Combined broadband volume diffusor and bass absorber
To be of real practical use, diffusers are wanted efficient over a broad frequency range and bass absorbers should (usually / often) only be wanted efficient in the low range. Mid and high frequencies are wanted scattered / diffused in the room, not absorbed creating a “dead sounding room”. A combination will require large width and depth. That combination can be accomplished with volumetric diffusers integrated into a deep bass absorber. If one has a long room and can spare the depth needed, both low bass modes can be treated while having a diffused “live” sound for mids and high frequencies. If one uses surround speakers / subs at the back wall, these can be integrated too.
In a document for Degree of Doctor of Philosophy at Salford University’s faculty for acoustics, Richard James Hughes presents his findings för Volume Diffusers in comparison to other type of diffusers like BAD, Schroeder, MLS etc. In short, the volume ones show better results. The paper is loaded with heavy math but it is not necessary to understand all that math, one gets by quite well from the text alone and the diagrams. The original paper is included and a “compilation” I made. In the paper "Geometric Shadow Zone and Back Scattering" is mentioned with a bunch of math. In the compilation some pictures for this is included. BADs aim for 50% reflecting areas and 50% absorbtive areas. Volume diffusers are based on a Golomb Ruler (google for explanation) and like BADs aim for a total of 50% reflecting area but for for 50% transparency (gaps between slats), seen from straight ahead. BADs have no temporal diffusion, volume diffusers have that. A bunch of identical Schroeder diffusers beside each other, forming a long total width, will give lobing. There will be no/less of lobing with volume diffusers. In short the volume diffuser consist of, say 5 layers / fences, behind each other. Each fence has a different slat width, narrowest at the front and widest at the back.
Negative about broadband volume diffusers with several layers:
They need to be large in length and depth.
Positive about broadband volume diffusers with several layers:
As they need to be large in length and deep, they are suitable to integrate with efficient bass absorbers and back wall speakers.
They are a lot easier / less time consuming to DIY with fewer and simple tools than say BADs, Schroeder or “skyline” diffusers.
If one wants to, one can integrate LED-strips for visual effects behind the slats.
Hughes gives a nice example for the construction of a complete volume diffuser to reach the wanted 50 % transparency and which has a broad range from 400 to 3378 Hz. I took that example and wrote an excel spread sheet from it. From the excel, one can to a certain amount customize other widths. Ideally: Start with total possible width, the 2 slats at the back should together form 1/3 of the diffuser width. Then keep the width ratios in the excel from back to front intact to keep 50% transparency. Check out carefully on the “drawing” where the slat should be positioned for 50% transparency. Example there are 3 slat 180 mm wide versus totalt width 1800 mm. It is quite easy to count 3x180 mm slats and 3 gaps with w= 180mm. As the chosen “N” = 10 x 180 mm = 1800 mm, it is easy to conclude that the largest gap in that “fence” is then 10-6 = 4x180 mm wide. One can chose other “N” depending on available width and depth. That will be a compromise and change the 50% transparency. If available width is small, the slats at the front will be very narrow with 5 “fences”. Diffusion at very high frequencies is quite useless unless you are a dog or a bat. I would use fewer fences in that case and skip the first at the front.
From some scrap wood I made some which are 70 cm wide x 30 cm deep x 120 cm high. I put some hinges on them so they can be swiveled sideways to have some control of where to steer the reflection in the room, towards listening position or towards side wall for longer delay time. (70 cm was a bit too wide, so I had to cut off a corner for a clearance vs the sub.)
The diffuser could start at about 600 mm above floor level and be 1-1,2 m high. Above and below and to the sides, there is place for bass absorbers. For integration with a good and efficient bass / slat absorber at the back wall I suggest to check out the thread at Gearspace: https://gearspace.com/board/studio-building-acoustics/1314238-different-helmholtz-slat-sizes.html and ostfisk’s video on Youtube:
(very informative on details).
To open the excelfile Volume diffuser-Adhoc, change the ending to xls (I hope that will work)
Edit: No it didn't 🙁 Someone who knows how to fix that, so it can be used?
