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Really trying to upgrade my room
Old 29th July 2019
  #31
Gear Head
Thanks for the link, reading now
Old 29th July 2019
  #32
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Synthpark's Avatar
Quote:
Originally Posted by Twiggy View Post
Thanks again for all the help so far guys, it's much appreciated! I spent the weekend making the soffits I discussed in my last post. So I now have soffit traps floor to ceiling in the rear corners, and along the rear wall/ceiling corner. I have soffits almost up to the ceiling of the corners to the left and right of the mix position (the gap is because I'm also building traps to fill the wall/ceiling corner on the left hand side of the room). And I have soffits in the front two corners up to about waist height (the speaker position stops me going any higher). I'll detail the building of these with pictures + descriptions if you guys think they've made a positive impact to my room!

I installed the small traps underneath the speakers, and traps almost floor to ceiling in the left/right mix position corners.

I then moved some traps around and took a few measurements:

1) GIK soffits at rear, DIY soffits in the left/right corners.
2) DIY in rear corners, GIK in the left/right corners
3) DIY in rear corners, GIK in the left/right corners, rear wall/ceiling soffits loosely positioned.
4) Added a roll of 170mm 5000 wool still in the wrapper along the front floor/wall corner
5) Added two 100mm rockwool RW3 (28000 pa.s/M2) panels on the walls directly to the sides of the speakers.
6) Removed the previous panels, and moved one to the middle of the room and one above an existing panel in the intended RFZ (this was purely as it was easy to place them here - I expect them to move)

REW file here: https://www.dropbox.com/s/q9h7jzxjjd...0REW.mdat?dl=0

Because I'm expecting the soffits to do most of the work in the low end, the measurements I've posted here are with both speakers running. I did take measurements of just L and just R speakers running, which I will happily post them for the others if people are interested / think it would be useful.

The most interesting thing is there seems to be a lot of movement in the 100-200hz range, and just above 300 on the frequency response (not always for the best) but I think the waterfall always shows improvements.

Edited to include link to REW file
Your big peak is the 1 0 0 mode. Move speakers more into the room or the sub away front the front wall, if you have one running, and the peak will reduce.

If this has negative side effects, you can try with DSP. Of course, proper bass trapping will help, but needs knowledge and some experience.
Old 29th July 2019
  #33
Gear Addict
 

Get a load of packages of mineral wool and place 'm at the front wall (covering the entire wall) and measure/listen.
If it doesn't do what I expect you can return 'm, else you can use it for serious treatment.
Old 29th July 2019
  #34
Gear Head
Quote:
Originally Posted by Synthpark View Post
Your big peak is the 1 0 0 mode. Move speakers more into the room or the sub away front the front wall, if you have one running, and the peak will reduce.

If this has negative side effects, you can try with DSP. Of course, proper bass trapping will help, but needs knowledge and some experience.
Cheers for the info. When you say try with DSP, do you meant try to treat the 1-0-0 mode with DSP or treat the side effects of moving the speakers with DSP?
Old 29th July 2019
  #35
Gear Head
Quote:
Originally Posted by bert stoltenborg View Post
Get a load of packages of mineral wool and place 'm at the front wall (covering the entire wall) and measure/listen.
If it doesn't do what I expect you can return 'm, else you can use it for serious treatment.
The bottom half of the front wall is a radiator, and the top half is a window. I've read a few times that low frequency energy will pass straight through the window?
Old 29th July 2019
  #36
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Quote:
Your big peak is the 1 0 0 mode. Move speakers more into the room or the sub away front the front wall, if you have one running, and the peak will reduce.
So you are saying that it is better to have SBIR than modal issues? And that moving a speaker several inches away from the wall will stop the first axial mode from ringing?

Bert is right: porous absorption is the answer.

- Stuart -
Old 29th July 2019
  #37
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Quote:
Originally Posted by Twiggy View Post
Cheers for the info. When you say try with DSP, do you meant try to treat the 1-0-0 mode with DSP or treat the side effects of moving the speakers with DSP?
Both of those won't work: SBIR is a phase cancellation, so it cannot be "fixed" with DSP, and modal resonance is an acoustic issue in the room, not the signal chain, so it also cannot be fixed with DSP (especially if your speakers are far away form the walls....)

