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Author Topic: Amazing (to me) Room Acoustics Success Story  (Read 8223 times)

Arnold B. Krueger

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Amazing (to me) Room Acoustics Success Story
« on: May 02, 2011, 08:33:56 am »

We've been living for about 4 weeks with the results of our first attempt at improving the acoustics of our sanctuary.  The room is 45 x 120 x 27. We put up sound absorbers made by a member, based on a single 2" layer of 705 high density fiberglass spaced about 4 inches away from the front of our balcony,  in a lightweight wooden frame and covered on the front with grille cloth. Total area covered is  under 180 square feet. Compare that to 145,800 square feet of walls, floor and ceiling.

Benefits:
Sound at mix position more strongly resembles the sound in middle of room.
People on the platform hear cleaner sound from their stage monitors.
Some members who relied on our hearing assistance system now say that they can hear fine without it.

Absence of Problems:
No complaints that we spoiled the sound of the pipe organ, congregational singing, etc.
Few people have visually noticed that there was a change.
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Brian Ehlers

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Re: Amazing (to me) Room Acoustics Success Story
« Reply #1 on: May 02, 2011, 06:35:35 pm »

Congratulations!  It's amazing you were so successful with only 180 sq. ft. of treatment.  That shows you had a VERY specific problem.

I bet you had to fight like a dog to get approval to do this.  Which means it took a lot of guts on your part to tackle the project.  Of course, there was almost zero chance it would make things worse -- but would it be worth it?  It's not easy to stick your neck out like that.  Good job.
 
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Brad Weber

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Re: Amazing (to me) Room Acoustics Success Story
« Reply #2 on: May 02, 2011, 09:36:39 pm »

Glad to hear it worked so well for you!
 
I always get a bit worried when I hear about people using the online calculators from acoustical panel manufacturers or basing acoustical treatments on some very basic calculation of reverberation time as it's a matter of not just the surface area you have of different materials but also of putting the right materials in the right places.  Sometimes less material used judiciously can be more effective than more material placed without much thought.
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Arnold B. Krueger

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Re: Amazing (to me) Room Acoustics Success Story
« Reply #3 on: May 04, 2011, 06:40:12 am »

Congratulations!  It's amazing you were so successful with only 180 sq. ft. of treatment.  That shows you had a VERY specific problem.

I bet you had to fight like a dog to get approval to do this.

You've been reading my posts here!  Yes, the church boards are loathe to do anything at all, especially make a change that might be real and noticable.

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Which means it took a lot of guts on your part to tackle the project.

With these guys, I'm beyond guts. They managed to turn the knife several times, though. The wouldn't let me officially recommend them. They had to hire a consultant. He and I talked and came up with this. They wouldn't let me design them, a board member who was an architect designed something with the fiberglass (He knows not about 703 or 705) firmly attached to new 3/4" ply with no space behind the fiberglass. He knows not about the benefits of spacing absorbers, but he is one of *them*.  They wouldn't let me build them, either. They found another church member who I secretly  convinced to change the design pretty dramatically. This worked out pretty well except that he felt that they would be improved by putting an arched piece of pegboard that bowed out about 2 inches in the middle. I showed him papers and formulas that said that regular pegboard has so few holes that it is just a piece of board, but he knows better than science. Since this didn't cause much loss of depth, I finally rolled over.

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Of course, there was almost zero chance it would make things worse

The board was so concerned that they would ruin the sound of the organ that they demanded that the panels be designed so that they could quickly be removed. This of course made them more complicated and further delayed things.

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-- but would it be worth it?  It's not easy to stick your neck out like that.  Good job.

One nice thing about having no credibility with your church board is that you can't hurt your credibility any further! ;-)
« Last Edit: May 04, 2011, 06:42:47 am by Arnold B. Krueger »
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Arnold B. Krueger

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Re: Amazing (to me) Room Acoustics Success Story
« Reply #4 on: May 04, 2011, 06:51:18 am »

Glad to hear it worked so well for you!
 
