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Natural Science Forum / Physics / Acoustics / November 2005? about properties of a room-within-a-room
I have just completed the construction of a drum practice room in my garage. The internal dimension of the room are 2.64m by 3.09m by 2.23m (W,L,H). This room fills the full width and height, but only about half of the internal length of the garage. The garage walls are constructed from light weight expanded clay blocks, and the inner walls have had 75mm Rockwool slabs (RWA45) applied to them, then there is an air gap of some 200mm and finally, the stud wall, lined with 75mm slabs of Rockwool. The inner walls of drum room have a double layer of 12mm plasterboard, followed by another 30mm layer of Rockwool for internal reverberation control. The interior walls of the room are finished by stretching Hessian over the Rockwool and stapling it to wooden battens. I had to start using the room before it was fully finished. All that was missing was the plaster board to finish only exterior wall of the drum room visible from within the garage. This exterior wall is some 500mm away from the inner wall. Here's my question: Before this final layer of plasterboard was put up, the stud frame only had Rockwool in it. All that could be heard from outside the garage was a faint low thud of the kick drum and a tiny amount on snare drum. Now that a double layer of plasterboard has been applied to the interior stud wall, there has been a noticeable increase in the transmission of higher frequency sound (the snare can be heard better, and the cymbals are now audible). This is really annoying! I had expected that the extra two layers of plasterboard would help reduce bass transmission. Instead, bass transmission seems unchanged, and there has been an increase in treble transmission. Any ideas? Chris W  SignatureThe voice of ignorance speaks loud and long, But the words of the wise are quiet and few. --- If I understand correctly, and couple of possibilities come to mind. With regard to the higher frequencies, did you use any kind of resilient material to attach the plasterboard to one side or the other of the studs? If not, the higher frequencies can transmit through the studs from one side to the other. Before you put up the second layer of plasterboard, the mineral wool was helping reduce these higher frequencies coming through the plasterboard, and the exposed studs were a small area. Once you put on the extra plasterboard, it became a sounding board for the higher frequencies coming through the studs. I would not have thought about this effect being so strong but can conceive of it. Regarding the low frequencies, whenever you create a cavity you create a resonance that weakens the wall at that frequency. Whenever you use a cavitity you improve the isolation above the resonance frequency, but weaken it at the resonance. >I have just completed the construction of a drum practice room in my >garage. The internal dimension of the room are 2.64m by 3.09m by 2.23m [quoted text clipped - 34 lines] > > Chris W > If I understand correctly, and couple of possibilities come to mind. With regard to the higher frequencies, did you use any kind of resilient material to attach the plasterboard to one side or the other of the studs? If not, the higher frequencies can transmit through the studs from one side to the other. No, the plasterboard is screwed directly to the outer stud frame. This stud frame forms the outer wall and is physically isolated from the stud frame of the inner wall. > Before you put up the second layer of plasterboard, the mineral wool was helping reduce these higher frequencies coming through the plasterboard, and the exposed studs were a small area. Once you put on the extra plasterboard, it became a sounding board for the higher frequencies coming through the studs. I would not have thought about this effect being so strong but can conceive of it. Neither did I think that the effect would be so strong... As far as correcting the problem is concerned, I am considering putting another thinner layer (30mm) of Rockwool on the outer face of the wall and covering it with Hessian. I expect that this treatment should greatly reduce the walls ability to radiate transmitted sound. If you know of another (simpler) solution, then I would appreciate hearing it. > Regarding the low frequencies, whenever you create a cavity you create a resonance that weakens the wall at that frequency. Whenever you use a cavitity you improve the isolation above the resonance frequency, but weaken it at the resonance. True. I think I can reduce bass transmission by adding a Helmholtz absorber inside the room, tuned to the offending frequency. I plan to make this from concrete flue lining. These are 9" tall pipe sections with an 8.5" internal diameter. I haven't done the calculations yet, but I believe if these are stacked on top of each other to the correct height, and wooden disc is tightly fitted into the top with a hole in it and some absorbent material, I can create an absorber tuned to relevant frequency. SignatureThe voice of ignorance speaks loud and long, But the words of the wise are quiet and few. --- >> If I understand correctly, and couple of possibilities come to mind. >> With regard to the