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Natural Science Forum / Physics / Acoustics / May 2006REQ: Low frequency sound absorbtion
Hello, I am trying to make a room as quiet as (reasonably) possible, especially below 200 Hz. The room is a laboratory about 6m wide by 8m long by 4m high. The floor is linoleum, the ceiling is exposed concrete (and lots of ducts and pipes), and the walls are 12mm sheetrock on metal studs. There is a scanning electron microscope in the room. The quieter we can make the room the better images we get. I have read older posts, especially by Eric Desart, about using rigid fiberglass panels, such as Owens-Corning 705. This appears to be the best solution so far. We are also concerned about flammability particle generation, and outgassing. I would appreciate any advice on methods and arrangement of applying the panels and alternative materials. Rebuilding the walls is not an option at this time but may be in the future. Thank You > I have read older posts, especially by Eric Desart, about using rigid > fiberglass panels, such as Owens-Corning 705. This appears to be the > best solution so far. We are also concerned about flammability > particle generation, and outgassing. That will help. At high frequwenies (500 Hz up) any acousical absorber works fine. But for lowe frequencies, the geomtry of theinstalled material beome increasingly inpotant as the frequency lowers. Ultimately, furred materals mus be used, and the furring depth can be 4"-6" and more at the lowest frequencies. "Wedges", though trndy, really con't help and are certainly never cost effective. > I would appreciate any advice on methods and arrangement of applying > the panels and alternative materials. The ceiling is usually an arguable choice because of logistics, though the conventional suspended grid acoustical ceiling is always welcome in these matters. That shuld be your first priority. > Rebuilding the walls is not an option at this time but may be in the > future. Well, there you have it. Let us know which ceiling configurations your management will tolerate. If not that, then hanging baffles are your only choice. Or you can operate late at night when no one is around and you can turn off the HVAC during critical runs... se la vie. Angelo Campanella > > I have read older posts, especially by Eric Desart, about > > using rigid fiberglass panels, such as Owens-Corning 705. [quoted text clipped - 34 lines] > > Angelo Campanella Thank you for your response. I suppose we can install any type of ceiling. Do you think a suspended grid ceiling would be better than hanging baffles? Can you suggest a link that discusses hanging baffle geometry? This is a lab so appearance is not very important. We only care about dampening low frequency sound. You say the geometry of the panels on the walls is important. Can you tell me what geometry is best or suggest a link? By furred, I assume you mean mounting the panels on spacers so there is space between the sheetrock wall and the acoustical panel (mounting the panels on furring strips). Is this correct? A note to Mr. Ghost: We have measured mechanical vibration and it is not so bad. The problem is mostly noisy machinery in adjacent labs. After we try acoustical dampening we are going to look at active vibration canceling. Thank You > A note to Mr. Ghost: We have measured mechanical vibration and it > is not so bad. The problem is mostly noisy machinery in adjacent > labs. After we try acoustical dampening we are going to look at > active vibration canceling. I think you should have an acoustic consultant on board, who should be able to save you money or at least avoid the risk of wasting money. For your background information, ceiling tiles can give very cost-effective low frequency sound absorption if you choose suitable ones and leave an appropriate spacing (usually 200 -300 mm) from the structural ceiling. However this will not damp all modes so you will also need absorption on the walls. The easiest way to make effective low frequency absorption is usually to have lightweight panels over an enclosed air space with some damping. I would reinforce what the Ghost wrote: make quite sure that the vibration is getting in via excitation from airborne sound, and not by direct mechanical coupling, before spending money on room treatment. One way to check this would be to produce sound from a loudspeaker in the room and find how the sound level relates to deterioration of the image, then compare this with the sound level from the interfering machinery. Also consider that unless the room is very reverberant in the relevant frequency range before treatment, it is not practical to make a large reduction in reverberant sound level by adding sound absorption. How much improvement do you need?  SignatureTony Woolf My e-mail address has no hyphen - but please don't use it, reply to the group. > > A note to Mr. Ghost: We have measured mechanical vibration > > and it is not so bad. The problem is mostly noisy machinery [quoted text clipped - 26 lines] > to make a large reduction in reverberant sound level by adding > sound absorption. How much improvement do you need? Thank you for your response. We have had measurements taken by the SEM manufacturer and they indicate that the mechanical vibration is O.K. (but not great) except for one peak around 8 Hz of 2 um/s, which is the maximum allowed. The noise is quite audible. We have taken measurements that show sound peaks of ~50 dB at ~60 Hz and ~120 Hz so we would like to minimize these. This is mostly from a large air compressor that we plan to move and insulate. Ideally, we would would like the sound level to be less than 35 dB A-wtd below 200 Hz. If I use Owens-Corning 705 panels I assume the best way to install them is to space them away from the wall as far as possible and place the panels in contact with each other. I can probably make the spacers out of the 705 panels. I would love to hire an acoustic counsultant but the administration is very resistant about hiring outside people. Thanks again > The noise is quite audible. We have taken measurements that show > sound peaks of ~50 dB at ~60 Hz and ~120 Hz so we would like to > minimize these. This is mostly from a large air compressor that we > plan to move and insulate. If you are planning to move the interfering sources, why are you proposing to spend money on other measures that will not be very effective? > Ideally, we would would like the sound level to be less than 35 dB > A-wtd below 200 Hz. The sound levels you mention add up to about 35 dB(A) already, unless those levels are already A weighted. If they are, you have quite a long way to go. You also wrote in your other post: > I think I understand what you are saying but there has to be some way > to isolate a room from external sound sources. We just have to do > this by installing material inside our room. Acoustic design or interior decoration, your choice. SignatureTony Woolf My e-mail address has no hyphen - but please don't use it, reply to the group. > Hello, > [quoted text clipped - 20 lines] > > Thank You You may want to consider the possibility that the cause of the problem is structure-borne vibration. If it is, reducing sound level will reduce an effect (sound) but will not reduce cause (vibration) of the problem. If low-frequency airborne sound is indeed the problem, rather than reducing the level of low-frequency sound in the entire room, you may want consider the possibility of using active noise cancellation around only the sensitive area of the microscope. > Rene Tschaggelarwrote wrote > A note to Mr. Ghost: We have measured mechanical vibration and it > is not so bad. The problem is mostly noisy machinery in adjacent > labs. After we try acoustical dampening we are going to look at > active vibration canceling First, I agree with Tony, you should have an acoustical consultant o board. At least spend a few hundred dollars to make sure you don' waste thousands of dollars on the wrong solutions Absorption does not improve isolation. This is a very commo misconception. If the room is overly live, then absorption is a goo thing, but I don't believe it will solve your problem > > Rene Tschaggelarwrote: > wrote: [quoted text clipped - 11 lines] > misconception. If the room is overly live, then absorption is a > good thing, but I don't believe it will solve your problem. Thanks for this information. I think I understand what you are saying but there has to be some way to isolate a room from external sound sources. We just have to do this by installing material inside our room. An acoustical consultant would be a big help. I will suggest hiring a consultant but I don't have much hope. To reinforce what others are saying It is usually very hard to make major improvements in low-frequency absorption in a room. It is difficult to get more than a few dB of noise reduction with added absorption. If sound is coming from elsewhere, the best route is to improve isolation - to prevent the sound from getting into the space in the first place. >> > Rene Tschaggelarwrote: >> wrote: [quoted text clipped - 20 lines] > An acoustical consultant would be a big help. I will suggest hiring > a consultant but I don't have much hope. |
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