How much absorption

Matthew J Poes said:

Sorry if I'm being dense here, what is a PAC?

How big is this venue? You mentioned the RT60 is too high, but without knowing its volume, its hard to judge that. Certainly getting a flatter RT60 is preferable.

If it is a large venue, I would take additional measurements. If you can measure in 4-6 locations in the room and share the mdat file, I could maybe say more.

I would go with 4" panels in at least some locations since 500hz is at the low end of the absorption peak in 2" panels.

RT60 calculators are not the most accurate thing for a number of reasons. First, they assume that the base room has no absorption, but that isn't true. The physical dimension may not match the "acoustic" dimensions of the room (rooms often act larger than they are). The surfaces are typically not as stiff as the calculators assume, their acoustical impedance is uneven and somewhat low at lower frequencies, unless the walls are block construction. Not all block construction is the same either. Is it one row or two? Hollow in the center? Filled with cement? Unused speakers (during the test) act as tuned absorbers (not necessarily tuned to anything good either).

If you need to improve acoustics temporarily or be able to take them down, curtains provide a lot of absorption and would improve things dramatically. That may be an option worth considering. What you need is to cover a lot of surface area, so that can be beneficial.

A very large venue is tough to treat. The number of panels needed is enormous. The scale is so different that people used to treating smaller spaces often underestimate what is needed for a larger space. A given amount of absorption in a small space goes way farther than it does in a large venue.

Let's say that a typical small room is 3000 cubic feet of volume. a 4'x8'x2" is something like 5.4 cubic feet of volume in absorption. Nobody would expect a single panel like that to do much in a room. Such a room might be dimensions as 25'x15'x8' and so we are talking about covering say 15-20% of any one wall. Typical suggestions would have you using more like 2 of those panels per wall at a minimum, covering 30% to 40% of a given wall, or more. It is not uncommon to see a suggestion of covering 1/3 of an entire room with absorption and have that be considered a small amount of absorption. Now if we move up into a space that is maybe like 30,000 cubic feet, ten times larger in volume, with wall dimensions that are perhaps 50'x30'x20' and you can see that to achieve the same 30% coverage of a single wall would need not 2 panels, but more like 9-10 panels per long wall (6-7 for the shorter walls). So where we could have gotten away with maybe 6-8 panels in a normal sized room for light absorption, we would need 30-35 panels in a large room to achieve similar results.

I suggested thicker panels and I want to explain that. In large venues as large as 30,000 cubic feet, you typically don't have room modes anymore because the dimensions of the room are nearly as long or longer than the lowest frequency being reproduced in the room (and lowest we can reliably hear). As such, low frequency absorption to deal with room modes isn't an issue. We still want to absorb some of the bass however to flatten the RT60. However, in practice panels don't absorb the way the absorption data would have us believe. Let's say all you need is absorption down to 300hz, you might find that a 2" panel is all that is needed, a 4" panel absorbs just as much at 300hz and above, it's only better below 300hz. Then why buy the thicker panel? Because in practice the absorption of the 2" panel at different angles of incidence is not even, but a thicker panel will even it out. That helps even out the frequency response of the reflections.

If the ceiling is very tall and you can do so, I would add some baffles. These are hanging absorbers. They help reduce standing waves and reduce bass buildup, they absorb some of the ceiling reflections, they generally lower the RT60. All good things. They often do this without negatively impacting the ambiance of the space. In a very reflective space, they also help reduce the perceived volume of the room too. Ever been in a really noise restaurant where the room's acoustics seemed to just make that noise obnoxious. Often it is because the room is highly reflective and treating the ceiling with baffles would make it much more tolerable. It will also improve clarity for the same set of reasons.

Click to expand...

bkeeler10 said:

I have EASE (acoustic modeling software) at work. If the room is not too complicated, I'd be willing to model it for you and tell you what effect the panels will have on RT60.

I would need a to-scale drawing showing some dimensions, and I would need to know what all the surfaces are in the room (sheetrock, carpet, seating areas, etc).

Click to expand...

Matthew J Poes said:

All sounds good. I think your recommendations seem good. the 4" panels are a good idea and 16 of them would be a good number if they also add curtains.

In my experiences with PAC's (I'm with you now) there was always a good amount of absorption in the form of curtains. Many had big cavities where the organ pipes once were or still are. Is this a more modern building with a lot of reflective surfaces instead?

What was your sound source for the measurements? In small rooms there is no real issue with using a directional speaker, it doesn't really matter much. In large spaces like this, the use of an Omni sound source becomes important. Since I'm more of an amateur acoustic consultant rather than a true licensed professional, I make due with what I can. I've done one of two things. If they have extra speakers, I've created an Omni array. This is nice as its a more full range source. I do this with just 6 speakers mounted so that they point up, down, forward back, left right. You probably get the idea. Since most places don't have such things laying around, what I do is use a large balloon taped to a stick and pop it. The balloon needs to be where the sound sources would normally be, so I would try a few stage locations and just above the orchestra pit. You can buy the balloons at a party store, bigger ones make a more full range impulse, so I would look for big ones and fill them up nice and big. That helps ensure the sound is loud and full. I don't use REW when I do this so I'm not sure if REW can handle this kind of external impulse. I would think so. I know it can handle an external sound source.

If you do the Balloons, you may want to then use their sound system as well, since that tells you specifically what their sound system is doing in the room, but the balloon test isolates the room issues and also is good if you have non-amplified sound sources.

I personally have very limited experience in spaces this large, I've typically worked with other experts rather than being in charge, so much of my suggestions are what I have seen recommended and make sense to me based on what experience I do have. It's possible a true professional would have something different to say.

There are a number of calculations that are common in performance spaces that REW doesn't support. You can do most of these using REW and some external calculations however. Some require different software, so I use a mixture of Audiotools on my phone and ARTA. Audiotools, ARTA, and REW collectively cover everything I need, and each allow files to be exported for analysis. Examples of measurements that may require something like ARTA and some external anlysis are IACC measures. These require two mics.

http://www.bnoack.com/index.html?http&&&www.bnoack.com/acoustic/concerthall.html
This is a good reference if you don't already have it. It may have some useful information.

Click to expand...