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n0nspaz
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31st December 2012
Old 31st December 2012
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locating wall panel absorbers

hi, i am trying to determine the best positions for hanging 6 additional 4" thick (with 4" airgap) hybrid rockwool/703 absorbers (2" of each inside).

i already have mounted 2 of these units (with 6" airgap) at the L&R first reflection points (see mix position photo).

here is my thinking, please confirm or explain a better way:
1. sweep room with REW. note largest modes & nulls.
2. play sines of these freqs*, walk around room perimeter (which is where the panels will be mounted, not in the middle of the room..). note loudest locations for each freq and mark it on the wall (see masking tape pieces).
3. mount a panel.

*relevant freqs here are those which this type of absorber can effect. my research (using 2 methods) shows approx .35 absorption @100Hz, .4 @200hz, .6@500hz, .8@1k (see attached graphs)

so i realize these panels wont be doing too much below say 500Hz but i thought if i could find clusters of loudest places for my room's low freq modes 100-500hz that it still wouldnt hurt to put my absorbers there. perhaps the null areas places around the room, 8" off the wall, where problem freqs are quietest, would work as well.

the front wall, centered btwn the spkrs is the loudest for all problem freqs (even louder than right near the speakers).

i probably need to listen up along the ceiling too, and not just at ear level

advice appreciated.
thank you all.

please forgive typos (iphone)
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Last edited by n0nspaz; 31st December 2012 at 07:11 PM.. Reason: higher qual photos
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31st December 2012
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i forgot to mention, this room will have floor to ceiling corner traps with a 35" face (across) and be filled with fluffy fiberglass

so those locations are already taken, so to speak.
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31st December 2012
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Hi

You are thinking in the right way and your plan of using REW to measure the room, or play sine waves and walk around the boundary to find the modal pressure peaks are a good start. Your "question" comes up time and again on GS, (occasionally prompting furious debate), most of your answers are to be found here:- Acoustics/Treatment Reference Guide - LOOK HERE!

The other important issue, which it is clear that you understand from your post, is the low frequency limitation of shallow porous absorbers. Just to add to that, remember that porous absorption works when placed at locations of high particle velocity for the wavelength (frequency) of interest. When you are walking around the boundary listening for loud peaks what you are listening to are pressure peaks. To absorb these peaks close to the boundary, at low frequencies, then pressure absorbers (membrane/panel absorbers) may be better (less depth required, for equivalent absorption, than when using porous absorber at specific low frequency)

From your research you will no doubt have seen that corner straddling of porous absorbers are considered the most efficient for porous bass traps, more so for the tri-corners (wall/wall/ceiling). This rule of thumb is due to the number of modes that the porous absorption is then exposed to and the depth of material/air-gap one can create across the corner. Have a look at this for your room dimensions hunecke.de | Room Eigenmodes Calculator It demonstrates how the pressure peaks are distributed for the lower modes, note how many have a peak in the corners. Finally, note for the fundamental axial modes, wall to wall, (length and width) and floor to ceiling how the pressure peaks are across the whole wall. I think this may be relevant for your front wall resonance that you mentioned. Does it coincide with the axial room mode in that direction? (or harmonic of it?)
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31st December 2012
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No mention of ceiling 'clouds' above the mix position ?
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31st December 2012
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Pressure

There has been a lot of insistence that fibre traps could not possibly work well due to being too thin compared to wavelength, or being in the wrong place. i.e. areas of High Pressure rather than Velocity.
IMO, it would be best to have the science try to follow the reality here. Strangely these velocity traps work really well, even in areas presumed to be predominantly High Pressure, even at 1/10 of the wavelength.
There are very good reasons for this, not commonly stated in the usual text books. http://www.johnlsayers.com/phpBB2/vi...cdbc1&start=15

I recommend playing sines. My main purpose here is just to confirm the obvious corner and whole boundary locations for the primary axials.
The sheer silence of the primary axial null and the wall shaking peaks are a great source of encouragement for treatment.

