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Author Topic: Cathedral Roof Bass Trap  (Read 6316 times)

ruffrecords

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Cathedral Roof Bass Trap
« on: October 25, 2006, 06:06:38 pm »

My studio is an old flint outbuilding with a cathedral roof that must be at least 15ft high at the apex. As bass traps work well in corners it occurred to me that the longest expanse of corner I have is the length of the apex of the cathedral roof (24ft long). This is unused space so would it be a good idea to build one long bass trap here - I could lose 3 or 4 feet off the apex and still have 11ft ceilings. What do you think?

Ian
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franman

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Re: Cathedral Roof Bass Trap
« Reply #1 on: October 25, 2006, 07:50:03 pm »

I think this could be a very good place to "cut the corner" as we discuss here regularly. .. Yes, give up three feet of space, frame out a flat cover.. Hang some Insulation blankets up in the space and friction fit some Rigid Insulation into the framing. Stretch fabric over the whole shebang and voila!! Broadband bass trapping... Of course you could do many membrane traps of Helmholtz resonators tuned for different low frequencies above the framing instead, or... you get the idea... but do it. It'll tighten up the bass and without exposing tooooo much surface area of absorption you won't deaden the top end too much.. I'm assuming this is a performance room???? Am I correct?
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ruffrecords

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Re: Cathedral Roof Bass Trap
« Reply #2 on: October 26, 2006, 05:17:19 am »

franman wrote on Thu, 26 October 2006 00:50

I think this could be a very good place to "cut the corner" as we discuss here regularly. .. Yes, give up three feet of space, frame out a flat cover.. Hang some Insulation blankets up in the space and friction fit some Rigid Insulation into the framing. Stretch fabric over the whole shebang and voila!! Broadband bass trapping...

Excellent, sounds like a really elegant solution. I am not sure what you mean by 'Insulation blankets' - are we talking loft insulation or bedding  Laughing  
Quote:

 I'm assuming this is a performance room???? Am I correct?

Performance and control in one. Presumably in bass terms what's good for one is good for the other?

Ian
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franman

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Re: Cathedral Roof Bass Trap
« Reply #3 on: October 27, 2006, 09:20:42 pm »

ruffrecords wrote on Thu, 26 October 2006 05:17

franman wrote on Thu, 26 October 2006 00:50

I think this could be a very good place to "cut the corner" as we discuss here regularly. .. Yes, give up three feet of space, frame out a flat cover.. Hang some Insulation blankets up in the space and friction fit some Rigid Insulation into the framing. Stretch fabric over the whole shebang and voila!! Broadband bass trapping...

Excellent, sounds like a really elegant solution. I am not sure what you mean by 'Insulation blankets' - are we talking loft insulation or bedding  Laughing  
Quote:

 I'm assuming this is a performance room???? Am I correct?

Performance and control in one. Presumably in bass terms what's good for one is good for the other?

Ian



Insulation not bedding!! LOL.... Yes as far as the bass control being good for both.
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ruffrecords

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Re: Cathedral Roof Bass Trap
« Reply #4 on: October 30, 2006, 03:33:48 pm »

OK I have ordered the Rockwool; just need a sanity check to make sure I am pointed basically in the right direction.

There is a convenient cross beam about 40 inches down from the apex of the cathedral roof so I am going to use this (and others similarly positioned) as the ends of the frame. This means the frame (and the bass trap) will be 2m (6ft 6ins wide). There are convenient beams which cut the length into two handy lengths of about 9ft. So basically I am planning to build two roof bass traps each 9ft long, 6ft 6in wide and 40 in deep. I intend to use 4in of Rockwool RW3 (60Kgm/cm or 4pcf) all the way along then 8in of regular roofing fibreglass insulation throughout.

I have done some room measurements and there seems to be significant resonances at the mix position of around 42Hz, 58Hz, 90Hz, 105Hz, 120Hz, 135Hz, 150Hz, 185Hz, 260Hz, 280Hz, 315Hz, 340Hz, 380Hz, 400Hz, 440Hz and 460Hz. According to my calculator 40in is a quarter wavelength at about 85Hz so it looks like I am in the right ball park.