higher frequencies, did you use any kind of resilient [quoted text clipped - 5 lines] > stud frame forms the outer wall and is physically isolated from the stud > frame of the inner wall. If you do have two sets of studs, and that is not very clear from the photos, then that blows my theory about the transmission through the studs. >> Before you put up the second layer of plasterboard, the mineral wool was >> helping reduce these higher frequencies coming through the plasterboard, [quoted text clipped - 9 lines] > covering it with Hessian. I expect that this treatment should greatly > reduce the walls ability to radiate transmitted sound. You may be onto something here to get back to where you were before with the rockwool exposed in the open area of the garage. It can have two effects. It can prevent some high frequency radiation from the wall by absorbing hte sound as it is trying to radiate. It also adds absorption to the space to reduce the reverberant level once the sound has radiated. If you only do 30 mm this will be primarily a high frequency effect without much significance for bass. > If you know of another (simpler) solution, then I would appreciate hearing > it. [quoted text clipped - 11 lines] > and wooden disc is tightly fitted into the top with a hole in it and some > absorbent material, I can create an absorber tuned to relevant frequency. > As far as correcting the problem is concerned, I am considering putting > another thinner layer (30mm) of Rockwool on the outer face of the wall > and covering it with Hessian. I expect that this treatment should > greatly reduce the walls ability to radiate transmitted sound. I think you are correct. I have experienced this phenomenon in other structures. Namely, this high frequecy wave radiation mode is NOT normal emergence. Rather it is a surface-launched wave, skimming at and near the surface, that occurs because of the the traveling wave nature of that mode. In that case, a surface absorber of moderate thickness will catpture and absorb it. >> Regarding the low frequencies, whenever you create a cavity you create >> a resonance that weakens the wall at that frequency. Whenever you use >> a cavitity you improve the isolation above the resonance frequency, >> but weaken it at the resonance. The first mode that comes to mind is the mass-spring panel resonance, of two-body style, where the definitive mass is the virtual mass of the wall pair (M1*M2)/(M1+M2). The second mode is that where the distance between the two walls is 1/2 wavelength. > True. I think I can reduce bass transmission by adding a Helmholtz > absorber inside the room, tuned to the offending frequency. I plan to [quoted text clipped - 4 lines] > and some absorbent material, I can create an absorber tuned to relevant > frequency. The basic device can be a stack of such hollow blocks with a heigght of 1/4 wavelength. A cover can have a hole sized to damp and absorb sound. The hole will also define a lower resonance frequency, AKA a "Helmholtz" resonant frequency. The Helmoltz regime is more likely to be strong when the volume is large and centralized, and the hole is small. When the volume is elongated, shaped like a pipe, etc, the Helholtz mode is weakened and the 1/4-wave mode becomes dominant. Angelo Campanella > I had to start using the room before it was fully finished. All that > was missing was the plaster board to finish only exterior wall of the > drum room visible from within the garage. This exterior wall is some > 500mm away from the inner wall. It's hard to accurately visualize where the rock wool, layers of gypsum, and garage wall are located, and where the sound observations were located as well. A graphic depiction will help. Also, you need to depict the location of all 4 walls of the drum room with respect to the garage walls. Might as well define the location n of the ceiling to the garage roof as well. This is a 3D problem! > Before this final layer of plasterboard was put up, the stud frame only > had Rockwool in it. All that could be heard from outside the garage was > a faint low thud of the kick drum and a tiny amount on snare drum. Are you concerned about the sound in the garage, or the sound escaping the garage. If so, what sounds were heard at the location where it is not desired to hear sound in both situations (without and with the final gypsum layers)? Two factors apply: The first factor, as Noral points out, attachment of the gypsum boards directly to the studs will transmit sounds of all frequencies. The gypsum boards should not be attached to the same stud frame as is carrying the inner wall gypsum. Rather it must be fastened to a second independent outer stud frame. > Now that a double layer of plasterboard has been applied to the interior > stud wall, there has been a noticeable increase in the transmission of > higher frequency sound (the snare can be heard better, and the cymbals > are now audible). The second factor is that gypsum board is notoriously efficient at transmitting sounds around 2 kHz (the "coincidence" effect). I the latter case one of the countermeasures is to make the second layer thinner than the first because the thinner layer will transmit only at a higher frequency, 2,500 to 3,000 Hz. when the board thickness is 10 or as little as 6 mm. In bygone