I tried that once. After this was taken I put heavy pleated double drapes over the wall with a gap behind them, and along the sides. The rockwool is 200-300mm thick at the ceiling with a gap behind it. There were more at the ceiling reflection points.
To hit 50hz:
Panel absorber as in mh-audio.nl,
200cmx50cmx12cm with a 12,5kg panel: 51,4hz resonnant freq by itself ( without porous absorber in the box), 42,8hz with.
One thing about most acoustic treatments when they are effective at a freq range, they'll most probably have the inverse effect for others. And beware it is tuned, so better located where it is effective and you might need other tuned ones if other modes are of concern: they are not as wideband as VPR. You should use by multiple of two to garanty symmetry of room ( less a concern behind listening point but still...).
Iow a membrane absorber will be reflective outside it's bandwidth of interest.
It means better have them behind your listening point and if possible not on rearwall ( better locate diffusion there imho).
If they have to be located on rear wall, then it is worth bending a piece of material ( small depth plywood) in arc shape and locate it in front of the panel ( without closing upper and bottom parts): you have a cylindrical diffusor for mid/high as well as basstrap.
Correction about Northward Acoustics: it 'Front to Back' principle, not 'Progressive Acoustic' which is the approach used by a French acoustician ( J.-C. B., designer of 'Zippy' loudspeakers too and a great electronician)
Panel absorber as in mh-audio.nl,
200cmx50cmx12cm with a 12,5kg panel: 51,4hz resonnant freq by itself ( without porous absorber in the box), 42,8hz with.
One thing about most acoustic treatments when they are effective at a freq range, they'll most probably have the inverse effect for others. And beware it is tuned, so better located where it is effective and you might need other tuned ones if other modes are of concern: they are not as wideband as VPR. You should use by multiple of two to garanty symmetry of room ( less a concern behind listening point but still...).
Iow a membrane absorber will be reflective outside it's bandwidth of interest.
It means better have them behind your listening point and if possible not on rearwall ( better locate diffusion there imho).
If they have to be located on rear wall, then it is worth bending a piece of material ( small depth plywood) in arc shape and locate it in front of the panel ( without closing upper and bottom parts): you have a cylindrical diffusor for mid/high as well as basstrap.
Correction about Northward Acoustics: it 'Front to Back' principle, not 'Progressive Acoustic' which is the approach used by a French acoustician ( J.-C. B., designer of 'Zippy' loudspeakers too and a great electronician)
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no worry, i like to experiment, that is some of the fun part of a project. but i must say that panels scares me a bit, they seems to be very difficult to get right and they seems difficult to tune too, helmholtz absorber on the other hand seems easier to build and tune. i must also say that i am mostly interested in freestanding movable items, i am not particularly fond of building any larger structures that are attached to walls and ceilings and so on, i am more into build stuff in my shop, carry them in the room, try to find a good location and do some tuning, if they do not work then out they go, and no harm is done to the room that needs to be restored
I get it but: helmotz are the most difficult and time consumming to do ( there is always something to adapt and it's not intuitive to me, you'll need at least 3 or 4 prototypes ime).
You fear panels but it's the most simple... as long as you don't try to do things they can't do! 😉
If you want to treat 50hz in low bulk , there is no escape it will have to be on a wall...
Tbh, the panel absorbers i gave will not weight more than 20kg finished and should be easily moved.
If you got the wood availlable then it would not be difficult ( a bit of MLV to bring the weight up, some glass dedicated silicone and a bit of fabric and it is done.
You fear panels but it's the most simple... as long as you don't try to do things they can't do! 😉
If you want to treat 50hz in low bulk , there is no escape it will have to be on a wall...
Tbh, the panel absorbers i gave will not weight more than 20kg finished and should be easily moved.
If you got the wood availlable then it would not be difficult ( a bit of MLV to bring the weight up, some glass dedicated silicone and a bit of fabric and it is done.