As frequency increases the hot spots and nulls get increasingly complex, appearing everywhere, almost random. They can be stimulated on the spot by voice. Tonal vowel sounds starting with a plosive. Ba Bo etc. I kid not, Newell speaks of this also. If you find a ring or a honk at say the listening position, get a friend to hold a panel over head or at the side wall ceiling corner. If the honk goes, QED. No theory or debate necessary.
One might say that such anomalies are positional and that the speakers may not cause the same honk. One can play back tonal bursts. The same ones recorded or boggy's very clever set. http://prorecordingworkshop.lefora.c...-liste/#post13

DD
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31st December 2012
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Quote:
Originally Posted by n0nspaz View Post
the front wall, centered btwn the spkrs is the loudest for all problem freqs (even louder than right near the speakers).

Really..?? Is this the case with Only Your Room, or all set-ups..?? Good point to consider though !!
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31st December 2012
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Quote:
Originally Posted by Jolida View Post
Really..?? Is this the case with Only Your Room, or all set-ups..?? Good point to consider though !!
yes i was surprised as well. you are closest to the speakers, but still the modes are VERY loud at this point in the room, suggesting a lot of front wall absorption (to my novice thinking)
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31st December 2012
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Quote:
Originally Posted by DanDan View Post
Strangely these velocity traps work really well, even in areas presumed to be predominantly High Pressure, even at 1/10 of the wavelength.
DD
yes i measured my room before and after adding only 2 L&R side absorbers (seen in my first studio photo above) and the low end became MUCH flatter *SOMEHOW* I will post the before & after graphs when i am back at the studio. [these side absorbers are 2" rockwool + 2" OC703 + 6" airgap.] nothing i have read confirms why they would have such an effect below even 100hz

Quote:
Originally Posted by DanDan View Post
I recommend playing sines. My main purpose here is just to confirm the obvious corner and whole boundary locations for the primary axials.
The sheer silence of the primary axial null and the wall shaking peaks are a great source of encouragement for treatment.
DD
yes it was a great exercise. you don't even need an SPL meter. i noticed that some freqs have their power in the middle of the wall, some at the 1/3 and 2/3 locations, etc. so at some point one has to decide where to place absorbers, without over dampening the room.

the purpose of these absorbers is usually to quell reflections for better imaging, but i am also thinking of them as you would think of placing resonant absorbers, to try to get some additional bang for the DIY buck.

Quote:
Originally Posted by DanDan View Post
As frequency increases the hot spots and nulls get increasingly complex, appearing everywhere, almost random.
DD
yes i also noticed that as freq increases, the pattern of modes gets denser (see the Eigenmodes link posted above for visuals of this)
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31st December 2012
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Quote:
Originally Posted by RJHollins View Post
No mention of ceiling 'clouds' above the mix position ?
i didn't mention...i am also in the process of building a 4' x 4' ceiling cloud from 4" Roxul rockwool (RHT80) and i will put 15-18" of fluffy above that. it will be spaced down a few feet off the 13' high ceiling.

i hope this is wide enough for a decent sized sweet spot. i was thinking of going 6' across instead of 4'
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Quote:
Originally Posted by Icecube1 View Post
Hi

You are thinking in the right way and your plan of using REW to measure the room, or play sine waves and walk around the boundary to find the modal pressure peaks are a good start. .......

From your research you will no doubt have seen that corner straddling of porous absorbers are considered the most efficient for porous bass traps, more so for the tri-corners (wall/wall/ceiling). This rule of thumb is due to the number of modes that the porous absorption is then exposed to and the depth of material/air-gap one can create across the corner. Have a look at this for your room dimensions hunecke.de | Room Eigenmodes Calculator It demonstrates how the pressure peaks are distributed for the lower modes, note how many have a peak in the corners. Finally, note for the fundamental axial modes, wall to wall, (length and width) and floor to ceiling how the pressure peaks are across the whole wall. I think this may be relevant for your front wall resonance that you mentioned. Does it coincide with the axial room mode in that direction? (or harmonic of it?)
thank you, i was following the Eigenmodes site and although the frequencies did not correspond to actual testing (e.g. Eigen calculated 68Hz i think it was, and my actual resonances heard around the room in this visual pattern was at 74Hz, not sure here, this is from memory). strange.

i am finding that almost all acoustic calculators i try (as a layman) are inferior and misleading compared to actual REW room sine-sweep measurements, which give you this great very real picture of your room.

