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Installs Gym: Sound Absorbtion / Diffusion Panels

Discussion in 'Sound, Music, and Intercom' started by jamsession, Nov 10, 2008.

  1. jamsession

    jamsession Member

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    I'm doing a Christmas concert production (piano, mic'd orchestra, choir) on stage. The back wall is 71' from stage, brick, 30' high.

    The back wall is 80' across, and can go from the floor (up to some existing panels which start at 14' up and are 8' high above that. They do help some with overall room sound, but dont reduce the slapback)... One Big Wall, no bleachers.

    There are 2 speakers Front of house on either side of the stage, about 15 feet up. (above the basketball stands. They are pointed down towards the middle of audience, and it sounds pretty good from most places on the stage.

    HOWEVER, there is a nasty slapback echo noticable to anyone on stage because of the energy bouncing off the back wall. We pointed the speakers down as much as possible, even beyond the mounting brackets pre-drilled holes.

    I'm looking to add an acoustic treatment on the back wall to absorb/diffuse that. it will make a big difference to the conductor and performers on stage.

    Since the room is also used daily for backetball games and athletics, I'm looking for something that can fold down when needed, or more likely. some panels to set up for concert performances, of which there are only 3 main ones a year. I looked at curtains to match the stage, with 50% or 75% fold to match the stage curtain, cost was a bit high.

    I'm currently looking into making some wood frame panels with rigid fiberglass insulation inside, black fabric stapled over.

    1) has anyone done that? got pics or plans I could see? I have Mitch Gallaghers book on home studio acoustics where he talks about making them.

    2) 14' is big, so I'll probably need to make them fold verticallly to fit through a door to a storage room. I'll make them as thin as effectively possible to minimize storage bulk.

    3) I'm considering angling them to provide most diffusion possible. would take more material, but making a big flat wall of panels will probably bounce back more to the stage than an angled (accordian) diffusion type approach.

    4) rigging: will probably use wheels on bottom, and secure to a row of eye bolts into the brick where they touch the wall. just below existing sound panels where will be visually minimal.

    5) size: if I make them 4 foot wide, and angle so they come out 3' from wall, that gets me a decent diffusion angle. A few hard surfaces at the hinges/connection point, but if frames are thin can live with that.

    6) design: I'm guessing if the panels are angled, it wouldn't require using acoustically transparent fabric( $$$) in front of the rigid fiberglas mat) and go for something cheap and black. thoughts?

    budget is an issue.

    a stage curtain with carriers, track, and pull rope (matching the stage curtain) is in 15-20K range.

    I suppose we could rent pipe and drape, that would help - but it wouldn't be near as big (maybe 10' high?)

    If this would provide the accoustic solution we need, it would be worth building it and keep material costs under a few hundred $.

    Have you made or seen panels like this? Experience / suggestions welcomed.

    Thx!
     
  2. cprted

    cprted Active Member

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    From my own gym to theatre-like-space conversions, I found the cheapest/easiest solution was the hang drapes along the side and back walls. Not perfect by any stretch, but it dampened the acoustic toilet bowl effect to a workable level.

    We rented a couple of 40'x40' drapes and hung then along the walls using a-frame ladders. Only took a crew of four students an hour to do. It would be even faster if you had a gennie/scissor lift (assuming there were enough adults available to operate said lifts).
     
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  3. museav

    museav CBMod CB Mods Departed Member

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    I know both of the experts Mitch references in that book, Jeff Szymanski and Russ Berger, in fact Jeff and I graduated from the same program. Both Russ and Jeff would probably be among the first to tell you that there are significant differences between small room and large room acoustics and that a gym is not a home studio. A combination of absorption and diffusion on the rear wall is probably god idea, just don't necessarily go by what is recommended for home studios.

    If the sound can't easily transfer through the fabric, then why have the fiberglass at all? If you have a less permeable covering then you are creating a membrane absorber, which may help or not depending on the specific needs of the space and the characteristics of the panels. One other relevant critical aspect in any such endeavour is that you can't use just any material, all materials should be properly rated for the use, including smoke and flame spread ratings.