Effectively absorbing something like 50 Hz is impossible in a small / normal room with resistive absorbers, -if you at the same time wish to have some wiggle room and also avoid damping too much of midrange and higher frequencies.
Panel / membrane absorbers along the walls requires much less size and volume to be effective at low frequencies.
The pictures show a tuneable prototype I made from scrap material. The idea is simple, it is the same as a slave bass driver = A soft flexible surround + a stiff membrane which can be weighted down so the box can be tuned to the wanted resonance frequency. The black part is very soft rubber (like a regular rubber band), the panel is veneered MDF 0,704 kg/m², 11 mm thick, the 3 long screws are used for adding weight to the panel and are sealed off with o-rings + washers + nuts. The grey cylinder on top is a cable gland for Ø 12 mm = the same as a REW microphone. The cable gland is classified as IP 67 = water tight to 1 m below water. Position of it places the microphone 1 cm behind the frontpanel / membrane and 2 cm in front of glass wool inside the box.
So, by playing a bass sweep and measuring with REW, the microphone will show the resonance frequency of the box and by adding weight to the panel the resonance will go lower in frequency. The back is 22 mm particle board sealed with screw and rubber strips, so if the tuning is too low in frequency the back can be taken off. One can then saw off a cm or two to decrease the internal depth and by this increase the tuning frequency while having the front intact.
The box is ugly, after all it is only a prototype for fine tuning to a specific frequency which means: What weight / surface area do I need with this particular size of surround material together with the depht behind the the panel and the glass wool inside the box? For some later nice looking boxes it is important to not change width of the surround or its material or depth or amount of glass wool inside. One can for later ready made boxes have a nice looking fabric in front with a little air gap.
MLV (mass loaded vinyl) has a surface weight of 3 kg / m² and upwards depending on thickness and added filler material (barium sulphate) and it is expensive. If one uses regular vinyl flooring (solid not foamed) 1 mm thick, it has a surface weight of roughly 1 kg/m². Left overs can be bought for cheap at any carpet store. I would use 1 large layer 1 mm thick vinyl flooring as membrane giving a surround width of about 2-4 cm and then add layers with a smaller surface area on top. Use double adhesive tape for the extra layers and check the resonance for a 1st box. For later boxes, glue could be used for the extra layers. The extra layers could be any material, steel, rubber, vinyl, MDF, etc but a flexible soft material would be preferred for a limp membrane with little / nothing of its own resonance.
A ps: I made an identical box with the same veneered MDF sheet as front but without the rubber surround. If I push the one with the rubber surround large puffs of air can be felt at the cable gland and next to nothing for the one without the surround. To be really effective the panel on a panel / membrane absorber must be able to move freely!
Panel / membrane absorbers along the walls requires much less size and volume to be effective at low frequencies.
The pictures show a tuneable prototype I made from scrap material. The idea is simple, it is the same as a slave bass driver = A soft flexible surround + a stiff membrane which can be weighted down so the box can be tuned to the wanted resonance frequency. The black part is very soft rubber (like a regular rubber band), the panel is veneered MDF 0,704 kg/m², 11 mm thick, the 3 long screws are used for adding weight to the panel and are sealed off with o-rings + washers + nuts. The grey cylinder on top is a cable gland for Ø 12 mm = the same as a REW microphone. The cable gland is classified as IP 67 = water tight to 1 m below water. Position of it places the microphone 1 cm behind the frontpanel / membrane and 2 cm in front of glass wool inside the box.
So, by playing a bass sweep and measuring with REW, the microphone will show the resonance frequency of the box and by adding weight to the panel the resonance will go lower in frequency. The back is 22 mm particle board sealed with screw and rubber strips, so if the tuning is too low in frequency the back can be taken off. One can then saw off a cm or two to decrease the internal depth and by this increase the tuning frequency while having the front intact.
The box is ugly, after all it is only a prototype for fine tuning to a specific frequency which means: What weight / surface area do I need with this particular size of surround material together with the depht behind the the panel and the glass wool inside the box? For some later nice looking boxes it is important to not change width of the surround or its material or depth or amount of glass wool inside. One can for later ready made boxes have a nice looking fabric in front with a little air gap.
