Bass Absorption in Auditoriums

[chickenfingers]

Hi

When designing an auditorium where sound will mostly be played through the PA system, the sound designer asks the acoustician for a pretty "dead" room, say Rev Times of 1sec. As always Mid and high Frequencies are easily delat with. However bass frequencies are far harder to absorb. Lets also think of a very good and expenssive PA system will be used hence Reverberation can be "recreated artificially" when required.

My assumption and question and here´s where I would like some input from some of you who know far more than I do:.
My thought would be to treat the whole ceiling by installing a dropped ceiling made entirely (or mostly) of 2" or 3" absorber. There would be an air gap of say 5" between the dropped ceiling and the roof. The principle behind this is that if you leave an air gap between the absorber and the hard surface your absorber deals better with Low frequencies (this has to do with the particle velocity of the of the sound wave approaching a wall and its wavelength, bla bla bla).
So far, is it a good idea?

Ok, say we are talking of a 20m (wide) by 30m (long) by 7m (h) room. If the whole dropped absorber ceiling cannot be built for whatever reason, but you are still allowed to treat the top side corners. Would it still be a good idea to install an absorber dropped ceiling along the side corners from front to rear? Taking into account the fact that Low frequency energy tends to cluster in corners (never understood this one but take it as a fact). And if possible may be do the same on the wall corners (as it is done in recording studios)?


I would really appreciate any input, or referrals to links, forums or books where this is dealt with.

Many thanks guys.

Chao

Sergio

 

[bpape]

You're moving into the realm of large space acoustics. In general, you want to launch a wavefront of bass and allow it to develop in the space. In general, those space are designed without parallel surfaces and the bass is left largely alone.

Bryan

 

[L3VY]

"My thought would be to treat the whole ceiling by installing a dropped ceiling made entirely (or mostly) of 2" or 3" absorber."

Then you wouldn't want to listen to music in that room because it will be silent that it will drive you crazy.

More info, and explenation why on corners is more bass in about 12 hours.

For now Good night.

 

[chickenfingers]

"My thought would be to treat the whole ceiling by installing a dropped ceiling made entirely (or mostly) of 2" or 3" absorber."

Then you wouldn't want to listen to music in that room because it will be silent that it will drive you crazy.

QUOTE]

Thanks for the input. However, I know for certain that professional PA companies ask for large auditoria (where their systems will be installed with their design), to have Rev Times of less than 1sec. How else can you achieve that at say 125Hz, having a full reflective ceiling? Their design wants to leave the room out of the equation and focus on the speaker´s direct sound. Different from Concert Halls where 2-2.5s REv Times is normal.

Furthermore, all the new cinemas in my country have full absorbing ceilings. I saw a link somewhere, of a newish auditorium in Mexico where I think they followed the same approach.

Sorry if I sound a bit challenging, not my intention at all . Just that I´m really interested in large space acoustics and want to hear other people thoughts on the topic.

Thanks a lot

 

[L3VY]

Sorry for delay.
My TT is always absorbing my time and money


20 x 30 x 7 meters:
Ideal Reverb Time to listen to stereo system is 0,6 - 1,3 sec.

Walls: 20cm concrete, painted.
Celling plasterboard.
On the floor thin looped carpet glued to the surface.
And we have at 125Hz 4,8sec, longest RT is 7,5sec at 500Hz.

Celling: gypsium plaster board, 20% perfored + mineral fiber behind.
Walls: 12mm gypsium plasterboard with 50mm space to the wall.
Floor: as earlier
And we have 1,3 sec at 125Hz, max RT is 1,65sec at 300Hz, above 3,7kHz TR is 0,95sec.

Furnitures not included, if it's gonna be a cinem then additional absorption you will get from chairs and peoples.

 

[highfigh]

"My thought would be to treat the whole ceiling by installing a dropped ceiling made entirely (or mostly) of 2" or 3" absorber."

Then you wouldn't want to listen to music in that room because it will be silent that it will drive you crazy.

More info, and explenation why on corners is more bass in about 12 hours.

For now Good night.

2"-3" material won't/can't absorb enough to make it silent.

