Audio

Gymnatorium Absorption: The Sweet Spot

Although acoustic absorption is not usually the focal point of an install, it should not be discounted. You may not get the comment, “I love the sound of these baffles…” but if you don’t take into account the acoustics of the room, you may not hear, “I love the sound of these speakers,” either.

Acoustics historically have been an afterthought in every respect; the architects were more concerned with building codes and functionality, the interior designers didn’t see acoustic treatments in their aesthetic vision, and perhaps the sound system designer either assumed the architect took care of it, maybe it wasn’t discussed or they didn’t think it was a problem. However, it happens that acoustics are frequently overlooked or unaddressed, and this can lead to some unhappy clients.

Why do I specifically mention absorption?

Absorption is the reverb killer, the RT60 sniper, the echo eliminator, the… you get the point. It refines the clarity of the space, which makes it so that everyone can clearly hear the intended source without too much coloration from the room itself. This is especially apparent in large spaces. Applying the appropriate amount of absorption to a space is both an art and a science: How will you know how much to use, of what type and where to put it? This changes from space to space, function to function and person to person. Treating a movie theater is different from treating a live music venue, or a playhouse, or church of the same size and dimension…even if the buildings are identical.

The reality of the situation is that you will only be 100% sure after it is all done and the client is happy. The good news is that you can get a decent idea of what the result will be if you can do some reasonably simple (however lengthy) math, and also by having a good idea of the final function of the space. Some people have begun doing full-blown acoustic simulations of spaces that haven’t even been built. This may be the future; however, acousticians are usually working with spaces that have already been built, as much as we would have appreciated having some input earlier in the game. So let’s skip over the world of the ideal, and focus on reality.

One treacherous acoustic realm that many have faced is that of the gymnatorium: part gymnasium, part auditorium, movie theater and all-around reverb chamber of doom. These buildings are constructed to save cost by having a multipurpose space that can handle many different functions, so is it any surprise that you are looking at a space that was built on a budget as well? Many of the common issues are metal and concrete construction, hard floors, exposed trusses, HVAC, lighting and sprinklers…and perhaps some glass, parallel walls, ceiling and floor. Starting to get the picture?

To complicate matters even further, the activities and seating arrangements may change daily or even hourly. You may have a gym class one period that uses all the floor space, then an assembly with folding chairs, guest speakers and videos, an afternoon basketball game with bleacher seats and, finally, an after-school concert with bleachers, floor seats and standing room. All have very different needs.

RT60Calc

All graphics: Joe Horner, Acoustics First Acoustic calculator.

The correct answer is the solution that will work for all of these, even if it isn’t the ideal solution for any of them. What we do know is that it’s tough to do anything with a 3.5- to 6-second reverb time, which is a fairly common RT60 measurement for these spaces. We would really like to have something like 1.5 seconds.

Quick review: RT60 time is the amount of time it takes for a sound to decay (drop) 60dB. I suggest doing this measurement if you can, or get someone to do it. Bring a portable recorder (phone), a balloon to pop and a popping implement (key, pin, etc.). Inflate the balloon, ask for a few moments of silence, start the recorder and pop the balloon. Make sure you record the entire reverb tail, and no one makes any extraneous noises during the recording.

Now you can import the recording into a computer, measure the waveform and figure out the RT60 time. This measurement will assist you in your calculation of materials by helping you correct for variations in construction, measurements, material quality etc. (aka correcting for the real world).

All the materials used in a room have an impact on the sound: Very hard materials, such as concrete, are very reflective, while softer materials, such as curtains and carpet, are somewhat absorbent. One formula used for RT60 calculation uses the Sabine Equation.

Where:

RT60V is the volume of the room in ft3
S is the surface area of the room in ft2
α is the average absorption coefficient
Sα together is the total sabins for the room

So, get your measurements, note the materials, look up the absorption coefficients and fill in the equation. Spreadsheets will be your friend on this one. Take the time to do it right; that work will save you time in the future. It will work like this:

Room dimensions Length(l), Width(w) and Height(h)
Volume = l x w x h
SabinsFloor = l x w x (sound absorption coefficient of floor material)
SabinsCeiling = l x w x (sound absorption coefficient of ceiling material)
SabinsWall1 = l x h x (sound absorption coefficient of Wall1 material)
SabinsWall2 = w x h x (sound absorption coefficient of Wall2 material)
SabinsWall3 = l x h x (sound absorption coefficient of Wall3 material)
SabinsWall4 = w x h x (sound absorption coefficient of Wall4 material)

Total all the Sabins calculations:
Open the calculator, type .049 x (Volume) ÷ (Sabins Total) = (RT60!)
(Note: This is for the Imperialists out there: If you use metric, replace 0.049 with 0.161, and use meters.)

Fun part number one: How did this calculated RT60 compare to the balloon pop RT60? If you’re close, you’re good to go. If you’re not, you may have to revisit your equations and adjust the values slightly to match your measurement. If you’re way off, then you may be missing something and you should do some investigating: A concrete wall may be coated with rubber sealant, sheetrock may be applied directly to concrete, flooring may not be hardwood…it may just appear that way.

After checking your calculations, it’s time to treat. The best place to start is to figure out how many sabins worth of material you need to reach your RT60 goal, in this case 1.5 seconds.

Easy:

Sα

That’s it. Look familiar? It’s the same equation as before, instead we know the RT60 time we want; we just want to solve for the amount of absorption we need. One of the great utilities of this calculation is that, if you already know the sabins of the material (as is common in hanging baffles), you just divide the sabins total by that and it will tell you exactly the number of baffles you need; don’t forget to subtract the absorption that your room already has! If you are treating a general-purpose room, this may be the fastest, easiest, most implementable and most cost-effective solution for everyone involved, allowing you more wiggle room with your budget to focus on the other project requirements.

A client comes forth and says, “I have a giant multipurpose room: It’s a gym, theater, stage, function hall, we do lectures….” (Sound familiar?)

Without even seeing the space, you can get a good idea of what has to be done. Ask for measurements, materials the room is made out of and a recorded balloon pop, then get to work.

Run the “as is” chart in the lovely Excel spreadsheet I told you to make, use the measurements and check with the balloon pop, then figure for about 1.5 seconds. (This is just a general guide for this type of space. Some rooms may have requirements for a shorter (or longer) RT60, so adjust accordingly.)

Calculate the before and after, and go.

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