decades, 1/2mm lead (Pb) sheet was used for this purpose. > This is really annoying! I had expected that the extra two layers of > plasterboard would help reduce bass transmission. Instead, bass > transmission seems unchanged, and there has been an increase in treble > transmission. Thus it has often been the complaint about apartment and condo walls of such unfortunate structure that conversations are readily heard through said walls! Angelo Campanella Hey Chris, I'm probably wrong as usual, but are you describing a construction with three walls/leafs? An outer wall of clay and a sheetrock wall with a gap and another sheetrock wall? Bert When you have an outer wall and an inner sheetrock wall 50 cm apart with no connections, air thight, you should definitely have a dramatic increase in high/mid freq TL, resonance should be far under 100 Hz. How about your ceiling construction? Tuned devices are very dangerous. You have to tune 'm by measurement, you cannot calculate 'm AFAIK. Often resonators store energy and act as an organ pipe, making things worse. When you then go damp 'm they don't work anymore. When you attach a resonator to a wall, you will create a triple leaf wall making things worse (as Noral pointed out). Saw that in a speaker test room, with a double brick wall with a large airspace. They build the walls on the same slab of concrete, thus flanking was terrible, and when they build some resonating pannels to tame room modes the triple leaf they created made things even worse. > Tuned devices are very dangerous. > You have to tune 'm by measurement, you cannot calculate 'm AFAIK. [quoted text clipped - 6 lines] > flanking was terrible, and when they build some resonating pannels to > tame room modes the triple leaf they created made things even worse. Hi Bert I was planning on measuring the internal response of the room and an using a waterfall plot to identify the offending the frequency. Then trying to build an absorber tuned to that frequency. That's my plan at least... It'll probably need to change though to account for this awkward thing called reality. ;-) Chris W SignatureThe voice of ignorance speaks loud and long, But the words of the wise are quiet and few. --- Guess you have acces to a measurement system like mls or sweep, as you can generate waterfalls. You could measure the sound levels in the room, using a speaker with mls or sweep. Measure on several locations, about a meter from the boundaries and average the data. Than measure the same levels outside. Compare the freq curves. That gives an indication of TL and could learn you what treatment you nedd, I guess. Listening could also tell where the weak spots are. By the way, to tame resonances you could go for broadband absorption like described on treatment of control rooms, as often discussed on studiotips or rec org or sayers. Could make TL for low freq's a couple of dB better. How heavy do you think your clay wall is, kg/m^2? If I know this, I could prolly give you some estimations of transmission loss and the increase of TL when using a decoupled sheetrock construction. And Eric Desart and Rod Gervois seem convinced that using Brian Ravnaas' (audioalloy) green glue between two sheets of sheetrock can increase TL quiet a bit. seems a bit messy, but extremely effective :-) > Guess you have acces to a measurement system like mls or sweep, as you > can generate waterfalls. [quoted text clipped - 18 lines] > Ravnaas' (audioalloy) green glue between two sheets of sheetrock can > increase TL quiet a bit. seems a bit messy, but extremely effective :-) OK, thanks for the hints. Gotta go now, got a plane to catch! Back next Thursday. Chris W SignatureThe voice of ignorance speaks loud and long, But the words of the wise are quiet and few. --- > It's hard to accurately visualize where the rock wool, layers of gypsum, and garage wall are located, and where the sound observations were located as well. A graphic depiction will help. Also, you need to depict the location of all 4 walls of the drum room with respect to the garage walls. Might as well define the location n of the ceiling to the garage roof as well. This is a 3D problem! Ok here are some photos. Before construction started: http://www.whealy.com/images/stage2_01.jpg Applying Rockwool to the inner walls of the garage: http://www.whealy.com/images/stage2_04.jpg After applying Rockwool to the inner walls of the garage: http://www.whealy.com/images/stage2_07.jpg Inner stud frame built: http://www.whealy.com/images/stage5_01.jpg Partial construction of inner walls http://www.whealy.com/images/stage5_02.jpg Room with inner treatment applied but Rockwool missing from outer stud wall. http://www.whealy.com/images/stage5_03.jpg The room was being used with the outer stud wall having only the Rockwool in it. Now the room looks like this: http://www.whealy.com/images/Finished_01.jpg > Are you concerned about the sound in the garage, or the sound escaping the garage. If so, what sounds were heard at the location where it is not desired to hear sound in both situations (without and with the final gypsum layers)? The sound in the garage is fine. I am concerned about the sound that escapes from the garage. > Two factors apply: > > The first factor, as Noral points out, attachment of the gypsum boards