# semi-cylinder diffusors



## bearr48 (Jan 26, 2014)

I want to experiment with two half-cylinder diffusors 24" diameter, abt. 6' high, (Sonotube cut in half), possibly placing them at left and right ends of back wall. My question is this: Do I need to make top and bottom panels for them, to avoid some weird Helmholtz resonance effect, or can I leave them open at top and bottom without suffering any ill effect?

All opinions welcome, especially educated opinions. Thanks much.

Jack


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## NBPk402 (Feb 21, 2012)

Years ago there were plans out for the RPG cylinders and I made some. The way the plans were was to cut 4 circles (2 the same size as the circle and 2 a little bit larger), and if you wanted it 3' high you cut some chicken wire fencing to that height and then cut it to the circumference of the smaller circles you cut. Then you attach the fencing with a stapler on the one end to the smaller circle. Once that is done you line the inside of the circle with fiberglass insulation in 2 pieces...with one half of the fiberglass having the paper side facing outwards, and the other side having the paper side going inwards. Next you staple the 2nd smaller ring to the top of the wire cage, and then wrap withan acoustical transparent cloth. Once that is done you attach the top and bottom pieces. To tune you rotate the tubes to where it is paper side out or paper side in...what ever sounds best.


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## bearr48 (Jan 26, 2014)

ellisr63 said:


> Years ago there were plans out for the RPG cylinders and I made some. The way the plans were was to cut 4 circles (2 the same size as the circle and 2 a little bit larger), and if you wanted it 3' high you cut some chicken wire fencing to that height and then cut it to the circumference of the smaller circles you cut. Then you attach the fencing with a stapler on the one end to the smaller circle. Once that is done you line the inside of the circle with fiberglass insulation in 2 pieces...with one half of the fiberglass having the paper side facing outwards, and the other side having the paper side going inwards. Next you staple the 2nd smaller ring to the top of the wire cage, and then wrap withan acoustical transparent cloth. Once that is done you attach the top and bottom pieces. To tune you rotate the tubes to where it is paper side out or paper side in...what ever sounds best.


Thank you for responding! Regards, Jack


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## rhtjytku (Jan 21, 2021)

that is done you line the inside of the circle with fiberglass insulation in 2 pieces





192.168.100.1 192.168.1.1


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## T-Dub (Dec 11, 2020)

ellisr63 said:


> Years ago there were plans out for the RPG cylinders and I made some. The way the plans were was to cut 4 circles (2 the same size as the circle and 2 a little bit larger), and if you wanted it 3' high you cut some chicken wire fencing to that height and then cut it to the circumference of the smaller circles you cut. Then you attach the fencing with a stapler on the one end to the smaller circle. Once that is done you line the inside of the circle with fiberglass insulation in 2 pieces...with one half of the fiberglass having the paper side facing outwards, and the other side having the paper side going inwards. Next you staple the 2nd smaller ring to the top of the wire cage, and then wrap withan acoustical transparent cloth. Once that is done you attach the top and bottom pieces. To tune you rotate the tubes to where it is paper side out or paper side in...what ever sounds best.


I give the OP a ton of credit if these directions made any sense to him. I’m just not seein it. That may lend to my stupidity tho hahaha


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## fokakis1 (Feb 29, 2012)

I agree with the fiberglasss. Make your "poly-fusers" and stuff them with roxul or OC and they will serve double purpose - diffuser with bass trapping. I would look to GIK for proper dimensions as this is critical. I built several for a 2 channel room and the results were impressive. Evolution PolyFusor © - Sound Diffuser / Absorber Combination


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## AEIOU (May 3, 2011)

bearr48 said:


> All opinions welcome, especially educated opinions. Thanks much.
> 
> Jack


If they are sitting on the floor, then technically one end will be closed. There were, are many plans available for making your own DIY diffusors, bass traps etc. Look for them, follow them.