my question remains should i mount my additional panels at ear level and 1. get additional benefit of reduced flutter & reflections at higher freqs, or 2. place them near wall-ceiling boundaries and absorb more bass? i suppose it depends on how much bass absorption you have already. maybe i should wait to mount these until i build my fluffy insulation corner traps.
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31st December 2012
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Quote:
Originally Posted by n0nspaz View Post
yes i was surprised as well. you are closest to the speakers, but still the modes are VERY loud at this point in the room, suggesting a lot of front wall absorption (to my novice thinking)
And the centre of the back-wall too..!!
Tried between a 4" panel vs a 13" tubetrap at that point, & the panel was clearly way ahead..
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31st December 2012
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Quote:
Originally Posted by DanDan View Post
There has been a lot of insistence that fibre traps could not possibly work well due to being too thin compared to wavelength, or being in the wrong place. i.e. areas of High Pressure rather than Velocity.
IMO, it would be best to have the science try to follow the reality here. Strangely these velocity traps work really well, even in areas presumed to be predominantly High Pressure, even at 1/10 of the wavelength.
There are very good reasons for this, not commonly stated in the usual text books. http://www.johnlsayers.com/phpBB2/vi...cdbc1&start=15

I recommend playing sines. My main purpose here is just to confirm the obvious corner and whole boundary locations for the primary axials.
The sheer silence of the primary axial null and the wall shaking peaks are a great source of encouragement for treatment.

As frequency increases the hot spots and nulls get increasingly complex, appearing everywhere, almost random. They can be stimulated on the spot by voice. Tonal vowel sounds starting with a plosive. Ba Bo etc. I kid not, Newell speaks of this also. If you find a ring or a honk at say the listening position, get a friend to hold a panel over head or at the side wall ceiling corner. If the honk goes, QED. No theory or debate necessary.
One might say that such anomalies are positional and that the speakers may not cause the same honk. One can play back tonal bursts. The same ones recorded or boggy's very clever set. http://prorecordingworkshop.lefora.c...-liste/#post13

DD
As been mentioned each time in this forum regarding spacing the panel off the wall, is it necessary that a 4" panel be spaced only 4" away from a flat wall, or will it benefit to increase this spacing to say 8" or 12" or even more ??
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Simple

Quote:
As been mentioned each time in this forum regarding spacing the panel off the wall, is it necessary that a 4" panel be spaced only 4" away from a flat wall, or will it benefit to increase this spacing to say 8" or 12" or even more ??
The question may be simple, but a full answer cannot be.
Simple answer is yes. 1 inch acoustic tiles mounted with a 16 inch gap perform much better than with a 1 inch gap. But that is assuming a large area of them, usually a fully covered ceiling.
I hope it is obvious that a single panel, alone on a large boundary, will have 'leakage' around it's sides.

I would happily increase to an 8 inch gap with a cluster of four inch panels fairly close together, e.g. a typical cloud.
If a wall was fully covered, say a whole back wall, I would have no problem including a larger air gap, even up to say 5 times the panel depth. But in such cases it is probably both cheaper and better to fill the whole depth with light cheap insulation.

This has been explored at length Q 4 Avare


Again though, Eric's two posts at the end of this thread say it all IMO.

http://www.johnlsayers.com/phpBB2/vi...cdbc1&start=15




DD
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1st January 2013
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Quote:
Originally Posted by n0nspaz View Post
maybe i should wait to mount these until i build my fluffy insulation corner traps.
+1

I think that would be a good idea. Listen, measure, tweak

In answer to your concerns regarding the accuracy of modal calculators, you are right, there are often small differences between calculated and measured modal frequencies in a room. Part of this can be due to the fact that "real" rooms are not infinitely rigid, infinitely dense or perfectly rectangular in all dimensions. Generally speaking the "measured" modal decay data is what you are trying to control. The calculated numbers are however usually close enough to associate the measured data with particular wall combinations (or wall/floor, wall/ceiling)?
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1st January 2013
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yes Ice i think i will wait to build the corner traps, measure/listen again for peaks&nulls, including areas up high around the ceiling, and mount my panels where i guess they will make the most difference for modal freqs btwn say 100-500hz.

i plan to add diffusers too over the next few months. probably a large rear wall N=35 stepped according to Tim Arqen's design, and smaller N7's around the room. if anyone needs plans for this i made a basic 2D Adobe Illustrator file of the cross section, to scale. currently seeking a vendor to make these