Bert is right: fix it with acoustic treatment.

- Stuart -
Old 29th July 2019
  #38
Gear Addict
 

Quote:
Originally Posted by Twiggy View Post
The bottom half of the front wall is a radiator, and the top half is a window. I've read a few times that low frequency energy will pass straight through the window?
No, sound energy has a logaritmic nature so even if a lot of energy seems to disappear through a thin window or lightweight wall you will hardly notice it in the room.

Stuart is right in warning for SBIR.
A standing wave is a pain but it is rather narrow banded.
SBIR can cause severe, wide band dips.
As in dips there is no energy due to airmolucules cancelling each others movement you can't manipulate anything with EQ, DSP etc.
Acoustical treatment will prevent the cancelling wave to occur.

In the audiophile society experts advocate the placement of speakers away from boundaries. This causes the infamous SBIR unless the room is large enough to place speakers 1/4 wavelength or more away from the boundaries. If you realize 20 Hz has a 1/4 lambda length of 4.25 meters you need a pretty big room.

Last edited by bert stoltenborg; 29th July 2019 at 06:50 PM.. Reason: stupidity
Old 29th July 2019
  #39
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Synthpark's Avatar
Quote:
Originally Posted by Twiggy View Post
Cheers for the info. When you say try with DSP, do you meant try to treat the 1-0-0 mode with DSP or treat the side effects of moving the speakers with DSP?
You can simple reduce the peak with an EQ prior to DA-conversion. Or analog: my Neuman 805 subwoofer has a peak/notch filter with variable gain (positive/negative), good enough to reduce a mode. The same can be achieved with a highpass filter. One has to check the group delay with this method.

The point is that your subwoofer does not need more power, but even less power to accomplish this, the mode is simply not excited. Of course this will not reduce the ringing of the mode, but will still sound much better.
Old 29th July 2019
  #40
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Synthpark's Avatar
Quote:
Originally Posted by Soundman2020 View Post
So you are saying that it is better to have SBIR than modal issues? And that moving a speaker several inches away from the wall will stop the first axial mode from ringing?

Bert is right: porous absorption is the answer.

- Stuart -
Do you have any experience at all?
Move a SUBWOOFER away from the front wall will REDUCE the 1 0 0 peak. I did this in my room as well as the rooms before with always the same result, without causing problems. You have to find an optimum. If you are running a 2+1 system, even IF your speakers suffer somewhere from SBIR, lets say at 60 Hz, but the sub does fine at that frequency, you may be able to combine them such that defficiencies in the frequency response are minimized.

And listen: DSP was given as an ALTERNATIVE. Nobody was arguying to remove a deep Null with DSP, causing problems with power limitation etc.

Why do I have my satellite speakers away from the front wall? Because the stereo image is better, the sound has more depth, something a colleague also acknowledged. And this is as important as the bass response.
Old 29th July 2019
  #41
Lives for gear
Quote:
Originally Posted by Synthpark View Post
Do you have any experience at all?
Move a SUBWOOFER away from the front wall will REDUCE the 1 0 0 peak. I did this in my room as well as the rooms before with always the same result, without causing problems. You have to find an optimum. If you are running a 2+1 system, even IF your speakers suffer somewhere from SBIR, lets say at 60 Hz, ´but the sub does fine, you may be able to combine them such that defficiencies are minimized.
The better position for the sub is under the seat. In yachting and car audio demonstrates it.

And generalize your room as all the room of the earth is abusive.

And in my room when i had a sub, do what you write was an error.
Old 29th July 2019
  #42
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Synthpark's Avatar
Quote:
Originally Posted by dinococcus View Post
The better position for the sub is under the seat. In yachting and car audio demonstrates it.

And generalize your room as all the room of the earth is abusive.
I have mine always under the table, but each to his own.
Old 29th July 2019
  #43
Gear Addict
 

Quote:
Originally Posted by Synthpark View Post
Do you have any experience at all?
Move a SUBWOOFER away from the front wall will REDUCE the 1 0 0 peak. I did this in my room as well as the rooms before with always the same result, without causing problems. You have to find an optimum. If you are running a 2+1 system, even IF your speakers suffer somewhere from SBIR, lets say at 60 Hz, but the sub does fine at that frequency, you may be able to combine them such that defficiencies in the frequency response are minimized.