I always get a bit worried when I hear about people using the online calculators from acoustical panel manufacturers or basing acoustical treatments on some very basic calculation of reverberation time as it's a matter of not just the surface area you have of different materials but also of putting the right materials in the right places.  Sometimes less material used judiciously can be more effective than more material placed without much thought.

Excellent point. The online calculators I've seen to date treat all areas the same. 

I've known for almost 50 years that this particular area was the major problem with the room. I first heard it talking back to me when I was 16 and sang with the adult choir. Believe it or not I've been complaining about it since then but almost nobody heard the same thing.

When it became clear that we might be able to actually do something with the room. I bought a toy cap pistol and set up my Microtrack and a couple of mics (1 omni, one cardioid) in the middle of the platform and did some tests. The impulse from the cap pistol timed out to exactly coincide with the front of the balcony, and its reflection was far and away the biggest thing in the room.  It was thus time to stake what was left of my life on fixing it first.

We're meeting today with the consultant to plot out our next move. Everybody on the inside knows that we had our one golden opportunity and probably did pretty well with it. We know that from here on in we'll have to cover far more area to make a similar difference.
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Dan Andrews

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Re: Amazing (to me) Room Acoustics Success Story
« Reply #5 on: May 05, 2011, 09:15:25 am »

Hi Arnold,

That sounds very interesting, and well done.

Any chance of some photos?  I would love to see what the construction of the sound material is like.

All the Best, Dan Andrews
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Arnold B. Krueger

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Re: Amazing (to me) Room Acoustics Success Story
« Reply #6 on: May 05, 2011, 10:33:35 am »

Hi Arnold,

That sounds very interesting, and well done.

Any chance of some photos?  I would love to see what the construction of the sound material is like.

All the Best, Dan Andrews


Since I didn't do the work and the panels are all in place. no pix of the construction and sound material is likely.

But, it is as simple as apple pie and can be summarized as approximate 4' x 6' x 6" wooden frames covered with light bown Guilford firepoof grille cloth material. Inside, there is a 2' thick surface of Dow Corning 705 that fills the frame's front just behind the grille cloth.

Since the fiberglass panels are 2' x 4' x 2", they are pieced together to cover the entire area. This was faciliated by cross braces that coincided with the lines where the pieces came together.

There is an approximate 4" empty space behind the fiberglass sheets, except for a piece of 1/4" pegboard materail  with a slight curve to it so that the middle of the pegboard is about 2" into the empty space, and its top and bottom edges are at the back.

I think that the grilled coth and the pegboard are gratuitous. Just about any cloth would do if it was fireproofed, which is pretty easy to do. After all, if the grille cloth absorbs sound before it reaches the 705, it still gets absorbed, right? ;-)

The frames fit between decorative upright posts on the front of the balcony.  One surprise was that none of the panels are identical or even symmetrical. The decorative posts were all spaced differently, as if by eye.  The front of the balcony is about 6' high and 45 feet wide. The total ceiling height is about 27' so only a fraction of the back wall off the church was actually covered.

It seems like diffusion due to the sloped pews in the balcony broke up many of the reflections from the back wall behind the seats in the balcony.  The pews on the flat main floor seem to have had a similar effect on the reflections from the back wall below the balcony.  The media booth's front surface is about 4' high, rus about half the width of the room, and may have some mitigating effect on the reflections off the back wall below the balcony.
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Brian Ehlers

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Re: Amazing (to me) Room Acoustics Success Story
« Reply #7 on: May 05, 2011, 01:36:01 pm »

One surprise was that none of the panels are identical or even symmetrical. The decorative posts were all spaced differently, as if by eye.
Don't you just love working on old churches?   :)

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It seems like diffusion due to the sloped pews in the balcony broke up many of the reflections from the back wall behind the seats in the balcony.  The pews on the flat main floor seem to have had a similar effect on the reflections from the back wall below the balcony.
Regularly spaced reflections can have a really bizarre effect on the sound.  I wonder if you could pick out the individual reflections from each row of pews when you performed your cap-gun experiment?