 

[highfigh]

"My thought would be to treat the whole ceiling by installing a dropped ceiling made entirely (or mostly) of 2" or 3" absorber."

Then you wouldn't want to listen to music in that room because it will be silent that it will drive you crazy.

QUOTE]

Thanks for the input. However, I know for certain that professional PA companies ask for large auditoria (where their systems will be installed with their design), to have Rev Times of less than 1sec. How else can you achieve that at say 125Hz, having a full reflective ceiling? Their design wants to leave the room out of the equation and focus on the speaker´s direct sound. Different from Concert Halls where 2-2.5s REv Times is normal.

Furthermore, all the new cinemas in my country have full absorbing ceilings. I saw a link somewhere, of a newish auditorium in Mexico where I think they followed the same approach.

Sorry if I sound a bit challenging, not my intention at all . Just that I´m really interested in large space acoustics and want to hear other people thoughts on the topic.

Thanks a lot

I'll post some links for you:
http://www.atsacoustics.com/item--ATS-Acoustic-Panel-24x24x2--1000.html

http://www.ethanwiner.com/acoustics.html#modecalc

Look through this site for some of the project photgraphs:
http://www.acousticsbydesign.com/index.htm

You may get an idea of what is needed by seeing something similar to your auditorium.

Here's a material that's commonly used in large spaces, like convention centers and auditoriums:
http://www.tectum.com/

 

[chickenfingers]

2"-3" material won't/can't absorb enough to make it silent.

I don´t really want to make it silent. Just want to absorb some energy. Putting these panels diagonal to corners will work as bass traps. At least that´s my thought.

Thanks

 

[chickenfingers]

You're moving into the realm of large space acoustics. In general, you want to launch a wavefront of bass and allow it to develop in the space. In general, those space are designed without parallel surfaces and the bass is left largely alone.

Bryan

Thanks for your reply. Sorry it has taken me a while to reply back, wasn´t anywhere near a PC n the past few days.

But, if I leave the bass "alone" then I could have a room with say 2.5s Rev Time at 125Hz and 1s at 1KHz. That would sound very uneven wouldn´t it?.

Do you know if there´s a forum where people discuss Large Space Acoustics. Generally all forums are oriented towards small room acoustics. Or perhaps a book or papers you can recommend. I am a member of the JASA, so I could probably get a few over there.

Thanks

chao

Sergio

 

[MidnightSensi]

Thanks for your reply. Sorry it has taken me a while to reply back, wasn´t anywhere near a PC n the past few days.

But, if I leave the bass "alone" then I could have a room with say 2.5s Rev Time at 125Hz and 1s at 1KHz. That would sound very uneven wouldn´t it?.

Do you know if there´s a forum where people discuss Large Space Acoustics. Generally all forums are oriented towards small room acoustics. Or perhaps a book or papers you can recommend. I am a member of the JASA, so I could probably get a few over there.

Thanks

chao

Sergio

Check out prosoundweb.com. The forum there has some good information and may be able to help you more.

You'll find a lot of the recommendations for small rooms aren't the same for large rooms. Also a lot of different considerations need to be made for a room with an audience in it.

 

[highfigh]

Thanks for your reply. Sorry it has taken me a while to reply back, wasn´t anywhere near a PC n the past few days.

But, if I leave the bass "alone" then I could have a room with say 2.5s Rev Time at 125Hz and 1s at 1KHz. That would sound very uneven wouldn´t it?.

Do you know if there´s a forum where people discuss Large Space Acoustics. Generally all forums are oriented towards small room acoustics. Or perhaps a book or papers you can recommend. I am a member of the JASA, so I could probably get a few over there.

Thanks

chao

Sergio

Just putting the panels across the corners won't do much. Thickness is better for absorbing bass and non-parallel surfaces (not likely) are good, too.

 

[chickenfingers]

Check out prosoundweb.com. The forum there has some good information and may be able to help you more.

You'll find a lot of the recommendations for small rooms aren't the same for large rooms. Also a lot of different considerations need to be made for a room with an audience in it.

Thanks a lot for the site´s address!!. It surely looks more oriented towards large acoustic spaces.

Sergio