Any comments?

Cheers

Ian
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franman

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Re: Cathedral Roof Bass Trap
« Reply #5 on: October 30, 2006, 07:16:51 pm »

I would hang the regular insulation batts from above, as opposed to laying them on top of the rigid... This ensures that the cavity is relatively full (over time) with porous absoprtion, and it would settle rather quickly if laid directly on top of the rigid and framing. Make sense?

Yes, 40.8" would be 1/4 wavelength at 83 Hz... you're punching the right bag!!

How did you "measure" the resonances at these frequencies?? Sound level meter??
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ruffrecords

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Re: Cathedral Roof Bass Trap
« Reply #6 on: October 31, 2006, 03:38:24 am »

franman wrote on Tue, 31 October 2006 00:16

I would hang the regular insulation batts from above, as opposed to laying them on top of the rigid... This ensures that the cavity is relatively full (over time) with porous absoprtion

OK, understood.
Quote:

Yes, 40.8" would be 1/4 wavelength at 83 Hz... you're punching the right bag!!

How did you "measure" the resonances at these frequencies?? Sound level meter??

ATM10 omni mic at mix position via dbx286 mic pre into Ferrograph RTS2 test set audio millivoltmeter. RTS2 oscillator out direct to power amp into Tannoy monitor golds.

Ian
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franman

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Re: Cathedral Roof Bass Trap
« Reply #7 on: October 31, 2006, 10:00:14 pm »

Just be careful "measuring" modes this way... if you move the mic 1 foot things will change. You really have to listen to confirm what you hear makes sense with what you measure... It is natural (and necessar) for any pure sine wave tone to behave as peaks and valleys throughout a room.. It's the distribution of the modal peaks and valleys that makes a room sound natural in the LF..

MOTS: just because you measure peaks in one spot with pure tones, doesn't mean they are 'problems'.. they may be, but let your ears help you.. If you don't have a way to do a 3D decay (Waterfall) type test, to confirm the decay time at LF, than be carefull..

In your case Ian, we are talking about a broadband trap and it will help tighten up all the LF in the room.. you really can't go wrong. I'm just reiterating what I always say: use your ears... they are your best test gear!! If we were talking about building tuned resonators, then we would have to be more careful and thorough about determining actual LF resonances.
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ruffrecords

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Re: Cathedral Roof Bass Trap
« Reply #8 on: November 01, 2006, 05:32:05 am »

franman wrote on Wed, 01 November 2006 03:00

Just be careful "measuring" modes this way... if you move the mic 1 foot things will change. You really have to listen to confirm what you hear makes sense with what you measure... It is natural (and necessar) for any pure sine wave tone to behave as peaks and valleys throughout a room.. It's the distribution of the modal peaks and valleys that makes a room sound natural in the LF..

Thanks for the input, Fran. The results pretty much match up with what my ears were telling me. I take your point about measurement position - I have clearly marked the point on the floor. As I think I said in another post, absolute measurements are less important then relative ones. I want to repeat them after adding the bass traps to see if there is a measureble difference. I am sure my ears will confirm this.

Quote:

In your case Ian, we are talking about a broadband trap and it will help tighten up all the LF in the room.. you really can't go wrong. I'm just reiterating what I always say: use your ears... they are your best test gear!!

One further point. I assume my cathedral roof trap will affect only some of the room modes so I should still place bass traps in the 'regular' corners to control the remaining room nodes?

And a last question, there are plenty of programs to calculate room nodes from room dimensions but they all assume a fairly regular box shape - what do you enter for height when you have a cathedral roof???

Cheers

Ian
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Ethan Winer

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Re: Cathedral Roof Bass Trap
« Reply #9 on: November 01, 2006, 11:08:12 am »

Fran,

> if you move the mic 1 foot things will change <

Forget one foot, how about one inch! I recently tested this for an article unrelated to acoustics, and the resulting graphs surprised even me. Look here:

www.ethanwiner.com/believe.html

--Ethan

ruffrecords

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Re: Cathedral Roof Bass Trap -Update
« Reply #10 on: November 16, 2006, 05:28:50 pm »

Did the basic construction of the cathedral roof bass traps today. Some quick subjective tests indicate a useful improvment. Objective tests to follow.