And listen: DSP was given as an ALTERNATIVE. Nobody was arguying to remove a deep Null with DSP, causing problems with power limitation etc.

Why do I have my satellite speakers away from the front wall? Because the stereo image is better, the sound has more depth, something a colleague also acknowledged. And this is as important as the bass response.
Maybe you should be a little less agressive when you advice people?

I think personally that when using a sub and satellites you need to crossover so the systems will not have output in the same frequency bands. And that makes your argument concerning SBIR moot.

And you absolutely cannot remove a null using EQ or however you name it, no matter how much power you put into the speaker. You cannot fix a dip caused by a phenomenon in the time domain with a tool that operates in the frequency domain.
Old 29th July 2019
  #44
Gear Head
Quote:
Originally Posted by Soundman2020 View Post
Both of those won't work: SBIR is a phase cancellation, so it cannot be "fixed" with DSP, and modal resonance is an acoustic issue in the room, not the signal chain, so it also cannot be fixed with DSP (especially if your speakers are far away form the walls....)

Bert is right: fix it with acoustic treatment.

- Stuart -
Thanks for the clarification. I've been reading so much info on acoustics recently my brains starting to melt a bit I think!!

I spent a good chunk of today tweaking the multi layer absorber calculator to try to get a trap that will be effective against 33hz and shortly I'll move around my room with an SPL meter playing a tone at the 1-0-0 frequency to find the best place for it.

Something like this? http://www.acousticmodelling.com/mli...4=170&v14=5000

Last edited by Twiggy; 29th July 2019 at 07:40 PM.. Reason: added link to panel idea
Old 29th July 2019
  #45
Gear Head
Quote:
Originally Posted by bert stoltenborg View Post
No, sound energy has a logaritmic nature so even if a lot of energy seems to disappear through a thin window or lightweight wall you will hardly notice it in the room.

Stuart is right in warning for SBIR.
A standing wave is a pain but it is rather narrow banded.
SBIR can cause severe, wide band dips.
As in dips there is no energy due to airmolucules cancelling each others movement you can't manipulate anything with EQ, DSP etc.
Acoustical treatment will prevent the cancelling wave to occur.

In the audiophile society experts advocate the placement of speakers away from boundaries. This causes the infamous SBIR unless the room is large enough to place speakers 1/4 wavelength or more away from the boundaries. If you realize 20 Hz has a 1/4 lambda length of 4.25 meters you need a pretty big room.
Yeah I hear ya. In fact, SBIR is why I started on this endeavour in the first place! I had severe dips in the low end, which really cleared up when I moved the speakers back against the walls.
The only downside is that's it's excited this HUGE modal problem which we're now all discussing, haha!

But hopefully I'll be able to devise a way to tame it, thanks to all the info and discussion here

And just to clarify, for everyone here..I don't have a sub. This is just two speakers on stands up against the wall. Thanks for all the help thus far, it's really appreciated!

Last edited by Twiggy; 29th July 2019 at 07:22 PM.. Reason: Added subwoofer info
Old 29th July 2019
  #46
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Quote:
Originally Posted by Twiggy View Post
Thanks for the clarification. I've been reading so much info on acoustics recently my brains starting to melt a bit I think!!

I spent a good chunk of today tweaking the multi layer absorber calculator to try to get a trap that will be effective against 33hz and shortly I'll move around my room with an SPL meter playing a tone at the 1-0-0 frequency to find the best place for it.

Something like this? http://www.acousticmodelling.com/mli...4=170&v14=5000
That should work, yes, but it needs to be fairly large (cover a good percentage of the problem area). And do try to figure some way to make it tunable, so you can correct any problems with construction, materials, the room, calculations, or even air temperature, that push the tuning off from where you think it will be. Also make allowances for the fact that the measurement itself might be off: just because REW shows the peak at 33 Hz doesn't mean that it really is exactly 33 Hz... it could be off by a bit. So try to build in some way of tuning your trap after it is in place, to make sure that you really are hitting the right frequency as closely as possible.