    Here's something to think about. The speakers are apparently 15' above the floor and ear level of someone on stage is probably around 7' above the floor, so your critical point for slap back from the speakers is around halfway between those or 11' above the floor. In addition, any treatment below 4' above the floor or so will be blocked by the audience. So how about treating from maybe 5' above the floor to 13' above the floor? That almost cuts the area of material in half yet probably gives you most of the effectiveness.

    Heavy draperies hung a foot or two out from the wall (which increases the performance at low frequencies) would typically be the most cost effective option.

    If you want a black fiberglass material, check out the Owens-Corning SelectSound board (Commercial Acoustic Systems: SelectSound® Black Acoustic Board) and batt (Commercial Acoustic Systems: SelectSound™ Black Acoustic Blanket) materials, they are already black.
     
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  4. bluemando

    bluemando Member

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    By your comments I would have to guess that you are guessing at a lot of this. For one thing, aiming loudspeakers down to avoid reflections off the back wall is not really a proper solution. Aiming the loudspeakers correctly and treating problem areas is the best approach.

    Angeling absorptive panels will not create diffusion. Diffusor panels can be amazing but they do cost.

    Curtains? Umm, well that would be better than brick in this case for sure, but curtains make the best curtains and acoustic panels make the best acoustic panels.

    You can build panels - even 4ft. sq panels and placed as a field (multiple rows and columns) on the wall where reflections are causing problems. 1x4 wood frames with a door skin back, filled with r-11 wall insulation, covered with fire rated fabric and suspended off the wall (1 inch or so) will deal with reflections and help absorb bass. You can build them 4ft x 4ft x 4 inches and leave about 4" between them. How many and where they should go is the question. There are lots of places on the WWW to find plans for panels.

    The more activities that go on in a room the more challanging it is to find the right treatment that will do the job and last and look good in the long run. Keep the Basketballs in mind and put something up that can stay up, there is nothing wrong with a good sounding gym.

    If you are really serious about this you will have to take a serious approach - hire an acoustical consulant that will help define the target and specify the areas for treatment - he/she can help you with details and perhaps some do-it-yourself treatment options.

    A great loudspeaker system in a room that is umm not good won't sound as good as an average loudspeaker system in an acoustically great room.

    Remember that if the goal was to make every surface dead then we wouldn't need acousticians, you would just cover the room in absorptive panels - the challange is to dial in the rooms acoustical properties to facilitate the performance style(s). That is - controlling the rooms "liveness" so it's fun for the performers and the audience.

    But it is fairly safe to assume by what you describe that putting absorptive material over the brick back wall of your room is a safe idea. I would be thinking in the range of 2" thick rigid fiberglass panels from a factory or, like I mentioned before, 4" thick mineral wool fiber or fiberglass insulation panels.

    You have reflections to deal with as well as the overall room RT to think about. Finally, again - if you are serious then take a serious approach and invest in some professional help. Do it right - it will be the gift that keeps on giving...
     
  5. tenor_singer

    tenor_singer Active Member

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    I was toying with trying to dampen the sound in my old high school's gymnasium. The bass would just roll around in the ceiling for an insufferable amount of time making all sound muddy.

    It was suggested to me by a friend in the industry that they make a paint product that can be sprayed onto hard surfaces that expands on contact (like a foam). He said from personal experience that it had very good acoustic dampening abilities.

    I never tried to push it through our B.O.Ed. because we had just been approved for a new building by the state and it seemed foolish to spend the money for a two year period, so I cannot give you my personal experience with the material.

    I do remember that the cost estimate per square foot made me take a deep intake of breath.

    Maybe some of our professional friends on the fora have heard of the material? I can't seem to remember the name.
     
  6. jamsession

    jamsession Member

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    Thx -

    Bass isn't the problem in this case - it's the midrange frequencies.