The exterior wall of the building in which I work consists of corrugated steel with a small, vertical notch maybe every 8 inches.  Several hundred yards away, at an oblique angle, is an industrial building with what sounds like rocks falling down a conveyer.  As I walk across the parking lot, I can hear the impulse response of each rock reflect acoustically off my building.  The small notches seem to reflect a very narrow range of frequencies, so there is that high-pitched tone.  But each notch reflects the sound at a different time, giving the sound a zipper-like effect of lower frequency.  It's totally bizarre.  Since the dimensions of church pews are larger, any similar effect would be at lower frequencies.  But I've always wondered how much of the clutter of sound I hear in our sanctuary is that  coming from empty pews.  It would be worst, of course, for those on stage.
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Dan Andrews

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Re: Amazing (to me) Room Acoustics Success Story
« Reply #8 on: May 06, 2011, 09:09:05 am »

Since I didn't do the work and the panels are all in place. no pix of the construction and sound material is likely.

But, it is as simple as apple pie and can be summarized as approximate 4' x 6' x 6" wooden frames covered with light bown Guilford firepoof grille cloth material. Inside, there is a 2' thick surface of Dow Corning 705 that fills the frame's front just behind the grille cloth.

Since the fiberglass panels are 2' x 4' x 2", they are pieced together to cover the entire area. This was faciliated by cross braces that coincided with the lines where the pieces came together.

There is an approximate 4" empty space behind the fiberglass sheets, except for a piece of 1/4" pegboard materail  with a slight curve to it so that the middle of the pegboard is about 2" into the empty space, and its top and bottom edges are at the back.

I think that the grilled coth and the pegboard are gratuitous. Just about any cloth would do if it was fireproofed, which is pretty easy to do. After all, if the grille cloth absorbs sound before it reaches the 705, it still gets absorbed, right? ;-)

The frames fit between decorative upright posts on the front of the balcony.  One surprise was that none of the panels are identical or even symmetrical. The decorative posts were all spaced differently, as if by eye.  The front of the balcony is about 6' high and 45 feet wide. The total ceiling height is about 27' so only a fraction of the back wall off the church was actually covered.

It seems like diffusion due to the sloped pews in the balcony broke up many of the reflections from the back wall behind the seats in the balcony.  The pews on the flat main floor seem to have had a similar effect on the reflections from the back wall below the balcony.  The media booth's front surface is about 4' high, rus about half the width of the room, and may have some mitigating effect on the reflections off the back wall below the balcony.

Thanks Arnold,

I am trying to imagine what you have described, but when you say "there is a 2' thick surface of Dow Corning 705"  I am thinking you meant a 2" thick surface, is that right? 

I haven't ever heard of Dow Corning 705, is that fibreglass batts used to insulate house walls with, or something else?

Thanks again.

All the Best, Dan
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Brad Weber

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Re: Amazing (to me) Room Acoustics Success Story
« Reply #9 on: May 07, 2011, 01:15:57 am »

I am trying to imagine what you have described, but when you say "there is a 2' thick surface of Dow Corning 705"  I am thinking you meant a 2" thick surface, is that right? 

I haven't ever heard of Dow Corning 705, is that fibreglass batts used to insulate house walls with, or something else?
I think it was a typo and Arnold was referring to this, http://commercial.owenscorning.com/products/pipe/fiberglas-700-series-insulation/.  The 700 series is compressed fiberglass board and the different models in the series (701, 703, 704, 705 and 707) relate to different densities (the last digit in the model number is roughly the density in pounds per cubic foot).  They're readily available in 1" or 2" thicknesses and either unfaced or with an FRK (Foil Reinforced Kraft) or ASJ (All Service Jacket) facing.
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Re: Amazing (to me) Room Acoustics Success Story
« Reply #9 on: May 07, 2011, 01:15:57 am »


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