directly to the studs will transmit sounds of all frequencies. OK, hence my idea of applying a 30mm layer of Rockwool to the outer face of the wall. > The gypsum boards should not be attached to the same stud frame as is carrying the inner wall gypsum. Rather it must be fastened to a second independent outer stud frame. Yes, the plasterboard is fastened to the outer frame. > The second factor is that gypsum board is notoriously efficient at transmitting sounds around 2 kHz (the "coincidence" effect). Bother! > I the latter case one of the countermeasures is to make the second layer thinner than the first because the thinner layer will transmit only at a higher frequency, 2,500 to 3,000 Hz. when the board thickness is 10 or as little as 6 mm. In bygone decades, 1/2mm lead (Pb) sheet was used for this purpose. If possible, I'd prefer to to rip down the plasterboard that I have already applied. Thanks for your comments. Chris W SignatureThe voice of ignorance speaks loud and long, But the words of the wise are quiet and few. --- > Ok here are some photos. > Applying Rockwool to the inner walls of the garage: > http://www.whealy.com/images/stage2_04.jpg So far, so good. > Inner stud frame built: > http://www.whealy.com/images/stage5_01.jpg This seems to be the mount for the inner barrier wall and the rock wool, both sides. > Room with inner treatment applied but Rockwool missing from outer stud > wall. > http://www.whealy.com/images/stage5_03.jpg OK. I see that the second stud wall is not attached to the first. BUT, the doorway has to be rationalized. In my opinion, two tandem doors must be used. The inner attached to and hinged from the inner wall, swinging inward; the outer door attached to and hinged to the new outer stud wall, swinging outward, all seams with weather-strips. If two such tandem doors are not used, the sound leakage you describe will occur. > The room was being used with the outer stud wall having only the > Rockwool in it. > > Now the room looks like this: > http://www.whealy.com/images/Finished_01.jpg > The sound in the garage is fine. Does that mean that the sound that is there is quiet enough (for unoccupied garages)? > I am concerned about the sound that > escapes from the garage. Does that mean that the sound heard outside is the same as or greater than that inside the garage (that you just called "acceptable"? Or do you mean that the outer sound, though less than the above mentioned garage sound, is still not low enough? If the latter is true, then there is some flanking going on, such as sound transmission trough the roof, or longitudinal waves that got into the primary masonry wall and is now radiating from that masonry. If so, connections to that masonry wall need to be broken; is it going into the floor of the garage, then conduced into the masonry? To test this, place a carpet down, lay plywood on the carpet (at least 95% of all drum room floor area) and re-test. > If possible, I'd prefer to to rip down the plasterboard that I have > already applied. First, search for flanking paths (framework, doors and floor) > Thanks for your comments. In my opinion it is important that we get it right, because this topic (home music practice rooms) is sadly in need of proper codification. When we get it good, we should tout it in the rec audio news groups, and make our world a better place in which to live! Angelo Campanella > OK. I see that the second stud wall is not attached to the first. BUT, the doorway has to be rationalized. In my opinion, two tandem doors must be used. The inner attached to and hinged from the inner wall, swinging inward; the outer door attached to and hinged to the new outer stud wall, swinging outward, all seams with weather-strips. > If two such tandem doors are not used, the sound leakage you describe will occur. This is exactly the door construction that I have - although I omitted to photograph the construction stages of the tandem door way. The two doors are about 500mm apart. I have not yet applied any absorbency to the space between the doors, neither have I fitted the weather strips. > Does that mean that the sound that is there is quiet enough (for unoccupied garages)? OK, I did not explain myself clearly enough. The sound inside the practice room is fine in terms of the reverberation time. I wear either ear plugs or in-ear-monitors to protect my ears when practising the drums, but I have also started learning the trombone (which does not require any ear protection). The RT of the room is short enough (about 0.2s) that it is also a very comfortable trombone practice environment without being too dead. > Does that mean that the sound heard outside is the same as or greater than that inside the garage (that you just called "acceptable"? > > Or do you mean that the outer sound, though less than the above mentioned garage sound, is still not low enough? What I meant to say was that prior to the addition of the plasterboard, all the could be heard from outside the garage was just the bass component of the drums being played, and its level was acceptably low from a noise pollution point of view. I don't think the volume level has increased, but now that the plasterboard transmits the treble component, the clarity of sound heard outside the garage is much greater. You can now