 40hz 100%. How many would you like??? There on sale for \$10 each.
#22
3rd August 2010
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Quote:
Originally Posted by avare
Of course the other factor there is material used and its gas flow resistivity.

Andre
Yes. And there are very interesting results to get from playing with gaps / sealed cavities and various flow resistance values / densities / materials.

Especially when the density of some specific materials gets to a certain point, their "mechanical" behaviour is changing in the LF, and the gap / cavity behind will start to greatly influence their oscillatory behaviour.

No surprise, the difficult part is to estimate that behaviour.

Thanks for the nicely written posts Andre.
#23
4th August 2010
4th August 2010
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Quote:
Originally Posted by Northward
Yes. And there are very interesting results to get from playing with gaps / sealed cavities and various flow resistance values / densities / materials.

Especially when the density of some specific materials gets to a certain point, their "mechanical" behaviour is changing in the LF, and the gap / cavity behind will start to greatly influence their oscillatory behaviour.

No surprise, the difficult part is to estimate that behaviour.
+1. The BBC absorbers are of great significance as the BBC has, as one acoustician once said "RD budgets the size of small countries'." They have a reverberation chamber in house for absorption and Transmission loss testing.

Some of the BBC absorbers I referred do use varying density material in the absorbers, but not as people here think of. The denser material is o hte outside. This is to provide some diaphramatic absorption. One unit also has ports on the side with (acoustically) specific cloth covering the port. Great and weird absorbers that can be developed IF you have the test facilities to develop them.
DanDan
#24
4th August 2010
4th August 2010
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Best Guess

There is a practical question which arose, around the same time as my OP here as it happens.
Cloud panel setup question...
A 12 foot high ceiling. The OP wants a recommendation as to what would be the best height to hang his cloud. He suggested 4 to 5 feet below, i.e. 7-8 feet over his head. The question seems to bring the theories here into play. Problem is they seem to be playing on opposite teams.
I eventually suggested a height of 4 feet which gives a more than decent gap, plus lies at a high velocity point of the second height mode.
How's that for committment? LOL
DD
#25
4th August 2010
4th August 2010
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This is great discussion and information, for porous, velocity type absorbers. I keep hoping to find this level of discussion on the pressure zone. There are references, but it seems that this is trade-secret, proprietary, black-science territory. Yet there is product growth in the area with all the plate resonators, etc. that have been coming out.

The references I have seen here seem to be limited to basic principles and "it can be effective, it takes trial-and-error, and I am not going to tell you what I know anyway." While I want to respect people's design and development work, there just doesn't seem to be the same depth of information available as in the porous absorber, and diffusor worlds. Is there anything else available without NDA?

Regards, Nathan
#26
4th August 2010
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Quote:
Originally Posted by avare
This is to provide some diaphramatic absorption
We use this technique at one point in almost all our designs these days. We managed to create a reliable series of systems based on pretty strict implementations (we basically concentrated on a handful of realistic possible implementations and dug from there). They are systematically big systems in terms of surface (many m²).

The hard part was to determine with enough accuracy where the resistance to flow's efficiency was dropping substantially and therefore where the 'membrane' behaviour was to be prioritized.

It resulted in systems based around various resistance to flow 'filters' which would narrow down the needed efficiency range of the membrane side of the system. These have a rather wide Q anyway.

We do not port these. Though generally speaking the idea is that those systems are de facto 'ported' (for lack of a better word to describe the behaviour) since the membrane and the other layers involved are still porous. Therefore we could calculate what that meant in terms of pressure variations in the cavity.

Quite a headache - but it works.

Where pressure is high and frequencies to be treated in the very low-end (typically the back of the room) other types of membranes are used (non-porous) but the system still bears some similarities in the philosophy of it.
#27
4th August 2010
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Quote:
Originally Posted by locutus
"it can be effective, it takes trial-and-error, and I am not going to tell you what I know anyway."

While I want to respect people's design and development work, there just doesn't seem to be the same depth of information available as in the porous absorber, and diffusor worlds. Is there anything else available without NDA?

Regards, Nathan
I understand what you say, and the frustration it can yield.

But you have to see it from our perspective. All our clients can ask any question they want about their design and we will answer them. Some do, some don't. They have all plans & detail. We don't hide anything from them.

NDA primarily protects their investment and then our work on the long term.

Spilling the beans on a forum is therefore difficult.

There are quite a few books that can give you the necessary tools to do the puzzle yourself. But expect it all to take years.

On the other hand Silvia Santafé (my colleague) and I can for sure be pressured. Usually good food and wine help a lot...
#28
4th August 2010
4th August 2010
#28
Lives for gear

Joined: May 2004
Posts: 4,858

Nathan:

As Thomas wrote, the physics are out there, but have to be both recognized (consider the data and designs I have referenced in this thread are all over 20 years old, yet still, unknown in their significance to the general studio design world), and confirmed with testing. In Canada, an absorption test costs \$1,000 for the lab. Add in test unit build, transportation, attending test, and the cost rises to about twice per unit tested.

Companies doing this have to recoup their investment.

Critical analysis of published designs, like BBC'c and studying of texts by Blaert, Fahy, Hopkins and Kuttruff for details on the physics to apply.

Andre
#29
4th August 2010
4th August 2010
#29
Lives for gear

Joined: May 2004
Posts: 4,858

Quote:
Originally Posted by DanDan
There is a practical question which arose, around the same time as my OP here as it happens.
Cloud panel setup question...
A 12 foot high ceiling. The OP wants a recommendation as to what would be the best height to hang his cloud. He suggested 4 to 5 feet below, i.e. 7-8 feet over his head. The question seems to bring the theories here into play. Problem is they seem to be playing on opposite teams.
I eventually suggested a height of 4 feet which gives a more than decent gap, plus lies at a high velocity point of the second height mode.
How's that for committment? LOL
You do not need more than ~5% of the lowest wavelength for good absorption. Recall the 4" 703 referenced earlier in the thread. What is the lowest frequency that you want to absorb? What is the absorption of the current ceiling? Can that be modified? How much dead is wanted of the acoustic in the room?

Andre

Last edited by avare; 4th August 2010 at 09:52 PM.. Reason: Corrected poor typing that made the 703 reference unintelligible.
#30
4th August 2010
4th August 2010
#30
Lives for gear

Joined: May 2004
Posts: 4,858

Quote:
Originally Posted by Northward
We use this technique at one point in almost all our designs these days...
Thanks for the information.

Andre