You might also want to consider membrane traps. That's another option for hitting low frequency modal issues.

- Stuart -
Old 29th July 2019
  #47
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Quote:
Originally Posted by Synthpark View Post
Move a SUBWOOFER away from the front wall will blah blah blah....
What sub would that be? The OP never mentioned one, and doesn't even have one. That's clear from the thread. Suggesting that he move something he doesn't even have, doesn't seem noticeably useful... (especially when the suggestion wouldn't fix the problem anyway, even if he did have one....)


- Stuart .
Old 29th July 2019
  #48
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Quote:
Originally Posted by Twiggy View Post
Yeah I hear ya. In fact, SBIR is why I started on this endeavour in the first place! I had severe dips in the low end, which really cleared up when I moved the speakers back against the walls.
Yup! That is, indeed, the best way to deal with SBIR in a small room, along with suitable porous absorption, as Bert pointed out.

Quote:
The only downside is that's it's excited this HUGE modal problem which we're now all discussing, haha
Don't you just love modes? Especially the lengthwise axials!

Quote:
But hopefully I'll be able to devise a way to tame it, thanks to all the info and discussion here
It's a tough one to deal with, for sure, down that low. It's going to take a lot of effort and careful tuning, but the good news is that it can be done.

Quote:
And just to clarify, for everyone here..I don't have a sub. This is just two speakers on stands up against the wall.
Yup. I got that early on in your thread. If you did have a pair of subs, you could set them up in a plan wave bass array to deal with your low frequency issues. It's a really good solution, but unfortunate it costs a lot of money to get a pair of good subs!

If only money grew on trees, like leaves...


- Stuart -
Old 29th July 2019
  #49
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Synthpark's Avatar
Quote:
Originally Posted by Soundman2020 View Post
(especially when the suggestion wouldn't fix the problem anyway, even if he did have one....)


- Stuart .
It would reduce the problem, this is just outside your imagination.
Old 29th July 2019
  #50
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Synthpark's Avatar
Quote:
Originally Posted by bert stoltenborg View Post
Maybe you should be a little less agressive when you advice people?
You may tell that soundman.

Quote:
I think personally that when using a sub and satellites you need to crossover so the systems will not have output in the same frequency bands. And that makes your argument concerning SBIR moot.
Your crossover filter is not infinite. Maybe you should eperiment a little more with subwoofers. Especially with 12 db / octave crossovers where there is a considerable overlap between sub and satellites (for example Dynaudio BM14). But that can cause cancellation problems. I fixed a problem at 60 Hz for Neumann 310 by adding a sub 805 which now takes the frequency over and 60 Hz is fully there. The additional degree of freedom with the sub (power, phase, cross over frequency) can be very beneficial.

Quote:
And you absolutely cannot remove a null using EQ or however you name it, no matter how much power you put into the speaker. You cannot fix a dip caused by a phenomenon in the time domain with a tool that operates in the frequency domain.
As everybody knows anyway.
Old 29th July 2019
  #51
Gear Head
Quote:
Originally Posted by Soundman2020 View Post
That should work, yes, but it needs to be fairly large (cover a good percentage of the problem area). And do try to figure some way to make it tunable, so you can correct any problems with construction, materials, the room, calculations, or even air temperature, that push the tuning off from where you think it will be. Also make allowances for the fact that the measurement itself might be off: just because REW shows the peak at 33 Hz doesn't mean that it really is exactly 33 Hz... it could be off by a bit. So try to build in some way of tuning your trap after it is in place, to make sure that you really are hitting the right frequency as closely as possible.

You might also want to consider membrane traps. That's another option for hitting low frequency modal issues.

- Stuart -
Ok great. Fantastic to know I'm on the right path! Since I'm drilling a number of holes in a piece of wood, I'm thinking the only way to alter the tuning would be to make the holes bigger / fill some of them up? I'll have a play about with the calc and see what both of those things do to the frequency.

Thanks for the heads up on not taking REQ/Fuzz as gospel, I'll bear that in mind. I'm planning to make several smaller boxes to cover the area so hopefully should be able to test with one and see if there is any improvement.