Dan i like your practical explanations. one must consider the whole picture and interpolate based on the little we do know from measuring acoustic responses. i am sure that panel depth interacts with surface area and other factors. eg. a single 2'x4' panel is different than a cloud is diff than a whole drop ceiling... thanks, that helps me get my head around it

happy new year all !
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6th January 2013
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Quote:
Originally Posted by Icecube1 View Post
Hi

You are thinking in the right way and your plan of using REW to measure the room, or play sine waves and walk around the boundary to find the modal pressure peaks are a good start. Your "question" comes up time and again on GS, (occasionally prompting furious debate), most of your answers are to be found here:- Acoustics/Treatment Reference Guide - LOOK HERE!

The other important issue, which it is clear that you understand from your post, is the low frequency limitation of shallow porous absorbers. Just to add to that, remember that porous absorption works when placed at locations of high particle velocity for the wavelength (frequency) of interest. When you are walking around the boundary listening for loud peaks what you are listening to are pressure peaks. To absorb these peaks close to the boundary, at low frequencies, then pressure absorbers (membrane/panel absorbers) may be better (less depth required, for equivalent absorption, than when using porous absorber at specific low frequency)
I have a room mode at 59.5Hz. I guess this is the floor-to-ceiling mode for the 9.5ft high ceiling. I have about 12 panels (4x2x4") , & four tri-corner traps in my room already. But I guess that the 4 inch panels are not doing anything below 200hz. Apart from these, I have another 12 panels of similar thickness already made & finished. Will it benefit if I get them into the room as well, or do I need to make the pressure-based absorber for this mode??
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6th January 2013
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Quote:
Originally Posted by Jolida View Post
I have a room mode at 59.5Hz. I guess this is the floor-to-ceiling mode for the 9.5ft high ceiling. I have about 12 panels (4x2x4") , & four tri-corner traps in my room already. But I guess that the 4 inch panels are not doing anything below 200hz. Apart from these, I have another 12 panels of similar thickness already made & finished. Will it benefit if I get them into the room as well, or do I need to make the pressure-based absorber for this mode??
Hi Jolida

First question. What porous material are the 4" panels made of (brand, density, gas flow resistivity? As much info as you have...).

It would seem as if the vertical, fundamental axial mode could be the problem. I say could as I don't know the full story. It could also be a harmonic due to another longer dimension? Have you tried measuring (with REW or whatever software you use) at the ceiling, or actually at floor level in your listening position? Alternatively, if you play a 60Hz tone through one of your speakers and you listen down at floor level does it get louder?

Regards
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6th January 2013
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Gaps

Quote:
But I guess that the 4 inch panels are not doing anything below 200hz.
They would if they have an airgap and sufficient number of them, i.e. surface area.

Relatively think fibre absorption does a lot more than many textbooks suggest. Look at the results obtained using GIK or RealTraps products.

1/10 wavelength is a useful yardstick IMO. Also the velocity regions are not as defined as one might think from the textbooks.

Thanks to Andre and Eric for this little gem. The two last posts.
John Sayers' Recording Studio Design Forum • View topic - Help with basement studio acoustic treatment

Jol, you can easily prove your mode using Sine waves. Use the Sig Gen in REW with Frequency follows cursor engaged. A height mode will be loud near the floor and ceiling with a null halfway.
Measuring Room Acoustics

DD
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7th January 2013
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Quote:
Originally Posted by DanDan View Post
They would if they have an airgap and sufficient number of them, i.e. surface area.

Relatively think fibre absorption does a lot more than many textbooks suggest. Look at the results obtained using GIK or RealTraps products.

1/10 wavelength is a useful yardstick IMO. Also the velocity regions are not as defined as one might think from the textbooks.

Thanks to Andre and Eric for this little gem. The two last posts.
John Sayers' Recording Studio Design Forum • View topic - Help with basement studio acoustic treatment

Jol, you can easily prove your mode using Sine waves. Use the Sig Gen in REW with Frequency follows cursor engaged. A height mode will be loud near the floor and ceiling with a null halfway.
Measuring Room Acoustics

DD
Surprisingly I tried it yesterday & Yes the spl was highest at floor level. Never tried half-way between floor to ceiling for the null though. After doing this, I tried keeping a pair of panels where the spl was highest, but it din't change things a BIT. So this got me wondering if my extra 12 panels would be wasted, as all are 4" thick rigid fibreglass....
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7th January 2013
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Quote:
Originally Posted by Icecube1 View Post
Hi Jolida

First question. What porous material are the 4" panels made of (brand, density, gas flow resistivity? As much info as you have...).

