Room Noise

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Room Noise

Postby Bob » Thu Jun 19, 2014 4:13 pm

You have a room, 75'x75'x15' (23m x 23m x 4.5m)
rectangular with hard reflective surfaces on six sides.

Ambient noise in the room is about 98dB(C). Noise is caused by World Soccer Cup on big screen tv, Muzak (pop music, elevator music), and the conversations of 200 people seated at open tables on movable wooden chairs.

How much quieter would the room get if seventy-five 2'x4'x4" acoustical absorbers were placed around the walls of the room (i.e. 1 every 4 feet, each with a 2' horizontal spacing between, mounted long vertically).

Volume of the room: 84,375 cubic feet. (2400 cubic meters)
Surface area of the room: 15,750 square feet (1450 square meters)
Surface area of proposed absorption: 600 square feet. (55 square meters)

I vaguely remember there exists a formula for this,
that presumes that people won't turn up their big screen tv, muzak, and voices.

(A local restaurant 'upgraded' -- removed the carpet, all the padded booths (tables with head-height padded leather couches), a dividing wall between the table restaurant area and the bar area (where the big screen tv is) -- and my goodness is it loud in there now)
Regards
Bob Golds
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Re: Room Noise

Postby drnelson » Fri Jun 20, 2014 4:54 am

DeltaL = 10 • lg (A2/A1) = 10 • lg (T2/T1) in[dB]

A1 is empty room
A2 is with absorption

In practice it is only a few dB most of time. However in a room of people it has a much greater effect. In my kids school they have the standard quad-torium. Whenever there was about 400-500 people in the room the noise was unbearable. The school had called me for advice on equipment selection to purchase with a grant they had received for the music program. During the the conversation I said something about how I could not stand being in that room when it was full of people to the point I had given thought of donating acoustic panels to install in the room just that the number needed was far more then I could afford. It turns out the grant had money left over that we could use to work on the acoustics. About 120 panel 2x4 panel of different thickness and types were installed, it dropped the noise level down with people in the room, I would say 10-12 dB because people are not fighting to be heard over each other.

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Re: Room Noise

Postby Yannick » Fri Jun 20, 2014 7:49 am

Exactly, the impact of people not starting to shout louder and louder will be much higher than the immediate impact by the absorbtion itself.
You will even be able to turn down the TV/music level.

Like an inverse snowball effect.

I think the (buildup) phenomenon actually has a proper term.
http://en.wikipedia.org/wiki/Lombard_effect
http://www.acoustics.asn.au/conference_ ... s/p133.pdf
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Re: Room Noise

Postby Andrey Smirnov » Fri Jun 20, 2014 1:05 pm

hi,

in case Bob described formula DeltaL = 10 • lg (A2/A1) = 10 • lg (T2/T1) in[dB] can be used.

But
A1 is room with people without absorption
A2 is room with people with absorption

Absorption of seated people is very large.
Thus additional absorption due 55 square meters of seventy-five 2'x4'x4" acoustical absorbers will very small relatively of absorption of 200 peoples.

In practice appreciable noise reduction will be a doubling of the absorption.
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Re: Room Noise

Postby Bob » Fri Jun 20, 2014 7:21 pm

Andrey Smirnov wrote:Absorption of seated people is very large.
Thus additional absorption due 55 square meters of seventy-five 2'x4'x4" acoustical absorbers will very small relatively of absorption of 200 peoples.


I figure a person is about 1.8 m^2 (20 ft ^2) surface area, with an absorption coefficient of about .8 at 1000hz, so 200 people should be about 3200 sabins (20 * 200 * .8).
But I'm not sure if I've done the math correctly. Googling turns up all kinds of wild numbers for people. I was expecting a difference between standing alone and seated in an audience, but even within those two categories there isn't much consensus.

75 panels should be about 600 sabins (75 * 8 * 1.0).

300 panels (two rings around the walls without gaps) would add .75 times the absorption at about 2400 sabins.

Combined with 5,625 ft^2 of outdoor carpet, at 0.15 per ft^2, would be another 850 sabins.
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Re: Room Noise

Postby bert stoltenborg » Fri Jun 20, 2014 10:12 pm

Bob,

reflections add 3 dB to the direct sound.
And they are delayed.

There is not a formula for this, it's to complex.

But is you tame reverb, you have a nice hall.
As stated people in these cirumstances are not likely going to scream, they tend to do it when they don't understand each other, as in a reverberant space.
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Re: Room Noise

Postby Andrey Smirnov » Sat Jun 21, 2014 9:11 am

bert stoltenborg wrote:Bob,

reflections add 3 dB to the direct sound.
And they are delayed.

There is not a formula for this, it's to complex.

But is you tame reverb, you have a nice hall.

I agree with Bob. This formula is very simplified approach.
There are more complicated methodologies described in local standards in each country.
Using absorbers possible to decrease only reverberant field but not direct sound.
This reason is limitation of noise reduction method using absorbers.
Besides more important surface for placing of absorbing materials is ceiling, not walls.
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Re: Room Noise

Postby Bob » Sat Jun 21, 2014 2:11 pm

Andrey Smirnov wrote:Besides more important surface for placing of absorbing materials is ceiling.

In one sense there's a lot of room in the ceiling, about 3' height could be completely converted to absorption.

In another, there's the usual 2' diameter round air ducts, sprinklers, and other mechanical stuff up there,
and more importantly they recently put up a false ceiling of brushed aluminum with pot lights.
Some of the brushed aluminum is a little like slats, in that they're sheets about 8" wide by 20' long (0.2 meters x 6 meters), with 4" (0.1 meter) air gap between them, hanging such that there is 3' (1 meter) above them. The rest is a solid false ceiling. I don't recall the percentages of slat/solid, as I didn't really pay attention to that at the time.

Aluminum isn't a fire hazard, and its washable every month to get the grease that probably rises off the burgers and fries and other steamy dishes they sell.
Whereas fabric absorber wrap would have to be replaced -- unless its those zippered bags that someone around here used to sell, which could be put into a washing machine.

The walls are currently covered in faux-brick -- 1/8" thick plastic brick pattern. Seemed to me an easy place to put absorber panels. They used to have photos of movie actors on the walls, so fabric with art seemed possible.
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