I have looked at 'Tims limp mass absorbers' but feel apprehensive about getting the tension of the membrane correct. It feels like the perf panel would be easier to execute - I'm probably wrong there though!
Old 29th July 2019
  #52
Gear Head
Quote:
Originally Posted by Soundman2020 View Post
Yup! That is, indeed, the best way to deal with SBIR in a small room, along with suitable porous absorption, as Bert pointed out.
Fantastic

Quote:
Originally Posted by Soundman2020 View Post
Don't you just love modes? Especially the lengthwise axials!
You know, I didn't actually think they would be this problematic. Given that I'd measured the room a few times I was honestly shocked when this giant one appeared, as if from nowhere! haha.

Quote:
Originally Posted by Soundman2020 View Post
It's a tough one to deal with, for sure, down that low. It's going to take a lot of effort and careful tuning, but the good news is that it can be done.
Every cloud, huh Hopefully I can get these traps right. If not I'll be looking at something like the GIK scopus trap.

Quote:
Originally Posted by Soundman2020 View Post
Yup. I got that early on in your thread. If you did have a pair of subs, you could set them up in a plan wave bass array to deal with your low frequency issues. It's a really good solution, but unfortunate it costs a lot of money to get a pair of good subs!
Yeah it does! I used to use an ADAM sub12 in my old studio but don't really fancy splashing about 4K on a pair of subs right now haha!

Quote:
Originally Posted by Soundman2020 View Post
If only money grew on trees, like leaves...
Tell me about it! Although the soffits were surprisingly cheap to build! Hopefully I got them right!
Old 29th July 2019
  #53
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Quote:
Originally Posted by Twiggy View Post
Ok great. Fantastic to know I'm on the right path! Since I'm drilling a number of holes in a piece of wood, I'm thinking the only way to alter the tuning would be to make the holes bigger / fill some of them up? I'll have a play about with the calc and see what both of those things do to the frequency.

Thanks for the heads up on not taking REQ/Fuzz as gospel, I'll bear that in mind. I'm planning to make several smaller boxes to cover the area so hopefully should be able to test with one and see if there is any improvement.
You could mount the perf panel itself on somthing that allows you to vary the depth of the cavity slightly. Something that can slide in and out, but that also maintains the airtight seal around the edges of the panel. Don't forget that Helmholtz resonators have to be sealed in order to work!

It is also possible to put broaden your Q a bit with a very thin layer of light cloth right behind the perf panel itself, like this.

http://www.acousticmodelling.com/mli...4=170&v24=5000

That spot directly behind the panel is the best place to get drastic effects, because the vibrating slug of air inside the hole interacts with it directly. So be careful with that: keep it very thin, and very light.

You can put it on the front of the panel, instead of behind it, if you prefer, so it's easier to experiment. So you could tune it like that, by trying out different types and thickness of cloth over the panel, after you finish building it. It doesn't change the tuning much, though: it only changes the Q. But still useful.

- Stuart -
Old 29th July 2019
  #54
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Quote:
Tell me about it! Although the soffits were surprisingly cheap to build! Hopefully I got them right!
Photos! Or it didn't happen...


- Stuart -
Old 30th July 2019
  #55
Gear Head
Quote:
Originally Posted by Soundman2020 View Post
Photos! Or it didn't happen...
Seems fair enough

The green wire is to stop the slabs sagging as I think I read that the more compressed they are the less effective they are?
Attached Thumbnails
Really trying to upgrade my room-soffit-trap.jpg  
Old 30th July 2019
  #56
Gear Head
Quote:
Originally Posted by Soundman2020 View Post
You could mount the perf panel itself on somthing that allows you to vary the depth of the cavity slightly. Something that can slide in and out, but that also maintains the airtight seal around the edges of the panel. Don't forget that Helmholtz resonators have to be sealed in order to work!
Hmm I think the sliding is a bit beyond me. But I have a friend that does woodwork so he might be able to make some sense of it!

Yeah airtight is one of the things I'm going to have to focus on. Because the box shouldn't resonate (apart from the front panel) I'm thinking 10/12mm MDF for the back and sides. These will be glued and screwed together and then the inside joins covered with some sort of sealant.