It would seem as if the vertical, fundamental axial mode could be the problem. I say could as I don't know the full story. It could also be a harmonic due to another longer dimension? Have you tried measuring (with REW or whatever software you use) at the ceiling, or actually at floor level in your listening position? Alternatively, if you play a 60Hz tone through one of your speakers and you listen down at floor level does it get louder?

Regards
Yes it's loudest at the floor. The panels are made using rigid 4" thick fiber glass. No matter where i keep my panels in the room, this mode remains unaffected. Guess I would need reasonable amounts of them forming some kind of a cloud at the ceiling to show some effect. Or as u said, a pressure based absorber (which I have no idea how to go about making)...
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7th January 2013
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Height

Two panels is really nothing. I have four in my cloud, with 6 inch airgap, small room. Plus two straddling the wall ceiling corners. In general 4 4 4 would be my minimum for anything. 4 Traps, 4 inches thick, 4 inch airgap. Having said that 60Hz is a bit of a stretch. VPR's should work well on the ceiling but good luck with attaching them!

If it is the height mode there should be a spectacularly silent null at half height.

Ultimately, most of us spend our time in the width null and in the height null.
Those big side traps we see at johnlsayers.com would help with the side but the vertical is often a niggling remainder. Sometimes it is worth turning the speaker upside down to get the woofer a bit out of the null.

DD
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7th January 2013
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Since you have panels for the early reflection points I would recommend straddling all of the remaining panels. That will give you the largest amount of low end absorption for those panels.
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8th January 2013
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Quote:
Originally Posted by DanDan View Post
Two panels is really nothing. I have four in my cloud, with 6 inch airgap, small room. Plus two straddling the wall ceiling corners. In general 4 4 4 would be my minimum for anything. 4 Traps, 4 inches thick, 4 inch airgap. Having said that 60Hz is a bit of a stretch. VPR's should work well on the ceiling but good luck with attaching them!

If it is the height mode there should be a spectacularly silent null at half height.

Ultimately, most of us spend our time in the width null and in the height null.
Those big side traps we see at johnlsayers.com would help with the side but the vertical is often a niggling remainder. Sometimes it is worth turning the speaker upside down to get the woofer a bit out of the null.

DD
Is there a way to avoid the Width null, as we are bound to sit in the exact centre of the side walls??
So as u said, the height mode can only be resolved by a cloud at the ceiling, & no panel put on any other wall can cure it..??
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8th January 2013
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That depends on the room geometry. Sidewall treatments are a huge priority.
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8th January 2013
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Quote:
Originally Posted by OpusOfTrolls View Post
That depends on the room geometry. Sidewall treatments are a huge priority.
At first-reflection points alone..?
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8th January 2013
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Nulls

Quote:
Is there a way to avoid the Width null, as we are bound to sit in the exact centre of the side walls??
So as u said, the height mode can only be resolved by a cloud at the ceiling, & no panel put on any other wall can cure it..??
Broadband absorbers on the side walls will do it. See johnlsayers.com
The VPR with a fibre or foam front is called a CBA, Compound Baffle Absorber. These would do a fine job as full range cloud or side wall treatment.

Fibre traps straddled across the ceiling wall corner at the sides seems to have a good go at the height modes. Do also remember there are many higher frequency modes. e.g. Second H mode will often be 145 or so.

DD
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8th January 2013
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Quote:
Originally Posted by DanDan View Post
Broadband absorbers on the side walls will do it. See johnlsayers.com
The VPR with a fibre or foam front is called a CBA, Compound Baffle Absorber. These would do a fine job as full range cloud or side wall treatment.

Fibre traps straddled across the ceiling wall corner at the sides seems to have a good go at the height modes. Do also remember there are many higher frequency modes. e.g. Second H mode will often be 145 or so.