To be of real practical use, diffusers are wanted efficient over a broad frequency range and bass absorbers should (usually / often) only be wanted efficient in the low range. Mid and high frequencies are wanted scattered / diffused in the room, not absorbed creating a “dead sounding room”. A combination will require large width and depth. That combination can be accomplished with volumetric diffusers integrated into a deep bass absorber. If one has a long room and can spare the depth needed, both low bass modes can be treated while having a diffused “live” sound for mids and high frequencies. If one uses surround speakers / subs at the back wall, these can be integrated too.
In a document for Degree of Doctor of Philosophy at Salford University’s faculty for acoustics, Richard James Hughes presents his findings för Volume Diffusers in comparison to other type of diffusers like BAD, Schroeder, MLS etc. In short, the volume ones show better results. The paper is loaded with heavy math but it is not necessary to understand all that math, one gets by quite well from the text alone and the diagrams. The original paper is included and a “compilation” I made. In the paper "Geometric Shadow Zone and Back Scattering" is mentioned with a bunch of math. In the compilation some pictures for this is included. BADs aim for 50% reflecting areas and 50% absorbtive areas. Volume diffusers are based on a Golomb Ruler (google for explanation) and like BADs aim for a total of 50% reflecting area but for for 50% transparency (gaps between slats), seen from straight ahead. BADs have no temporal diffusion, volume diffusers have that. A bunch of identical Schroeder diffusers beside each other, forming a long total width, will give lobing. There will be no/less of lobing with volume diffusers. In short the volume diffuser consist of, say 5 layers / fences, behind each other. Each fence has a different slat width, narrowest at the front and widest at the back.
Negative about broadband volume diffusers with several layers:
They need to be large in length and depth.
Positive about broadband volume diffusers with several layers:
As they need to be large in length and deep, they are suitable to integrate with efficient bass absorbers and back wall speakers.
They are a lot easier / less time consuming to DIY with fewer and simple tools than say BADs, Schroeder or “skyline” diffusers.
If one wants to, one can integrate LED-strips for visual effects behind the slats.
Hughes gives a nice example for the construction of a complete volume diffuser to reach the wanted 50 % transparency and which has a broad range from 400 to 3378 Hz. I took that example and wrote an excel spread sheet from it. From the excel, one can to a certain amount customize other widths. Ideally: Start with total possible width, the 2 slats at the back should together form 1/3 of the diffuser width. Then keep the width ratios in the excel from back to front intact to keep 50% transparency. Check out carefully on the “drawing” where the slat should be positioned for 50% transparency. Example there are 3 slat 180 mm wide versus totalt width 1800 mm. It is quite easy to count 3x180 mm slats and 3 gaps with w= 180mm. As the chosen “N” = 10 x 180 mm = 1800 mm, it is easy to conclude that the largest gap in that “fence” is then 10-6 = 4x180 mm wide. One can chose other “N” depending on available width and depth. That will be a compromise and change the 50% transparency. If available width is small, the slats at the front will be very narrow with 5 “fences”. Diffusion at very high frequencies is quite useless unless you are a dog or a bat. I would use fewer fences in that case and skip the first at the front.
From some scrap wood I made some which are 70 cm wide x 30 cm deep x 120 cm high. I put some hinges on them so they can be swiveled sideways to have some control of where to steer the reflection in the room, towards listening position or towards side wall for longer delay time. (70 cm was a bit too wide, so I had to cut off a corner for a clearance vs the sub.)
The diffuser could start at about 600 mm above floor level and be 1-1,2 m high. Above and below and to the sides, there is place for bass absorbers. For integration with a good and efficient bass / slat absorber at the back wall I suggest to check out the thread at Gearspace: https://gearspace.com/board/studio-building-acoustics/1314238-different-helmholtz-slat-sizes.html and ostfisk’s video on Youtube:
To open the excelfile Volume diffuser-Adhoc, change the ending to xls (I hope that will work)
Edit: No it didn't 🙁 Someone who knows how to fix that, so it can be used?
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That excel spread sheet I tried to add in the above post can now be down loaded on Gearspace forum. I added a cell (J13) for any other diffuser width than 1800 mm, -if one wants it smaller or larger, while still aiming for a 50% transparency diffuser according to Hughes doctors thesis.
Link: https://gearspace.com/board/showpost.php?p=16280575&postcount=11
Link: https://gearspace.com/board/showpost.php?p=16280575&postcount=11