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Natural Science Forum / Physics / Acoustics / August 2006Soundproofing and relation of absorption inside an enclosure
Can anyone direct me to a reference that describes the effect of adding absorption inside an enclosure (like a pump room) on sound transmission outside of the enclosure? thanks Andrew The most significant effect is that the absorption reduces the sound level inside the enclosure. Suppose you have a source sitting in the open. The sound level drops as you move away from the source. Now suppose you put a box over the source and leave all the surfaces inside hard. The sound reflecting from those surfaces causes the sound level inside to increase so it is about the same everywhere inside except possibly very close to the source. This is called the reverberant sound. Because of this build up, the sound coming through the walls is more than might have been expected. Now, if you put sound absorption on the inside surfaces, less sound reflects as much is converted into a small amount of heat in the absorbing material. The sound level inside is reduced to something closer to what it would have been without the box present. If there is enough absorption, the remaining sound is primarily that coming directly from the source. That sound is reduced a little as it passes through the absorption before reaching the blocking wall. If the wall is not very heavy and the absorption is thick, this could be a more significant effect. The absorption in contact with the wall may in some cases also damp vibrations in the wall. The wall will not do quite as good a job blocking this direct sound from the source as it does the reverberant sound, but the overall effect is that you get a better result. The benefit of the absorption is even more important if the enclosure has any holes such as for ventilation or just leaks. If there are no holes, the lack of absorption inside could be overcome by making the walls much heavier or otherwise designing them to do a better job of blocking the sound. It is usually less expensive to add the absorption, and you also usually need the benefit because of leaks being present. > Can anyone direct me to a reference that describes the effect of adding > absorption inside an enclosure (like a pump room) on sound transmission [quoted text clipped - 3 lines] > > Andrew > The most significant effect is that the absorption reduces the sound > level inside the enclosure. Suppose you have a source sitting in the [quoted text clipped - 32 lines] >> >> Andrew Noral, While you're the expert, I was going to say that the application of sound absorbing material, that is uniformly distributed inside the enclosure, would decrease the sound pressure inside the enclosure and outside the enclosure by the same amount. In other words, the transmission loss through the walls of the enclosure remains the same. Please correct me if I am wrong. >> The most significant effect is that the absorption reduces the sound >> level inside the enclosure. Suppose you have a source sitting in the [quoted text clipped - 40 lines] > through the walls of the enclosure remains the same. Please correct me if > I am wrong. There are some situations where adding material to the wall changes the TL significantly, and others where the effective change in sound level becomes more complex though the TL is not actually changing. For an example of a change in the TL, consider for instance first a plain sheet of 18 gauge steel. Now add 4 inches of high-density fiberglass covered by perforated metal on one side. If you test the TL of that panel, it will be significantly higher than the plain sheet of steel. Various factors contribute to this including the fact that the weight of the added material is significant compared to the initial weight of the steel. However, if I was starting with a heavy concrete wall and adding an absorptive facing, I would not expect a significant increase in the TL. In most situations where we are concerned with isolation between rooms, the addition of absorption reduces the sound as you say in that it reduces the reverberant field that is controlling the sound in the room. In the typical indoor situation between two rooms you are going from one diffuse field to another, or you are assuming you are even if they are not fully diffuse. This is the most common situation encountered in practice. However, if you take an extreme case of absorption addition and a source close to a wall, you can create a situation where the direct field dominates until you get very close to the wall. This often occurs when creating enclosures that are not "close fitting" but also are not a lot larger than a machine source. The situation then becomes more complex. For one thing you have to consider that the TL is actually a function of angle of incidence, though the usually measured and quoted TL is for a diffuse field. The other factor is that you have created a situation more like a source outdoors impinging on a facade. You must be very careful about how and where you measure the source level and thus the difference in sound levels on the two sides of the wall. The effect is that the perceived benefit of the wall is often not as great, though the TL is not changed. My statement " The wall will not do quite as good a job blocking this direct sound from the source as it does the reverberant sound" was not exactly correct. It was based on the normally found situation where the observed level reduction from a free-field level to a diffuse-field level through a given partition is less than when the same partiton is put between two rooms with diffuse fields and the diffuse level in the source room equal to that original free-field level. If you do not have a diffuse field in the source room and do not measure the source sound right at the wall surface, the apparent noise reduction between the two rooms could be less than it would have been if the field in the source room was diffuse. As I think through this a little, the presence of absorption on the partition surface would reduce the buildup at the surface to reduce this effect. The situation becomes more complex than with diffuse fields in both spaces. > >> The most significant effect is that the absorption reduces the sound > >> level inside the enclosure. Suppose you have a source sitting in the [quoted text clipped - 88 lines] > this effect. The situation becomes more complex than with diffuse fields in > both spaces. Thank you very much for the answers and for writing so much. Andrew >>> The most significant effect is that the absorption reduces the sound >>> level inside the enclosure. Suppose you have a source sitting in [quoted text clipped - 93 lines] > situation becomes more complex than with diffuse fields in both > spaces. I think that the most important thing to be learned from your reply is that there is no general answer and that the applicable answer depends on the specifics/details of the situation. >I think that the most important thing to be learned from your reply is that >there is no general answer and that the applicable answer depends on the >specifics/details of the situation. That's true about so many things. That's also why the best answer to so many of the "How do I solve a noise problem" posts is "Hire a local consultant." I used to do a modest amount of noise control and architectural acoustics consulting work. Most of the architectural work was in new buildings, where the architect had just done what he usually did, but failed to notice what was different that time. Ken Plotkin |
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