DD
Since the height mode will have the offending frequency at its highest spl at both the ceiling & floor, so does the length & width mode at their boundaries..?? Seems like a shortcut to pin down the culprit..
Secondly, as I had read somewhere, a null at the listening position is always caused by a Peak elsewhere in the room, & finding the peak & trapping it will clear out that null as well. Is that always the case?
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8th January 2013
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Eureka

Quote:
Since the height mode will have the offending frequency at its highest spl at both the ceiling & floor, so does the length & width mode at their boundaries..?? Seems like a shortcut to pin down the culprit..
Secondly, as I had read somewhere, a null at the listening position is always caused by a Peak elsewhere in the room, & finding the peak & trapping it will clear out that null as well. Is that always the case?
Yes, bass builds up at the boundaries. Corners share two (or three) boundaries. That is why they are such good value places to trap.
These first lowest modes are usually the strongest by far. But there are higher modes which bounce between several or all the boundaries. Tangential and Oblique. They can have hot spots mid wall and all sorts of unexpected places.

Here is a calculator which shows the areas of High and Low pressure graphically. hunecke.de | Room Eigenmodes Calculator

And Yes, absorbing the peak will soften the null. But they are tough nuts to crack. Two waves adding in phase will be +6dB. Two waves subtracting out of phase can be -30dB or worse.

DD
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8th January 2013
Old 8th January 2013
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Quote:
Originally Posted by Jolida View Post
Yes it's loudest at the floor. The panels are made using rigid 4" thick fiber glass. No matter where i keep my panels in the room, this mode remains unaffected. Guess I would need reasonable amounts of them forming some kind of a cloud at the ceiling to show some effect. Or as u said, a pressure based absorber (which I have no idea how to go about making)...
Ok, so it sounds like it probably is the fundamental, vertical axial mode, you could do as Dan suggests and try to confirm the null at halfway between floor and ceiling, just to be certain.

When you say... "No matter where i keep my panels in the room, this mode remains unaffected.". If your expectation is to totally flatten the frequency peak of this mode then prepare for a reality check, you won't. Effective absorption (of whatever description) will reduce the "ringing" (resonance) in the time domain of your targeted mode ie you would expect to see the decay time reduce on say a waterfall plot. However, the mode will still exist, the structural walls (..or floor/ceiling for the vertical) are still there. So, you may only see a small reduction in amplitude on the frequency response

Yes, you could construct a cloud. With 9.5 feet to play with I guess you could afford to lose some height? Could you afford to lose up to 20".? That is the sort of depth you are probably going to need for a porous type solution.

There are many ways you could go from here depending on how important it is to you to tame the vertical mode. Have you measured at your listening position to check how significant the decay time for the 60Hz mode is? I would do this first.

You could certainly look at building membrane (panel) absorbers and I can give you some design ideas, but before you go down that route which will be a significant amount of work if you are going to DIY, I recommend you exhaust the potential of what you already have. Let's say you could position 4 of your panels side by side to create an area of 8'x4' and suspend the whole assembly slightly forward biased of your listening position, at least 200mm (~8"), from the ceiling. That would be a starting point.

Before you start suspending things from the ceiling however, (which in itself is a lot of work), as a confidence booster try this... Leave your measuring mic set up, in the listening position@ear height, on a boom stand with mic stand behind the listening position. Lay out the 4 panels on the floor, mirroring where you might hang the cloud assembly. Get 4 lengths of 4"x2" timber cut to 8 feet, and put them on top of each other to create two 8" high support lengths to lay the rigid fibreglass panels across. Now, remeasure the room. You will see if there is any noticeable difference to the decay time at 60Hz. (I know it's not a totally valid experiment but it will show you whether your panels are worth the effort of hanging as a cloud for your perceived problem)
#30
8th January 2013
Old 8th January 2013
  #30
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Quote:
Originally Posted by DanDan View Post
Yes, bass builds up at the boundaries. Corners share two (or three) boundaries. That is why they are such good value places to trap.
These first lowest modes are usually the strongest by far. But there are higher modes which bounce between several or all the boundaries. Tangential and Oblique. They can have hot spots mid wall and all sorts of unexpected places.

Here is a calculator which shows the areas of High and Low pressure graphically. hunecke.de | Room Eigenmodes Calculator

And Yes, absorbing the peak will soften the null. But they are tough nuts to crack. Two waves adding in phase will be +6dB. Two waves subtracting out of phase can be -30dB or worse.

DD
Fantastic. Thanks for the info Dan..
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