The 170mm insulation will be placed on the back panel and held in place with twine, and the front panel will be glued and screwed on the front. This can then be drilled with holes measuring as I go to check the tuning.

Quote:
Originally Posted by Soundman2020 View Post
It is also possible to put broaden your Q a bit with a very thin layer of light cloth right behind the perf panel itself, like this.

http://www.acousticmodelling.com/mli...4=170&v24=5000
Wow that does do quite a bit for the bandwidth!

Quote:
Originally Posted by Soundman2020 View Post
That spot directly behind the panel is the best place to get drastic effects, because the vibrating slug of air inside the hole interacts with it directly. So be careful with that: keep it very thin, and very light.

You can put it on the front of the panel, instead of behind it, if you prefer, so it's easier to experiment. So you could tune it like that, by trying out different types and thickness of cloth over the panel, after you finish building it. It doesn't change the tuning much, though: it only changes the Q. But still useful
Very interesting. I may do that then, experiment with cloth on the front. Double winner because the unfinished MDF probably wouldn't look too good exposed so that gives it a nice touch.

I'd written these off because I assumed they would be too big for my room but I do have the chance to get some huge poly-cylindrical diffusers (http://www.studiopeople.com/products...Poly-Diffuser/) - would these be of any use?
Old 30th July 2019
  #57
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Quote:
Hmm I think the sliding is a bit beyond me. But I have a friend that does woodwork so he might be able to make some sense of it!
Another option is to angle the panel a little, so it is slightly deeper at one edge, slightly shallower at the other. That broadens the bandwidth slightly, but also lowers the Q. The simple model doesn't allow you to do that, but you can get a rough idea by modeling for the maximum depth and minimum depth, then mentally extrapolating, but with a lower peak.

Quote:
These will be glued and screwed together and then the inside joins covered with some sort of sealant.
Ordinary bathroom/kitchen caulk is good for that. The type that remains soft, rubbery, and flexible even after it has fully cured. Get one that sticks like crazy and does not shrink as it dries (some do). Be generous as you apply it! Even tiny cracks can rob the device of performance... and even de-tune it.

Quote:
This can then be drilled with holes measuring as I go to check the tuning.
If you drill holes bigger, you can only change the tuning upwards, to a higher frequency, and probably with a lower Q. However, you can also seal up some holes, thus reducing the overall open area, to bring the frequency down again... at the cost of efficiency.... (there's no such thing as a free lunch in acoustics! )

Quote:
Wow that does do quite a bit for the bandwidth!
Yup! Pretty impressive, isn't it, considering that it is such a small, thin, light change! TH reason this works so well is because Helmholtz resonators work on the principle of the "slug" of air trapped in the "neck" of the device (each hole, in the case of perf panel) moving more or less as a single unit of mass, and vibrating in and out of the hole, along with a little extra on each end. That's the moving mass that actually does the resonating part. Some people get this wrong and think that the air in the cavity behind is what resonates, but in reality the air in the cavity is acting as the "spring", and it's the air in the holes that does the "bouncing" on top of that spring (there's some resonance back there yes, but that air is mostly just a spring). So if you interfere with the movement of the actual slug in the hole, you get a pretty big change in the results, whereas if you interfere with the air "spring" in the cavity, the change is a lot less exciting: you need fair amount of absorption to get a big change in the cavity, but just a tiny amount to affect the air slug.

Quote:
Double winner because the unfinished MDF probably wouldn't look too good exposed so that gives it a nice touch.
Right, but you might find that you can only put cloth over some of the front surface, not the entire area, as the effect might be too much... so don't count on that as your final finish. Rather, you could put some nice looking finish fabric on a frame that is spaced away from the perf panel itself by maybe a couple of inches. (That frame should not be sealed to the box, of course.)

