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Acoustic Absorption

05/23/2011 8:55 PM

In our church the stools stand on platforms over the granite floors. The platforms have dimensions 60x180x7 cm and stand 2 cm above the granite ground, ie the top surface is a total of 9 cm above the ground. They have a solid 2 cm top of hard wood but are empty underneath. This leaves 7 cm of air space between the floor and the underside of the hardwood top. How effective would the placement of 3 cm of acoustic cotton under the wooden top be in absorbing sound as compared to putting it over the top ? (We cannot put it over the top since this is where the church goers stand! ) I have measured the sound pressure under the panel and it is actually higher than over the panel. This is not surprising as we know that the pressure on the surfaces of an enclosure and the corners have higher pressures. Please give me your informed opinion. Thanks phosforx

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#1

Re: Acoustic Absorption

05/23/2011 9:05 PM

Acoustics is an extremely complex "black art".

I assume you are trying to kill echos? High frequency, maybe.

How did you measure the sound pressure level? What instrument did you use? Spectrum analyzer?

You seem to have answered your own question.

More info please.

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#4
In reply to #1

Re: Acoustic Absorption

05/23/2011 10:40 PM

You wrote, "Acoustics is an extremely complex "black art"."

Well, that was true 30 to 40 years ago, but advances in physics and the advent of computers have taken both the black and art out of the field.

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#5
In reply to #4

Re: Acoustic Absorption

05/23/2011 11:05 PM

Maybe. It's still complex. And an art.

Seriously, your advice is good and well informed. And probably will accomplish the goal of reducing the reflections of sound.

However, advances in technology and the ability to use them are still out of reach of many non-technical users. And over the head of many "experts".

I readily admit that I have been a stranger to both church, and acoustics, for some long time now.

Cheers.

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#8
In reply to #1

Re: Acoustic Absorption

05/24/2011 3:31 AM

Yes lyn. The church is large and extremely reverberant. T60 is 9.3 seconds ! We are limited with areas to put absorption because most of the church is to be covered with icons painted directly on the walls. I know I can convert each platform either to a resonant panel absorber or to a perforated absorber by laboriously drilling holes on the top panel, putting a second panel underneath and porous absorber inside. But if I do not do the above how would the porous absorber put on the underside perform,as compared to putting it on the top, where the sound would be statistically more likely to be incident? This is the question to which I need to know the answer before trying it out !

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#11
In reply to #8

Re: Acoustic Absorption

05/24/2011 8:48 AM

I'll defer to AH here, because I agree that wavelength and reflected energy are the two factors most important.

Drop a stone into a quiet pool of water and watch the ripples propogate, amplify, reflect, interfere and even cancel each other and you'll get a small taste of acoustics. I think you have a solid understanding of the subject.

Hopefully this discussion won't get bogged down by other opinions from others.

Good luck.

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#2

Re: Acoustic Absorption

05/23/2011 9:13 PM

You are dealing with a drumbox. The biggest problem you will have is contact sound, drum beats, created by the shoes - feet - legs - people. If you want to approve the acoustics, or better - deal with this phenomenon, the best is to cover the surface with pretty thick carpet or (not so effective- hard rubber). This will reduce the source intensity. Isolation underneath will be very difficult, because you are dealing with low frequencies and big amplitudes. Compare with the base drum in a orchestra. The lower the frequency, the more omnidirectional the sound expands. You'll need to make the platform "insensitive". This can be done with heavy materials, like lead in rubber, thick roofing e.g. Putting something underneath might be discouraging. Mass and structural strength is the way to go. Filling up the gap between the board and the floor will help a little also.

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#3

Re: Acoustic Absorption

05/23/2011 10:33 PM

Here is what I would do and it is the cheapest route.

Buy a roll of 30# roofing felt and staple the felt every 6" to the inside top of the platform. Apply 2 to 3 layers in this fashion.

Fill the remaining airspace underneath with fiberglass insulation (remove the paper backing). It should not be a tight fill, just enough to loosely fill the void.