MLV (mass loaded vinyl) has a surface weight of 3 kg / m² and upwards depending on thickness and added filler material (barium sulphate) and it is expensive. If one uses regular vinyl flooring (solid not foamed) 1 mm thick, it has a surface weight of roughly 1 kg/m². Left overs can be bought for cheap at any carpet store. I would use 1 large layer 1 mm thick vinyl flooring as membrane giving a surround width of about 2-4 cm and then add layers with a smaller surface area on top. Use double adhesive tape for the extra layers and check the resonance for a 1st box. For later boxes, glue could be used for the extra layers. The extra layers could be any material, steel, rubber, vinyl, MDF, etc but a flexible soft material would be preferred for a limp membrane with little / nothing of its own resonance.
A ps: I made an identical box with the same veneered MDF sheet as front but without the rubber surround. If I push the one with the rubber surround large puffs of air can be felt at the cable gland and next to nothing for the one without the surround. To be really effective the panel on a panel / membrane absorber must be able to move freely!
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for easy to build helmholtz absorbers i thought of sonotubes, about 100 litres each and as many ports and length as the design allows, by changing port length the absorber is easily tuned. from what i have read helmholtz absorbers are tricky to get in action, solution seems to having the port(s) to cover as much area as possible, but better to use a long slot then a single large point
If you are close to Gothenburg-Sweden, you can take over the prototypes. I made them out of curiosity and for experiments only and have no real use for them really. They will end up in the recycling container next week or so.
thank you for your kind offer, i am relative close but still too far to pick em up, but thank you anyway 🙂
regarding panels absorbers, i thougt the working principle to be to be quite the same for all but that does not seems to be the case, some uses a heavy plate between two absorbers and straddle them in room corners, other use a panel over an airtight enclosure mounted on walls. so, would an leaky panel with box work also? say two sheets of masonite sandwiched between 10-20cm lowest density mineral wool and mounted to walls?
"To be really effective membrane/panel absorbers must be able to move freely!"
Yes. But does it need to have a suspension with this much movement allowed as a loudspeaker?
I mean the windows ( and the stuctural walls) in the last control room i've been involved with relyed on the silicone joint as the suspension*, and the walls i described in previous message on 2x 2mm depth neoprene strip ( one at ceiling one on floor).
The amplitude of movement is related to energy which have to be dissipated in my understanding, but maybe i'm wrong.
May i suggest to keep things simple as a start Celef. Multilayered material can bring more effectiveness ( more often than not to widen the bandwidth of treatments) but the initial 'simple' principle are the one which matters the most.
As you are experimenting and prototyping ( which is imho the best way to make your own mind) base this experiment on already prooven answers/solution before switching to complicated (needing tuning with different materials, holes whatever) stuff.
I've been quickly fed up of shot in the dark in my initial efforts with this. Such a relief when i saw there was some formulae and theorical understanding to be able to reach at least a 'ball park' results.
* the glass's frame was someting like 1,5/2cm larger than the glass itself and same depth as the glass ( laminated double layer glass with a special plastic film sandwiched to extend acoustic properties), maintened by wedges before application of this kind of silicon:
https://www.amazon.fr/Rubson-Mastic-Vitrier-Silicone-Blanc/dp/B0092MEEYA
The one used wasn't this one ( we brought it directly from the glass maker, it was their dedicated stuff for heavy use in outdoor condition: it doesn't stiffen over time and is one of the more sticky stuff i ever seen).
It was aplyied in multiple pass ( layers) of maybe 2mm each max, left curated then reapeated until cavities was fully filled ( doing parts between wedge, wait them to cure, pull wedge out, do it again below them... egyptian pyramid build i tell you!). Took ages to be done but once in place it was very strong ( the glass was something like 50kg by itself- one side as it was 'dual leaf' built) and still allowed a bit of movement.
This is what i had in mind for the 'suspension' of panel. Maybe not as extreme but same principle.