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## NBPk402 (Feb 21, 2012)

T-Dub said:


> I give the OP a ton of credit if these directions made any sense to him. I’m just not seein it. That may lend to my stupidity tho hahaha


Very simple to make. Here how about this... just cut 2 circles out of wood, Get some chicken wire screen (lets say 3'x3'). Take one of the circles, and take the screen...then staple the screen to the edge of the circle. Staple it all the way arourd the circle and cut off the excess screen. Now you have what looks like a wire trash can with a wood bottom. Next you take the fiberglass insulation (or Roxul rockboard), and line the inside of the basket you made. Next you insert he 2nd wood circle on the top and staple. Last step is staple material around your basket. Now you are done and just saved yourself hundreds of dollars on buying one from ASC.


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## bearr48 (Jan 26, 2014)

fokakis1 said:


> I agree with the fiberglasss. Make your "poly-fusers" and stuff them with roxul or OC and they will serve double purpose - diffuser with bass trapping. I would look to GIK for proper dimensions as this is critical. I built several for a 2 channel room and the results were impressive. Evolution PolyFusor © - Sound Diffuser / Absorber Combination


Thanks, I'll check it out.


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## bearr48 (Jan 26, 2014)

AEIOU said:


> If they are sitting on the floor, then technically one end will be closed. There were, are many plans available for making your own DIY diffusors, bass traps etc. Look for them, follow them.


Thanks, will start looking.


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## AVoldMan (May 15, 2011)

bearr48 said:


> I want to experiment with two half-cylinder diffusors 24" diameter, abt. 6' high, (Sonotube cut in half), possibly placing them at left and right ends of back wall. My question is this: Do I need to make top and bottom panels for them, to avoid some weird Helmholtz resonance effect, or can I leave them open at top and bottom without suffering any ill effect?


There may be some resonance effect as you have questioned and the absorption with fill material should reduce that risk. One quick method (assuming a REW measurement setup is available) to see if this might be an audible problem is to measure with and without the half rounds in place and look at the frequency spectrum for any peaks or dips. 

Your interest in using the empty half round tubes for diffusors would probably have some audible effect depending on your placement. The back of the room at the first reelection point of the front speaker to the listener position might be a good spot to try. Once again for evaluating this purpose looking at REW ETC data may give you a clue as to any measurable efforts whether positive or negative. If you have a reflective room you may not "see" anything in the ETC as you make changes (with and without tubes). 

I have a tall (about 7 foot) glass front Ikea book case behind the LP and have measured both frequency spectrum changes as well as ETC changes depending on the opening of the doors at different angles and being closed probably due to the reflections from surface and cabinet volume resonances. As with most things the changes may be small or masked by other effects but under the right conditions they are measurable. Now the big question is how does it affect the sound? This is the most important question and takes a lot of careful listening with constant objective measurements.

Let us know if you try some experiments and your outcomes either objective measurements or subject listening tests. Good luck, nothing is really easy in audio.


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## bearr48 (Jan 26, 2014)

AVoldMan said:


> There may be some resonance effect as you have questioned and the absorption with fill material should reduce that risk. One quick method (assuming a REW measurement setup is available) to see if this might be an audible problem is to measure with and without the half rounds in place and look at the frequency spectrum for any peaks or dips.
> 
> Your interest in using the empty half round tubes for diffusors would probably have some audible effect depending on your placement. The back of the room at the first reelection point of the front speaker to the listener position might be a good spot to try. Once again for evaluating this purpose looking at REW ETC data may give you a clue as to any measurable efforts whether positive or negative. If you have a reflective room you may not "see" anything in the ETC as you make changes (with and without tubes).
> 
> ...


Thanks, I appreciate your input. As with so many other things (speaker placement, subwoofer integration, etc), REW is my friend...but a substantial learning curve is involved. "The true path is never easy".


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## fokakis1 (Feb 29, 2012)

The decay plot in REW is an even better friend when dealing with absorption and diffusion.


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