Ian
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franman

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Re: Cathedral Roof Bass Trap
« Reply #11 on: November 17, 2006, 10:41:31 pm »

Bring it on Ian.. we're all waiting with baited breath for your report!!!
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ruffrecords

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Re: Cathedral Roof Bass Trap
« Reply #12 on: November 18, 2006, 03:02:41 pm »

franman wrote on Sat, 18 November 2006 03:41

Bring it on Ian.. we're all waiting with baited breath for your report!!!

There is a bit more work to do yet. At the moment a criss cross of steel cored plastic covered wire (clothing line) is what supports the Rockwool so most of it is exposed. We had to cover all the equipment is the studio with dust sheets to keep Rockwool debris out of it. I have just acquired two super king size flat bed sheets from the wife so on Monday we will staple these over the underside to finish it off. Then we can remove the dust sheets and carry out some tests. I have taken some pics of before and after which I'll post when I have made some measurements.

Iani
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ruffrecords

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Re: Cathedral Roof Bass Trap
« Reply #13 on: November 19, 2006, 03:23:53 pm »

Here are some pics of the progress so far in building the cathedral roof bass traps. Enjoy.

One end before:

http://i103.photobucket.com/albums/m124/ruffrecords/bass%20traps/basstrap1.jpg

And after:

http://i103.photobucket.com/albums/m124/ruffrecords/bass%20traps/basstrap5.jpg

Middle before:

http://i103.photobucket.com/albums/m124/ruffrecords/bass%20traps/basstrap4.jpg

And after:

http://i103.photobucket.com/albums/m124/ruffrecords/bass%20traps/basstrap6.jpg

Overall before:

http://i103.photobucket.com/albums/m124/ruffrecords/bass%20traps/basstrap3.jpg

and after:

http://i103.photobucket.com/albums/m124/ruffrecords/bass%20traps/basstrap9.jpg
And lastly a view inside:

http://i103.photobucket.com/albums/m124/ruffrecords/bass%20traps/basstrap10.jpg



Overall it is 6ft 6in wide, nearly 18ft long and 40in deep at the apex.

Ian
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franman

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Re: Cathedral Roof Bass Trap
« Reply #14 on: November 19, 2006, 03:27:37 pm »

Ian,

Looking good.. but what happened to the hanging Insulation blankets above the flat layer?? We did discuss that (above).. I checked..
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ruffrecords

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Re: Cathedral Roof Bass Trap
« Reply #15 on: November 19, 2006, 03:42:24 pm »

franman wrote on Sun, 19 November 2006 20:27

Ian,

Looking good.. but what happened to the hanging Insulation blankets above the flat layer?? We did discuss that (above).. I checked..

Indeed we did. I confess I am still not sure exactly what you mean. We don't use the term 'insulation blankets' in the UK. I have some low density fibreglass insulation that comes in a roll that I was thinking of filling much of the cavity with. It is the sort of stuff you roll out in the attic/loft between the ceiling beams to keep the heat in. Presumably any generally absorptive (acoustically) stuff will do?

Ian
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franman

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Re: Cathedral Roof Bass Trap
« Reply #16 on: November 19, 2006, 07:40:40 pm »

Yes.. that is the stuff... but instead of just laying it in there (where it will settle over time) we prefer to hang "batts" (pieces of insulation) from the hard ceiling above, like hanging "blankets"... make sense?
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ruffrecords

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Re: Cathedral Roof Bass Trap
« Reply #17 on: November 20, 2006, 06:25:55 pm »

franman wrote on Mon, 20 November 2006 00:40

Yes.. that is the stuff... but instead of just laying it in there (where it will settle over time) we prefer to hang "batts" (pieces of insulation) from the hard ceiling above, like hanging "blankets"... make sense?

OK, I got that and I understand the reasoning behind it.

Cheers

Ian
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