Quote:
I'd written these off because I assumed they would be too big for my room but I do have the chance to get some huge poly-cylindrical diffusers - would these be of any use?
Possibly... I do like polys, and use them in various forms in many studios! The ones in your link look quite nice, but it would be good if you could get the actual acoustic performance specs from the manufacturer to find out exactly what they do. I looked around the website, but could not find that at first glance, so you'll probably have to contact them to get the actual independent lab test report. If they don't have such a report, or won't give it to you, then that's a Big Red Flag! In that case, forget it and just go with your own DIY treatment. If a manufacturer won't tell you how their products perform in an acoustic lab test (or never even bothered to get them tests in the first place!), then that's not a good sign at all... if a product performs well, they should be proud of it and publish the results... Unfortunately, there's more than just a few manufacturers of "acoustic products" that are really nothing more than snake-oil dealers, so do be careful what you buy: if there's no technical data to back it up, then walk away... go elsewhere, or do it yourself.

- Stuart -
Old 13th August 2019
  #58
Gear Head
Sorry for the lack of response, I've been researching materials a lot, as well as building some more soffits! - With an interesting test (I think) results below...

Quote:
Originally Posted by Soundman2020 View Post
Another option is to angle the panel a little, so it is slightly deeper at one edge, slightly shallower at the other. That broadens the bandwidth slightly, but also lowers the Q. The simple model doesn't allow you to do that, but you can get a rough idea by modeling for the maximum depth and minimum depth, then mentally extrapolating, but with a lower peak.
That's an interesting idea. There's no way to calculate that on the perf panel absorber webpage though is there? I'll have a dig and see if theres anything I can find online.

Quote:
Originally Posted by Soundman2020 View Post
Ordinary bathroom/kitchen caulk is good for that. The type that remains soft, rubbery, and flexible even after it has fully cured. Get one that sticks like crazy and does not shrink as it dries (some do). Be generous as you apply it! Even tiny cracks can rob the device of performance... and even de-tune it.

If you drill holes bigger, you can only change the tuning upwards, to a higher frequency, and probably with a lower Q. However, you can also seal up some holes, thus reducing the overall open area, to bring the frequency down again... at the cost of efficiency.... (there's no such thing as a free lunch in acoustics! )
Great, I've picked up some flexible caulk that dries fairly quickly so hopefully that will do the trick.

Ahhh, yeh I'm starting to find that out What would be a decent way to fill the holes up? I figure if I try to build a 30 ish hz one if I mess it up completely I could always increase hole diameter to try to 'convert' it into up around 100hz.... (or whatever other frequency I need to target).

In terms of efficiency, would that decrease because the % of perforation changes? If so, is there a certain amount of perforation % I should be looking around? I have 20% in my head for some reason.

Quote:
Originally Posted by Soundman2020 View Post
Yup! Pretty impressive, isn't it, considering that it is such a small, thin, light change! TH reason this works so well is because Helmholtz resonators work on the principle of the "slug" of air trapped in the "neck" of the device (each hole, in the case of perf panel) moving more or less as a single unit of mass, and vibrating in and out of the hole, along with a little extra on each end. That's the moving mass that actually does the resonating part. Some people get this wrong and think that the air in the cavity behind is what resonates, but in reality the air in the cavity is acting as the "spring", and it's the air in the holes that does the "bouncing" on top of that spring (there's some resonance back there yes, but that air is mostly just a spring). So if you interfere with the movement of the actual slug in the hole, you get a pretty big change in the results, whereas if you interfere with the air "spring" in the cavity, the change is a lot less exciting: you need fair amount of absorption to get a big change in the cavity, but just a tiny amount to affect the air slug.
Yeah it's crazy how much it does. Just to check, the very thin cloth is ok to be touching the perf panel itself? (unlike the thicker mineral wool which severely decreases the performance if it touches the panel).

Quote:
Originally Posted by Soundman2020 View Post
Right, but you might find that you can only put cloth over some of the front surface, not the entire area, as the effect might be too much... so don't count on that as your final finish. Rather, you could put some nice looking finish fabric on a frame that is spaced away from the perf panel itself by maybe a couple of inches. (That frame should not be sealed to the box, of course.)
Yeah my mistake - it goes behind the perf panel not in front, right? Yes not sealed to the box but touching somehow is ok? I'll do some research into varnishing perf panels but I imagine this will affect the rigidity of the panel and thus change its performance!

