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Natural Science Forum / Physics / Acoustics / December 2004Acoustic cinder block (properties?)
Hi The gym in a local elementary school was constructed with "acoustic cinder blocks" where every other block or so has three slits (by eye, slits are maybe 10 cm by 1 cm) Acoustics in the room are terrible, the gym teacher effectively can't teach without a bullhorn. The room also serves as lunch room and general purpose room, but they can't use it for anything that requires communicating... Out of curiosity I'm wondering what the acoustic properties of these acoustic cinder blocks might be. [OK, the scientist in me requires that follow-up, ...umm and the school board donated the property for my venue just across from the school. ] I may be asked to do a preliminary study of the room and then seek out an appropriate acoustic professional. [However, I'm sure the room needs more remedial work than they can fit in their budget... ] They have an installed sound system that (I've been told) only makes things worse. Ron Capik NJ Pinelands Cultural Society < www.alberthall.org > -- Ron, > every other block or so has three slits < I've seen blocks like those used in school band rooms, and I believe they are Helmholtz resonators tuned to absorb the bass range around 100 Hz. As you observed, such blocks do nothing to reduce mid and high frequency echoes that plague intelligibility. --Ethan > The gym in a local elementary school was constructed > with "acoustic cinder blocks" where every other block > or so has three slits (by eye, slits are maybe 10 cm > by 1 cm) These are the common "SoundBlox", > Acoustics in the room are terrible, the gym teacher Then there are an insufficient number of them. I have often designed them into gyms, and I can say that the area of such blocks must be at least 10% to 25% of the projected area of each wall. > effectively can't teach without a bullhorn. > The room also serves as lunch room and general > purpose room, but they can't use it for anything > that requires communicating... In addition, the ceiling must carry a similar amount of sound absorption. "Acoustic Roof Deck" is preferred, but it must be designed in at the outset.. Otherwise, hanging sound absorbing baffles up there is the only easy solution here. > Out of curiosity I'm wondering what the acoustic > properties of these acoustic cinder blocks might be. The NRC of SoundBlox is perhaps o.7 or better, but the specific absorption type must be selected, for voice in this case. > [OK, the scientist in me requires that follow-up, ...umm > and the school board donated the property for my > venue just across from the school. ] > They have an installed sound system that (I've been told) > only makes things worse. The only sound system that will do any good is one with high directivity horns, aimed at the seated audience positions. > Ron Capik > NJ Pinelands Cultural Society > < www.alberthall.org > > --  Signature--------- www.CampanellaAcoustics.com --------- "I have simply studied carefully whatever I've undertaken, and tried to hold a reserve that would carry me through." - Charles A. Lindbergh. "As for background noise level; 35 dBA is a good classroom; 45 dBA is a sound masking system!" - Anthony K. Hoover > > ..snip.. > > [quoted text clipped - 3 lines] > them into gyms, and I can say that the area of such blocks must be at > least 10% to 25% of the projected area of each wall. Thanks Angelo. I've located the SoundBlox web page. I'll read up on their technology. > < ..snip.. > > In addition, the ceiling must carry a similar amount of sound > absorption. "Acoustic Roof Deck" is preferred, but it must be designed > in at the outset.. Otherwise, hanging sound absorbing baffles up there > is the only easy solution here. ...didn't notice any absorption in the ceiling. > < ..snip.. > > [quoted text clipped - 3 lines] > The only sound system that will do any good is one with high directivity > horns, aimed at the seated audience positions. I noticed horns, but they seemed to be directed at the back wall. I only got a brief look at the room. Didn't want to interrupt the class. I'll get a better look when/if they call me back. > --------- www.CampanellaAcoustics.com --------- > [quoted text clipped - 3 lines] > "As for background noise level; 35 dBA is a good classroom; 45 dBA is a > sound masking system!" - Anthony K. Hoover Later... Ron Capik -- A propper line array loudspeaker system sometimes even works better than a horn system. > > > ..snip.. > > > [quoted text clipped - 39 lines] > Ron Capik > -- > A propper line array loudspeaker system sometimes even works better > than a horn system. It depends on the