That will dampen your resonance noises. If that is not enough, make everyone park their shoes at the door. ;-)

What the felt does is converts the acoustic vibration energy to heat. Every time someone steps on the platform it vibrates. That vibration couples to the air under the platform and the resulting sound waves bounce around in the void and cause the top panel to re-resonate.

The felt effectively absorbs some of that vibrational energy from the top panel and converts the energy into heat before it is coupled to the air again. The top panel is like the head of a drum. By adding damping material to the top panel you muffle the drum head. The remaining sound waves are further absorbed by the fiberglass and deadens the noise.

Another way to attack the problem is to close off the bottom of the platform with a wood panel and fill the void with sand. Unfortunately, the sand will settle and leave a small air gap that will still make the top panel vibrate. The results will dampens some of the sound, but it will still make noise.

The ultimate method is to make what is called constrained layer damping. Essentially, you make a 3-layer top panel. The top and bottom layer is wood. Sandwiched in between is a special layer of rubber that converts vibration to heat.

There is a product used in the construction industry called Green Glue. It comes in tubes and is applied to one surface and then another panel is screwed over the first with the Green Glue in between.

Green Glue is typically used in between two layers of drywall to make sound proof walls, but works excellently with other building materials.

However, back to reality, I would try the roofing felt since it is cheap, easy to install, and will do an excellent job of deadening that top panel. There have been a number of scientific studies performed in the world of acoustics that proves this works.

Oh, by the way, it may be tempting to try other materials instead of roofing felt, but you will be surprised how much superior the felt works. Avoid using roofing tar since it will stink, just stick with the felt and staples.

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#7
In reply to #3

Re: Acoustic Absorption

05/24/2011 3:17 AM

Thanks anonymous hero, The problem we are trying to solve is the excessive reverberation which is 9.3 seconds. We have taken many other steps of acoustic treatment in other parts of the church. We are limited as to where we can add absorption because most of the church is to be covered with frescoes (religious icons, as it is the trend nowadays in Greek Orthodox churches). As there are 220 platforms each with an area of 60x180 cm then it would be good to attack this area converting each bench to either A) a resonant panel absorber by sealing the bottom with a thin panel with absorbing cotton wool fibre inside or B ) a perforated panel absorber by sealing the bottom and opening 2.8 mm holes on the top, again with absorber inside. I see no problem with A but B would be laborious. THE QUESTION IS THIS : Would I get a wide band sound absorber by just putting the porous absorber on the underside of the panel? Would it work just as well as if we had put it over the top surface? I would hesitate because even though the church is very reverberant one would think that the sound would be by far incident on the top of the surface much more than the bottom . Yet, the sound pressure measured under the panel is greater than that over the panel as meaured with the audio tools analyser. Obviously, i would like to know the correct answer before we go to the the expense and trouble to try and see! Thanks for your informed answers.

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#10
In reply to #7

Re: Acoustic Absorption

05/24/2011 7:17 AM

Probably not.

You have to consider the wavelength of the spectrum you wish to suppress.

The best way to do this is with a sound absorbent material suspended in front of and away from the incidental walls and surfaces.

The sound wave is attenuated as it passes through the material, then strikes the wall or surface behind it and arrives out of phase with the next wave as it rebounds toward the acoustic absorption material.

Many people settle for heavy drapes, but again, you want to hang them slightly away from the wall surface.

You best route would be to see if you can find someone who is an acoustics expert in the field of building construction and see if they would volunteer their time to analyze your situation and suggest ways to improve the acoustics.

Normally, these things are done at the early planning stages of the building, so you are at a disadvantage here since you are long past that stage along with the desire to use lots of bare hard surfaces in the building design.

It's just too hard to get a reliable understanding of the whole picture of your building environment using the internet and txt messages. You can ask for good advice here, but you would also spend a lot of labor time trying things that do not serve your intended purpose.

Employing an expert to visit your church and inspect the environment is the best method I can think of, but given the need to keep so many bare hard surfaces you are constrained to live with at least a generous portion of that reverberation.

