Acoustic absorption under a platform

phosforx

New Member
May 1, 2011
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Hope this is not out of place:
Our extremely reverberant church has wooden stools on wooden platform bases. The platforms have a height of 8 cm. Their wooden base is 2 cm thick so, this leaves just 6cm between the marble floor and the bottom of the base. Would sticking a porous absorber which is 2.5 cm thick under the base, which would leave 3.5 cm of air space between the floor and the bottom of the material , prove an efficient absorber ?
phosforx
 

JackD201

WBF Founding Member
Apr 20, 2010
12,308
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Manila, Philippines
Hi Phosforx,

If you did that you would be making a form of resonator. The frequency absorbed would be affected by the size of the platform, the size of the openings and the density of the stuffing. Personally I think that if reverberation is the problem and by problem I mean the congregation is having a difficult time making out what the Priest or Pastor is saying the more efficient solution would be to use main arrays with tighter directivity aided by delay arrays. This won't do anything for the choir though. For the choir you'll have to clad a fair percentage of the very hard surfaces with absorbent material and this would also benefit the new PA system. While I wish filling the platforms would do the trick, especially since it requires minimal expense and will not change the aesthetic of the house of worship, sadly my opinion is that it won't be sufficient. Just my 2 cents.

Jack
 

phosforx

New Member
May 1, 2011
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Acoustic absorption under platform

Thanks JackD201,
RT60 is 9.36 seconds. We have absorption for many different areas to bring the RT60 down to 1.9 seconds. NO choir. We would like to keep sound reinforcement to a minimum. We must fix the acoustics first. The area under the platforms is about 400 m2. So if the acoustic cotton that we are thinking of putting under the platforms is effective we would have about 250 Sabines of absorption. We need total absorption in excess of 800 Sabines including the people in the church, to accomplish the feat of 1.9 seconds. Under the platforms is a good idea if this would work. This problem is quite unusual since the reverberant sound can reach the acoustic cotton only through the 2 cm of space remaining between the floor and the underside of the cotton which is above the floor.

I do not think that the platforms are behaving as a resonant panel absorber. The wood is too thick and rigid and the air space underneath the wood is not enclosed so it has hardly any stiffness, so any resonance would be at a very low frequency to be of benefit.
I have measured the sound pressure level under the platforms and not surprisingly is higher than the average in the church. IF SOUND ABSORPTION UNDER A PLATFORM IS EFFECTIVE THEN OBVIOUSLY THIS OPENS UP MORE POSSIBILITIES?
One way to find out is to try it! But can we predict theoretically what we should expect?
Phosforx

We need a total of about
 

JackD201

WBF Founding Member
Apr 20, 2010
12,308
1,425
1,820
Manila, Philippines
9.36 seconds. That is challenging alright. Typically the first thing done on a tight budget is to fly a simple horn array and fire it down at the warm bodies thereby using the crowd as the primary absorbers. It's the same thing done in gyms and budget auditoriums. I'm in the Philippines and Catholic which means old Churches with ceilings at least 20 feet high, 30 feet to 40 feet being more typical. Can you give us more details on your Church's dimensions and layout?

My thinking is that the sound absorbed under the platform would be pretty low in frequency. Maybe 80Hz and below? As these lower frequencies have the best chances of finding their way to the openings. I'm not so sure it will do much between 1k and 400k :(
 

Art Noxon

WBF Technical Expert (Room Acoustics)
Mar 29, 2011
38
1
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Eugene, OR
www.acousticsciences.com
If I understand correctly, you have a huge surface a few inches above another surface. You want to put sound absorption between them and supply a huge amount of sabines to the room. However if sound can't get to the absorber then the absorber doesn't absorb any sound. You can add acoustic panels around the perimeter of the platform, say 12" wide and 2" thick. This will provide all the absorption you can get out of this system.

If you can drill an infinity of holes in the platform say 1/4" every inch pattern, then the platform becomes acoustically transparent for all bass and mids up t0o maybe 600 Hz, ane it becomes reflective above this. This is approximated. Then add the sound panel under the floor, remember to leave at least 1/4" air space between the back side of the plywood floor and the front face of the sound panel. Otherwise you will chole the air movement in the hole.

You can use carpet over top of this but make sure you can put the backing of the carpet to your lips and blow through it. You want acoustically transparent carpet.

In highly reverberant spaces the speaker choice is a "distributed sound system". Lots of small low power speakers, probably 8' apart and 10' off the floor.

What is the volume of the church? What are its dimensions? Why such a huge platform? How do you know how many sabines you need? And don't forget the bandwidth of your absorption. If you absorb all highs then you will have a boomy, dead space, for example. Usually there is a curve, that has longer reverb in bass and top end and less reverb in the middle range, 250 through 2k octaves.

What type of service to you have? Is there a praise band on the platform?, no choir so just lead singer?

Churches are complicated, you want low RT60 so the minister can be understood, but then the room is so dry the congregation can't sing and the music minister is not happy or visa versa. I have written many article on church acoustics, reverberant yet intelligible. It's not an RT60 fix. google acoustic sciences, go to church and then to articles. Read. ART Noxon
 

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