Yes. But does it need to have a suspension with this much movement allowed as a loudspeaker?
I mean the windows ( and the stuctural walls) in the last control room i've been involved with relyed on the silicone joint as the suspension*, and the walls i described in previous message on 2x 2mm depth neoprene strip ( one at ceiling one on floor).
The amplitude of movement is related to energy which have to be dissipated in my understanding, but maybe i'm wrong.
May i suggest to keep things simple as a start Celef. Multilayered material can bring more effectiveness ( more often than not to widen the bandwidth of treatments) but the initial 'simple' principle are the one which matters the most.
As you are experimenting and prototyping ( which is imho the best way to make your own mind) base this experiment on already prooven answers/solution before switching to complicated (needing tuning with different materials, holes whatever) stuff.
I've been quickly fed up of shot in the dark in my initial efforts with this. Such a relief when i saw there was some formulae and theorical understanding to be able to reach at least a 'ball park' results.
* the glass's frame was someting like 1,5/2cm larger than the glass itself and same depth as the glass ( laminated double layer glass with a special plastic film sandwiched to extend acoustic properties), maintened by wedges before application of this kind of silicon:
https://www.amazon.fr/Rubson-Mastic-Vitrier-Silicone-Blanc/dp/B0092MEEYA
The one used wasn't this one ( we brought it directly from the glass maker, it was their dedicated stuff for heavy use in outdoor condition: it doesn't stiffen over time and is one of the more sticky stuff i ever seen).
It was aplyied in multiple pass ( layers) of maybe 2mm each max, left curated then reapeated until cavities was fully filled ( doing parts between wedge, wait them to cure, pull wedge out, do it again below them... egyptian pyramid build i tell you!). Took ages to be done but once in place it was very strong ( the glass was something like 50kg by itself- one side as it was 'dual leaf' built) and still allowed a bit of movement.
This is what i had in mind for the 'suspension' of panel. Maybe not as extreme but same principle.
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great posts!
for loudspeakers and a certain spl we can either choose big cones and have a short stroke or a small cone with long stroke or multiple small cones with short strokes, can this thinking be applied to panels?
so a huge sized panel must not move much but a small panel do?
and how much room area needs to be treated for 50 hz?
for loudspeakers and a certain spl we can either choose big cones and have a short stroke or a small cone with long stroke or multiple small cones with short strokes, can this thinking be applied to panels?
so a huge sized panel must not move much but a small panel do?
and how much room area needs to be treated for 50 hz?
I agree Adhoc1 posts i always read very carefuly!
Yes it can be scaled down by increasing weight of membrane ( for a given freq) but it'll have drawbacks as location will have to be more accurate than a larger one.
How much area is difficult to say as it depend of mode intensity ( and why cubic shaped room are so difficults)... which bring us back to measurement and 'pressure polar map'.
I've always seen this kind of treatments by mutiple of 2.
Yes it can be scaled down by increasing weight of membrane ( for a given freq) but it'll have drawbacks as location will have to be more accurate than a larger one.
How much area is difficult to say as it depend of mode intensity ( and why cubic shaped room are so difficults)... which bring us back to measurement and 'pressure polar map'.
I've always seen this kind of treatments by mutiple of 2.
🙂
It is a method to measure your room and define it's real behavior in the low end.
The idea is to take a serie of measurements (at predifined location using a sweep) to determine where to locate your bass trap.
This way you can expect them to have best efficiency and there is no trial and errors as you know which freq and where to locate.
So how to define location: make an above view plan of your room then you divide it by 30cm lines in width and length. As outcome you have your floorplan divided in 30 cm side squares. Each crossing line is a point of measurement.
From there we have a basis for elevation ( height) for which we will have to do the same....
Makes a huge number of measurements! Yes i agree but as we are only interested in what happen on proximity of walls ( and maybe ceiling) and we have a design criteria to make the treatments not too bulky we can greatly limits them to 30cm and 60cm from each walls.