    We did position the speakers for the best dispersion, as mentioned, things sound great on the floor... but if you are on stage, you hear the slapback

    And since it is a gym and there are already some 8' square sound panels at 14-22', additional temporary treatment is needed just for concert occasions at the lower heights (in range of basketballs on other days).

    So it's thin fiberglass panels or drapes.

    The drapes would be a bit trickier rigging, for that size, (not insurmountable) and less bulky to store - renting might be a suitable option. But for the cost of heavy (flame retardant) fabric stage drapery (I've already priced that out) I could build panels which will probably do a better job absorbing. I'll compare both options and decide.

    I'm not worried about side reflections. It's a pretty lively space already, fixing the back wall issue should make it fine overall.

    Thx
     
    Last edited: Nov 11, 2008
  7. museav

    museav CBMod CB Mods Departed Member

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    Not a paint products, but there are several spray-on acoustical treatments and they are actually typically quite cost effective. Ones that I am familiar with include K-13, SonaSpray fc and Sonakrete. However, there are limitations with them regarding the thickness of the treatment and where it can be used. You can do things like have them modify the mix to add extra binder to allow for greater thickness and durability, but at the cost of less high frequency absorption. One of the biggest advantages is some applications is that the same material provides significant thermal insulation and can be tinted from bright white to black or even matched to a desired color (for a cost).

    Thin curtains directly over a surface are of limited use but heavy drapes spaced off the wall surface can actually make a very effective absorber. The cost for acoustical panels can vary greatly based on the finish material, edge treatment, etc., but $10 to $15 per square foot would probably be typical, although on another forum someone was recently quoted $7.15 per square foot for about 1,100 square feet of 2" thick, fabric covered panels from a manufacturer I did not know.
     
  8. jamsession

    jamsession Member

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    Thanks for the info -

    I read the specs on the Owens Corning SelectSound Black Acoustinc Board and Blanket in the links Brad provided (thx) - and its pretty impressive for absorption rates. one quote I got on 4x8' 1" thick was $58- great product but also a bit out of range for this project, where I'll need estimated 20 of them (I may be able to get by with less panels if I space them.)

    The other building supply places carry 1/2" "sound board" which is designed to go under (behind) gypsum board in wall construction. I believe made by celotex, Georgia Pacific, and BlueLinx. It's $10-13 for 4x8', 1/2" I'm looking for acoustic specs on that,if anyone has, please advise..... otherwise I might try making panels, filling with roll-in insulation, and covering with fabric.

    Of course gyms and home studios are different beasts, but most principles translate, and he says in his book that "any cloth you can breathe through will be open enough to allow sound waves behind it to be absorbed by the glass." Most sound panels I've seen use burlap, etc. So I don't have to use expensive speaker grill cloth in order to make sound panels.

    He also says "the go-to rigid glass fiber for many do it yourselfers is the Owens Corning 700 series, specifically types 703 and 705,"
    I'll check specs for those.

    I wonder how how above mentioned soundboard (celotex, GP, Bluelinx) compares with 700 series and regular fluffy pink insulation. Thoughts?
     
    Last edited: Nov 11, 2008
  9. museav

    museav CBMod CB Mods Departed Member

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    That is a material made for sound isolation and has little or nothing to do with absorption, you probably won't even find absorption data on it.

    Yes, but they are not public spaces, which have different code requirements. Look up The Station or Great White and see what happens when the wrong products are used in public spaces. You could use burlap, but it would likely have to be treated.

    Beyond that, small rooms are usually modal controlled and many don't even technically have a reverberant field while large rooms tend to be reverberant controlled with most room modes occurring at low frequencies.

    The SelectSound products match up pretty well with the equivalent 700 series products. As noted above, the "sound board" products are intended for a completely different purpose. By the way, for music applications you'll probably want a minimum 2" thickness and 4" or so greater be even better.
     

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