Quote:
Originally Posted by Soundman2020 View Post
Possibly... I do like polys, and use them in various forms in many studios! The ones in your link look quite nice, but it would be good if you could get the actual acoustic performance specs from the manufacturer to find out exactly what they do. I looked around the website, but could not find that at first glance, so you'll probably have to contact them to get the actual independent lab test report. If they don't have such a report, or won't give it to you, then that's a Big Red Flag! In that case, forget it and just go with your own DIY treatment. If a manufacturer won't tell you how their products perform in an acoustic lab test (or never even bothered to get them tests in the first place!), then that's not a good sign at all... if a product performs well, they should be proud of it and publish the results... Unfortunately, there's more than just a few manufacturers of "acoustic products" that are really nothing more than snake-oil dealers, so do be careful what you buy: if there's no technical data to back it up, then walk away... go elsewhere, or do it yourself.
Agreed! I've emailed asking for info but haven't heard anything back yet. We shall see what they come back with, but it might be too big for the room anyway! Soffit results in the next comment.
Old 13th August 2019
  #59
Gear Head
So in order to try to measure the effectiveness of the DIY soffits I built I wanted to test them against the same mineral wool just left wrapped as it comes from the factory. So I ran a few more sweeps. As I'm mainly interested in the low end these are with both monitors running. I also made sure the microphone was in absolutely the correct position which is why I believe I've sorted the high end silliness in the later measurements (once I started moving the speakers).....

1. A roll of 170mm Knauf on the floor between the monitors. (This stuff: https://www.amazon.co.uk/KNAUF-Roll-.../dp/B076H4913P).

2. A DIY soffit on the floor between the monitors.

Seems to me that there is a big increase at efficiency at 178Hz, with the soffit looking like it's reducing that null by about 15dB, which looks encouraging.

So to this end, now that I have floor to ceiling soffits in most corners and rear wall/ceiling soffits on the back wall, I wanted to see what would happen if I moved my speakers forward again - would this solve the SBIR issue I was experiencing before? So......

3. Speakers moved forwards with the front soffits extended almost floor to ceiling (window wall) - no Soffit on the floor between the speakers now.

Predictably this brought back some of the low end nulls I was experiencing before (round 70 and 90Hz), but did seem to improve a lot of the low mid problems. So back against the wall we go back against the rear wall. However, the speakers had to be closer together because the front wall soffits are sitting where some of the speaker was. Which led me to thinking, how close is too close for a pair of midfield monitors? I don't have the measurements in work but I think they're about 115cm apart (tweeter to tweeter - with the tweeter being in the middle of the monitor). Listening, I'd like a little more stereo spread I think so I'll adjust this positioning more to like it was before.

4. Speakers moved back against the front wall but narrower than previous. A slab of Rockwool RW3 (60kg/m3) 28000 G.S.M straddling the floor/wall corner between them.

Now the soffits that are stopping me having the speakers that little bit wider are intended to go along the left ceiling/wall corner, where I was measuring a lot of 90Hz as I moved the mic around, so I'm hoping they will help with some of that 100Hz null that's going on.

There's also a lot of 90Hz on the rear wall which is where I'm going to place these perf panel absorbers once they're made.

Here is the REW Mdat file: https://www.dropbox.com/s/r7shisyr4m...Roll.mdat?dl=0
Attached Thumbnails
Really trying to upgrade my room-1.-l-r-170mm-roll-wrapping-freq.jpg   Really trying to upgrade my room-1.-l-r-170mm-roll-wrapping-water.jpg   Really trying to upgrade my room-2.-l-r-soffit-freq.jpg   Really trying to upgrade my room-2.-l-r-soffit-water.jpg   Really trying to upgrade my room-3.-mons-fwd-freq.jpg  

Really trying to upgrade my room-3.-mons-fwd-water.jpg   Really trying to upgrade my room-4.-rear-wall-narrow-rw3-slab-freq.jpg   Really trying to upgrade my room-4.-rear-wall-narrow-rw3-slab-water.jpg  

Last edited by Twiggy; 14th August 2019 at 10:07 AM.. Reason: Added Mdat file.
Old 13th August 2019
  #60
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Quote:
Originally Posted by Twiggy View Post
4. Speakers moved back against the rear wall ...
If the wall you face is called the front wall and the wall behind you is called the rear wall, do you have your speakers behind you, against the rear wall, or in front of you, against the front wall?
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