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#6

Re: Acoustic Absorption

05/24/2011 12:03 AM

Hardwood gives practically no sound absorption and once vibrating won't be damped by any kind of acoustic cotton because the mass ratio is crushing. Understand you don't want to dump the air vibration (sound) under it, that's easy, but the annoying accumulating resonances of the board at it's natural frequency and harmonics induced by the hall sounds. You could consider an MDF piece securely bonded under it at about half or more of the hardboard surface and enough width for each platform. S.M.

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#9
In reply to #6

Re: Acoustic Absorption

05/24/2011 3:46 AM

Yes I do want to absorb the excessive reverberant sound in the church. The RT60 is 9.3 seconds. The cheapest way would be to just put porous absorber on the underside of the platform where it would not be visible. I assume that this would work but how well would it work? Failing this I can convert each platform into a resonant panel absorber or to a perforated sound absorber but obviously the former would be cheaper and easier. Please help if you can save me the trouble . Thanks

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#12
In reply to #9

Re: Acoustic Absorption

05/24/2011 11:06 AM

The absorber (rock wool or variations on this theme) under the panel will work exactly the same straight under the panel or in the area between the panel and the floor.

It will NOT absorb the sound, you want to get rid of.

These products work for high frequencies. Felt # whatever will help only when it "participates" in the mass build up, - attached to the panel. The rule for this is thickness, compared to the panel. This is not a thin metal car roof of door panel.

Having dealt with plenty of churches, auditoriums, big empty rooms, sport halls and more of these, it is important for sound and sound systems not to add materials that are active in the low mid range of the resonance frequencies that are particular amplified and range between about 130 and 500 Hz.

Do not believe me, but do a audio test with pink noise and a good measuring microphone, that is what we before did with each project, even 30 years ago acoustics were not black or grey area.

All by all I have read lots of hints for the DIY but the problem has not been analyzed yet, what is the matter? Reverb time T60 - 9.3 seconds. Spectrum? Source? The panel floor box does not start singing by itself. Is it knocking, or humming?

Acoustics in the first place is not only damping the problems, but not generate them, and often just making use of reflectors and traps, to create transparency.

I have answered your question but need a lot more info as input.

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#13
In reply to #12

Re: Acoustic Absorption

05/24/2011 1:13 PM

Thank you dvmdsc, The RT60 was 9.3 seconds when the church was empty because it has a volume of 10500 m3 and nothing to absorb sound. The floors are granite and in fact so is part of the walls to a height of 1.5 meters all round. The rest is at the moment plastered concrete. The aim is to bring this reverberation down to 1.9 seconds. Greek orthodox churches allow no music and have no choir. We are using various kinds of absorbers to achieve this , applying the absorption at places that will not be covered by religious icons. We are already down to about 5.5 seconds on average and this is in the absence of people in the church. Our design for RT60 =1.9 seconds in the presence of people involves utilising the platform for absorption . We do not want carpets over the platforms that support the wooden stools. We like the bare wood esthetic of the carved wood. Putting the porous material under the platform is good since it can not be seen there. We have 7 cm of air space that is available for putting absorbing material. This will consist of 3 cm cotton wool fiber 2cm of empty air space above it and 2 cm beteen the granite floor and the bottom of the material. Great! But will it absorb the reverberant church sound just as effectively on the underside of the top panel as when put over the top of the panel. This is not an easy question even for acoustic engineers. Thanks again for your contribution. Phosforx

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#14
In reply to #13

Re: Acoustic Absorption

05/24/2011 3:41 PM

It would be interesting to have the spectrum of the second half of the RT60 test time on white noise in several positions in space. That would give you info on what and how much is absorbed and then look for solutions. I believe your 'problem' is at frequencies well under 500Hz. Only practical way to control that is via DSP cancelling and not damping, plus this sound behavior is responsible for part of the atmosphere of an orthodox church and it's not necesarilly a bad thing. S.M.

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#15
In reply to #13

Re: Acoustic Absorption

05/25/2011 1:27 AM

The most reliable results, I achieved with a "dummy head" microphone on a stand at average "ear height". The dummy head of Sennheiser GMBH has two microphones in the ears (one on each side). This stereo sound image builds up in the ear shapes.