distance from the speaker to the seating section being served. One popular stadium speaker arrangement is to have one speaker for each seating section. This results in perhaps a dozen speakers on a ring that traces the front edge of the grandstands, and each located several meters above the lowest of those seats, etc. If one is trying to project across space, say, from a scoreboard position over the longest dimension of a football stadium, your array thought is appropriate. The sound so projected, if through only a few large horns, may be distorted via the finite amplitude distortion accompanying high amplitude sound reproduction; it will sound 'harsh'. Angelo Campanella > > A propper line array loudspeaker system sometimes even works better > > than a horn system. [quoted text clipped - 11 lines] > > Angelo Campanella Also, I'm talking about an elementary school (grades K to 4) gym, just guessing maybe 80 feet by 80 feet by about 20 feet high; no grandstands or even pull out seating. Hmmm, one or two of those Bose L1 systems might work for basic assemblies an such... Maybe I'll get a better look at the space after the holidays. ...and thus: Merry Christmas and Happy Holidays to all! Later... Ron Capik -- >> > A propper line array loudspeaker system sometimes even works better >> > than a horn system. [quoted text clipped - 11 lines] >> >> Angelo Campanella >Also, I'm talking about an elementary school (grades K to 4) gym, just >guessing >maybe 80 feet by 80 feet by about 20 feet high; no grandstands or even pull >out seating. Hmmm, one or two of those Bose L1 systems might work for >basic assemblies an such... >Maybe I'll get a better look at the space after the holidays. You could do a hell of a lot better at 1/10th the cost with some basic PA speakers. Those L1 speakers are so lame, Bose won't even say what their power handling is. The L1 is only for rich idiots. > < ...snip.. > > >out seating. Hmmm, one or two of those Bose L1 systems might work for [quoted text clipped - 6 lines] > > The L1 is only for rich idiots. OK, for the most part I was just thinking out loud there, but if you can recommend a nice compact "system" that might meet their needs for under $200 I'd like to hear more... Later... Ron Capik -- >> < ...snip.. > >> >out seating. Hmmm, one or two of those Bose L1 systems might work for [quoted text clipped - 6 lines] >> >> The L1 is only for rich idiots. >OK, for the most part I was just thinking out loud there, >but if you can recommend a nice compact "system" that >might meet their needs for under $200 I'd like to hear more... The L1 is $1600. At least two, more like four, sets would be needed to fill a gym. Any of the big names like jbl, klipse, pioneer, infinity, etc. A flea market car stereo box would blow away the L1's. Ron, You probably wont find adequate loudspeakers for a space of this size for under $200 each, let along a sufficient quantity to handle the space for only $200. I'd suggest you look into JBL professoinal seres, or EAW or Renkus-Heinz. As a last resort, I might consider Community. You should consider some professional help. Go to http://ncac.com and serach for consultants in New Jersey. Good luck, Dave Fagen in Florida > Ron, > [quoted text clipped - 10 lines] > Dave Fagen > in Florida Just to clear things up, that $200 price is only in response to TCS's statement that one could do better than the L1 for one tenth the price. That L1 thing sells for under $2000 this one tenth is under $200... Thus far the project is only an item on the principal's wish list. I do know my limits, and intend to seek proper professional help as the need arises. Thanks for the link. Later... Ron Capik -- Hello Angelo, Over here we use small line arrays (2-4 meter) for example in historic churches. You cannot treat them acoustically because them are monuments. A propper compact line array with good wave coupling between the speaker units can provide very high directivity and easily limit side lobing to under 15 dB, which is sufficient for good STI when you make the main lobe shear just over the public. you can even use digital beam stearing etc to make things better. Not for 200 bucks, of course :-) > > A propper line array loudspeaker system sometimes even works better > > than a horn system. [quoted text clipped - 3 lines] > speaker for each seating section. This results in perhaps a dozen > speakers on a ring that traces the front edge of the grandstands, and > each located several meters above the lowest of those seats, etc. If one > is trying to project across space, say, from a scoreboard position over [quoted text clipped - 4 lines] > > Angelo Campanella |
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