Here is a video which explains it ( sorry it's in french and the guy have a strong accent, but subtitles should work, if not just watch it and tell me if things are unclear, i could help translate):
I've done other much coarse technique:
If you identified the frequency of issue at listening point ( let's say 50hz) then you play it from loudspeakers. With a microphone and a RTA ( Real Time Analyzer) you monitor the 50hz level while slowly moving the mic along the walls trying to keep a constant distance (to the wall) and height ( i divide by layers in height). It'll be easy to identify area where level increase. You have then a basis to fine tuning using a pressure map or a constricted range for trial and errors.
I've tried a panel absorber of 1m x 50cm with 10kg panel and 7cm depth: 50,7hz in simulator ( non damped using absorbers). I would use 4 of them.
What kind of material are your walls? Plasterboard?
Roommap ( works with REW and is free):
https://github.com/Tunetown/RoomMap
It is a method to measure your room and define it's real behavior in the low end.
The idea is to take a serie of measurements (at predifined location using a sweep) to determine where to locate your bass trap.
This way you can expect them to have best efficiency and there is no trial and errors as you know which freq and where to locate.
So how to define location: make an above view plan of your room then you divide it by 30cm lines in width and length. As outcome you have your floorplan divided in 30 cm side squares. Each crossing line is a point of measurement.
From there we have a basis for elevation ( height) for which we will have to do the same....
Makes a huge number of measurements! Yes i agree but as we are only interested in what happen on proximity of walls ( and maybe ceiling) and we have a design criteria to make the treatments not too bulky we can greatly limits them to 30cm and 60cm from each walls.
Here is a video which explains it ( sorry it's in french and the guy have a strong accent, but subtitles should work, if not just watch it and tell me if things are unclear, i could help translate):
I've done other much coarse technique:
If you identified the frequency of issue at listening point ( let's say 50hz) then you play it from loudspeakers. With a microphone and a RTA ( Real Time Analyzer) you monitor the 50hz level while slowly moving the mic along the walls trying to keep a constant distance (to the wall) and height ( i divide by layers in height). It'll be easy to identify area where level increase. You have then a basis to fine tuning using a pressure map or a constricted range for trial and errors.
I've tried a panel absorber of 1m x 50cm with 10kg panel and 7cm depth: 50,7hz in simulator ( non damped using absorbers). I would use 4 of them.
What kind of material are your walls? Plasterboard?
Roommap ( works with REW and is free):
https://github.com/Tunetown/RoomMap
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One thing which is great with REW/ROOMMAP is you don't have to empty room if you have furniture. That is why i include 60cm measurements.
The 30cm dimension set up resolution limit of measurement of 250/300hz iirc. This should cover up to Schroeder frequency and a little above.
About polar pressure map:
you never seen one before because you didn't have to deal with an acoustician: they make simulations like this to have an idea of what to expect in real life. Then they confirm once build is done by measurements. These are included ( as well as others analysis) in a manual when room is purchased/finished.
The 30cm dimension set up resolution limit of measurement of 250/300hz iirc. This should cover up to Schroeder frequency and a little above.
About polar pressure map:
you never seen one before because you didn't have to deal with an acoustician: they make simulations like this to have an idea of what to expect in real life. Then they confirm once build is done by measurements. These are included ( as well as others analysis) in a manual when room is purchased/finished.
Gearspace has a lot of information, but the information there is for studio mixing room which is drier than average domestic room. It's OK if you're are listen to mostly studio recordings, but classical recordings are considered to sound better (subjectively) in more lively room. The bass management is always about compromise and preference, and there is no correct answer, I mean there is no absolutely right nor wrong.
krivium: i thank you very much for your posts, i appreciete them a lot 🙂
i will try to do some pressure maps as soon i get a chance, namely when my family and the neighbors is out
i will try to do some pressure maps as soon i get a chance, namely when my family and the neighbors is out
yes, gearspace has a lot of info, even art noxon, the inventor of tube traps, has posted over there, i find that to be very cool 🙂