The different posters go for colors: we had already green glue, I came up with "pink noise" and our friend S.M. with white noise, not to speak of blue, red, brown (all noises).

This still doesn't answer the question: what is bothering you? I come from the historical churches and cathedrals places too. The builders made these the way they are, because of the amount of people that have (had for the RC's) to be addressed by one single speaker (person), without microphone, and without audio system. So people learned to speak slowly while using monotone speech, ranging in the acoustic "feedback frequency" range and intervals, longer than RT60.

Even the music, produced by harmoniums and organs was composed in Gregorian style.

Back to your boxes: you'll need a dense composition absorber. I have some ideas, but would like some specifics.

What exactly happened with the spectrum content after the podiums were added? Do they resonate spontaneously or only after been kicked by shoes?

Can you supply a photo how they look like, also from the inside (upside down)

Thank you.

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#16
In reply to #15

Re: Acoustic Absorption

05/25/2011 10:55 AM

Plus you'll be supprized to know that the most sound absorbing 'objects' in a church are the humans populating it. So an empty church mesurement is not of much use. I wouldn't be suprized if the sound damping ratio of a full vs empty church was over 10:1 S.M.

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#17

Re: Acoustic Absorption

05/25/2011 4:29 PM

Have you ever tried some basic trouble shooting techniques such as; checking your speakers placements or sound (audio output) distribution, orientation or sound direction, physical size and adjusting each for the amount of power amplification coming out of every speakers? Acoustics is just one aspect of a sound system that is commonly being blamed in church environment. It deals mostly on room layout, furnishings, kind of materials used and building architecture, which by the way may be cost prohibitive, if one has the money to waste! /p

I would suggest to go back to the basics, take several measurements of sound levels (in db) all around and in every section of the room. Conduct your surveys when church is filled to capacity and when it is half filled! Remember that your church attendees are part of your acoustics system, they will absorb and dampen most of your amplified sound! Also, as an example approach to assist you, by using a small laser light as a pointing tool in combination with some mirrors to reflect / bounce those light(s) will help you in orienting all speakers as well as point to you where the sound it produce will bounce! Be also aware that inside every speaker cabinets or enclosures, is an impedance matching multi-tapped transformer that you can you use to balance/ match its output level.

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#18
In reply to #17

Re: Acoustic Absorption

05/25/2011 7:04 PM

Or, you can just walk to the center of the room and clap your hands once. ;-)

I agree that things change once you get people inside and sitting down.

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#19
In reply to #18

Re: Acoustic Absorption

05/25/2011 9:03 PM

Don't know if they sit down in Orthodox churches. Certainly NOT in Russian Orthodox churches.

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#20
In reply to #19

Re: Acoustic Absorption

05/25/2011 9:40 PM

Nothing dogmatic against it for most of the church service. Still, not enough chairs is common. S.M.

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#21

Re: Acoustic Absorption

05/25/2011 11:11 PM

Could we please address the original question? Let me rephrase it ! The church has very high reverberation which we would like to reduce from 9.3 seconds on average when empty to 1.9 seconds when there will be 300 people inside the church, which is the usual congregation expected. We have done all the calculations to achieve this. We measure RT 60 with professional impulse response equipment and using interrupted pink noise and the Schroeder method. Sound is very diffuse. We have a peak in the RT60 of over 11 seconds in the region 250 Hz to 500 Hz. We would like to reduce the reverberation over all the frequency range from 50 to 6000 Hz, especially though in the 250Hz to 1000Hz. In achieving this we would like to use the 300 platforms that support the stools to convert 100 of of them into resonant panel absorbers, 100 into perforated panel absorbers and in the case of the remaining 100 exploit the underside of the top cover on which people stand to add porous absorber. The question is simple: in the last case of the porous absorbers, is the addition of the porous material on the underside of the platforms equally effective as if it had been put over? The platforms consist of a 2 cm piece of hardwood on wooden feet of height 7 cm, thus making the top of the platforms stand a total of 9cm above the granite church floor. When we add the porous absorber which is 3cm thick we will have starting from the top, 2cm hardwood + 2 cm of air space + 3 cm of porous Absorber = 7cm This leaves 2 cm of air space between the granite floor and the bottom of the porous absorber. The question is this: How effective would this be compared to turning the platform upside down? Please answer this question explaining the physics on which your opinion is based.