perceive each part of the drum kit being played: the kick (as before), but now also the snare and cymbals are audible, whereas previously, they were completely inaudible. Consequently, I consider the performance of the room to be unacceptable. > If the latter is true, then there is some flanking going on, such as sound transmission trough the roof, or longitudinal waves that got into the primary masonry wall and is now radiating from that masonry. If so, connections to that masonry wall need to be broken; is it going into the floor of the garage, then conduced into the masonry? To test this, place a carpet down, lay plywood on the carpet (at least 95% of all drum room floor area) and re-test. Fortunately, the entire practice room (including inner and outer stud walls) is resting on 19mm tongue-in-groove chipboard which is in turn, resting on 25mm of expanded polystyrene. I do not expect any significant levels of transmission down through the floor into the masonry of the building. > First, search for flanking paths (framework, doors and floor) The only flanking that I can identify is the following: o The lining of the tandem doorway. o Due to the need to keep the construction simple, the tops of both the inner and outer stud walls are anchored to the roof members. (Whilst this is less than ideal, I don't think it creates a serious problem). > In my opinion it is important that we get it right, because this topic (home music practice rooms) is sadly in need of proper codification. When we get it good, we should tout it in the rec audio news groups, and make our world a better place in which to live! Absolutely! This is why I have taken photos all through the construction of my practice room because I want to demonstrate that it can be done effectively without having to spend a king's ransom. I am currently in the process of putting the photos you have seen (plus many others) together on my website. Here, I will list all the construction details of both the existing garage, and the practice room. However, it has become apparent that a few in-flight corrections are needed. Nonetheless, these should be identified, solutions discussed and implemented, and the final result documented. I think its just as important to document how problems are fixed, as it is to propose the correct solution in the first place. Otherwise, people will just be left guessing, which opens the door for well meaning, but otherwise ignorant pundits to voice their persuasive, but flawed opinions. (Hence my choice of sig) Chris W SignatureThe voice of ignorance speaks loud and long, But the words of the wise are quiet and few. --- > What I meant to say was that prior to the addition of the plasterboard, > all the could be heard from outside the garage was just the bass [quoted text clipped - 11 lines] > (Whilst this is less than ideal, I don't think it creates a serious > problem). It is a possibility. I know it is difficult to do, but it would be informative to remove the connections to the rafters. Since the framework is now more interconnected, you likely can remove some of the rafter connections. But one more thing: Was a plasterboard also attached as a horizontal surface to the rafters? This may be another flanking path. The separation recommended, as for the side walls which you seem to have followed well, includes the ceiling surface; they are all equally important. It is acceptable to use a soft connection, as a square of old tire to maintain horizontal stability. > Absolutely! This is why I have taken photos all through the > construction of my practice room because I want to demonstrate that it [quoted text clipped - 12 lines] > well meaning, but otherwise ignorant pundits to voice their persuasive, > but flawed opinions. It is well to write it up as a short essay, including history, to keep it interesting and coherent. Angelo Campanella It seems to me that The inner ceiling and walls are rigid connected to the outer walls and roof through the roof trusses. That connections should have been elastic. This goes for the whole Circumferense of the outer division wall as well. And several thinner layers of the plasterboard would be better than one thick layer at high frequencies. You want mass, not stiffness, and no rigid connections between inner and outer surfaces. Asbjørn >> It's hard to accurately visualize where the rock wool, layers of >> gypsum, and garage wall are located, and where the sound observations [quoted text clipped - 69 lines] > > Chris W I agree with Asbjorn advise. From your pictures I observe that exists a lot of rigid connections between leafs. Insted of pictures would be better to have detailed schematic drawings. Elastic mountings are absolutely necessaries between leafs, also to the floor and ceiling. Check the flooring transmission. I observe that your studs are rigidly fixed. Also at least you need three plasterboards leafs of one inch each elastically fixed to the inner one with minimun two inches of rock wool inside the chambers. Better if you use diferent densities. I think this would be enough for a garage room. Luck > It seems to me that > The inner ceiling and walls are rigid connected to the outer walls and roof [quoted text clipped - 81 lines] > > > > Chris W |
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