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#22
In reply to #21

Re: Acoustic Absorption

05/25/2011 11:54 PM

Your absorption wool does not work at all. Nix, null, nada. Wrong material. Not enough mass, Am I a parrot? If you want to keep it serious explain where the sound comes from. A mattress, the size of your interior underneath, filled with sharp gravel and chipped rubber from e.g old or new auto tyres. The void should be overfilled, so that a permanent compression deadens the panel underneath. You can do a knock test - the ruling frequency will be the one around the peek before.

Do not expect a miracle......that takes longer.

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#25
In reply to #22

Re: Acoustic Absorption

05/26/2011 1:17 AM

Hi dymdsc. The material has an NRC of. 0.71 so it should absorb 71% on average between 250 to 2000 Hz when stuck onto a surface. With 2 cm air gap between it and the wooden top the absorption would increase further at the lower frequencies which is precisely what I want. But can I have your opinion please on the question I asked ? Thanks dvmdc

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#28
In reply to #25

Re: Acoustic Absorption

05/26/2011 9:54 PM

Phosforx,

check "acoustical surfaces.com".

They print a frequency curve of their material. You can see that the materials are poor performers for less than 500Hz. At these low frequencies panels, like roof surfaces, walls, resonate. I also guess your church has domes or dome like directional structures.

They can catch and hold the sound for a long time. Your granite floor will not be very active in that low part - resonant like - unless there are plenty of hollow spaces underneath - but it will have a roller effect. (Easy road to transmit the waves to more resonating areas).

Some churches have crypts, basements, catacombs and here starts the headache for acoustical engineers. I remember noise problems in houses next to dancing places, where the noise was 12 dB louder 200 feet further, than in the house next door.

Anonymous Hero came up with the clap hint, that almost all the PA guys use before unpacking the equipment. An other one is the resonance detection, knocking the hand on surfaces and listen how they sound. A more reliable test is to do it with a rubber hammer.

If you have a panel it sounds completely different than the same panel cut in strips with some space between. When absorption is required, the strips have irregular but well designed angles to direct part of the wave between the strips to drop behind in a void.

Sometimes "pitch" is added between with massive stone. Eastlake monitor rooms look the same everywhere.

Damping or absorbtion in % for audio doesn't tell a lot either.

50% should mean -3dBA (weighed) on paper, but in the field it doesn't make much sense. Your task is very difficult with all the restrictions in place.

Bringing life in room is a lot easier than making one dead when nothing can be covered. I wish for you the icons and paintings were moveable and on hard surfaces.

(maybe glass)

I am afraid, without putting my tent in your church for some days under different acoustic loads (BTW more clothes is more damping) that I can not help you further out. I wish you good luck with your project and I am hoping to read you more.

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#23
In reply to #21

Re: Acoustic Absorption

05/26/2011 12:44 AM

Wave length of RMS of 250 and 500 Hz is suspiciously close to 10m. Is this by any chanse close to the average hight of your church and this measure was empty? S.M.

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#24
In reply to #23

Re: Acoustic Absorption

05/26/2011 1:07 AM

Not so. The wavelength at 250 Hz is is 1.4 m and at 500 Hz it is 0.67m. The church is byzantine rhythm of various heights and has an average of 12m height. So, nothing to do with room modes. The RT60 is about 11 to 8 seconds at all frequencies from 50 to 1500 Hz and agrees with the calculated values based on the absorption coefficients of the surfaces.

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#26
In reply to #24

Re: Acoustic Absorption

05/26/2011 1:56 AM

Yep. Lost a zero on calculation. S.M.

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#27
In reply to #21

Re: Acoustic Absorption

05/26/2011 9:45 AM

Well, you clearly have gone out of your way to get excellent equipment to define the problem.

If you put half that effort into getting a professional acoustic expert to analyze the data you would already have your answer.

Why haven't you asked for professional help? Many companies will also donate their time to non-profit organizations, but you need to ask.

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