question for the experts

sound absorption in a room using two diferent acoustic materials and coverage. room # 1, 100% coverage - Material NRC 0.20 across all freq. room # 2, 20% coverage - Material NRC 1.00 across all freq. the two room above will have the same reverberation time after treatment? room #1 will have a more uniform sound all arround the room?

Interesting question. It sounds logical to assume there's a difference in sound, although the total sum of energy absorped is the same. Am I understanding your question correctly? Do you want to know which of the two approaches results in the most uniform room sound, nearly regardless of microphone placement?

B.D.R. wrote:

room # 1, 100% coverage - Material NRC 0.20 across all freq. room # 2, 20% coverage - Material NRC 1.00 across all freq. the two room above will have the same reverberation time after treatment? room #1 will have a more uniform sound all arround the room?

1) NRC is by an average over bands (the NRC over all bands has no real meaning):

The NRC of an acoustical material is the arithmetic average to the nearest multiple of 0.05 of its absorption coefficients at 4 one-third octave bands with center frequencies of 250, 500, 1000, 2000 Hertz.

If you speak about absorption and reverberation then you refer to geometric acoustics, meaning somewhere higher than the Schroeder frequency. In smaller rooms the RT is more an energetic concept because the soundfield will be dominated by MODAL acoustics. Assuming we speak about a larger room where a diffuse field is present and the relation absorption and RT is more clear and evenly spread: No both rooms will NOT have the same RT. Most related calculation models are based on the Sabine approach and formulas. Sabine assumes a lot of statistical boundary conditions, which includes an equal number of sound incidences on all boundary surfaces with absorption equal distributed over all surfaces. In fact this formula is a simplified approach assuming a lot of things. There are a number of alternative formulas developped by different authors, to cover deviating circumstances. To keep it simpel: normally your absorption efficiency will be larger in option 2. Still it is possible that you measure higher RTs at certain spots. To cover complicated things one uses simulation models (numerical), not simple formulas anymore. If you use ceiling tiles with relative high absorption in an open office, where the ratios are as such that one speaks about an acoustic flat room (low hight versus length and width), the same tiles will show higher real live absorption coefficient then when used in a room with a more cubical shape. Summary: No your option 1 and 2 will not be the same, and in as far the diffuse field is the dominant factor (which in studios seldom is), the option with lower absorption but 100 % distribution should give you the most stable soundfield (is closest to Sabins intent, but with 20 % average absorption closer by Eyring's formulae). But important: If you think about studios with measures causing them to be mainly dominated by the modal soundfield, not much remains of the above described appraches.

B.D.R. wrote:

room # 1, 100% coverage - Material NRC 0.20 across all freq. room # 2, 20% coverage - Material NRC 1.00 across all freq. the two room above will have the same reverberation time after treatment? room #1 will have a more uniform sound all arround the room?

1) NRC is by definition an average over bands (the NRC across all freq. has no real meaning):

The NRC of an acoustical material is the arithmetic average to the nearest multiple of 0.05 of its absorption coefficients at 4 one-third octave bands with center frequencies of 250, 500, 1000, 2000 Hertz.

If you speak about absorption and reverberation then you refer to geometric acoustics, meaning somewhere higher than the Schroeder frequency. In smaller rooms the RT is more an energetic concept because the soundfield will be dominated by MODAL acoustics. Assuming we speak about a larger room where a diffuse field is present and the relation absorption and RT is more clear and evenly spread: No both rooms will NOT have the same RT. Most related calculation models are based on the Sabine approach and formulas. Sabine assumes a lot of statistical boundary conditions, which includes an equal number of sound incidences on all boundary surfaces with absorption equal distributed over all surfaces. In fact this formula is a simplified approach assuming a lot of things. There are a number of alternative formulas developped by different authors, to cover deviating circumstances. To keep it simpel: normally your absorption efficiency will be larger in option 2. Still it is possible that you measure higher RTs at certain spots. To cover complicated things one uses simulation models (numerical), not simple formulas anymore. If e.g. you use ceiling tiles with relative high absorption in an open office, where the ratios are as such that one speaks about an acoustic flat room (low hight versus length and width), the same tiles will show higher real live absorption coefficient than when used in a room with a more cubical shape. Summary: No your option 1 and 2 will not be the same, and in as far the diffuse field is the dominant factor (which in studios seldom is), the option with lower absorption but 100 % distribution should give you the most uniform soundfield (is closest to Sabin's intent, but with 20 % average absorption closer to Eyring's formulae). But important: If you think about studios with measures causing them to be mainly dominated by the modal soundfield or boundary interference (which cover most discussed in studio groups), not much remains of the above described approaches since one applies absorption not mainly in function of RT but controlling that specific soundfield. In such cases RT is more a result, providing some additional information about the type of control room, than a target.

Well, that about wraps it up. :lol:

But important: If you think about studios with measures causing them to be mainly dominated by the modal soundfield or boundary interference (which cover most discussed in studio groups), not much remains of the above described approaches since one applies absorption not mainly in function of RT but controlling that specific soundfield.

Since it sounds so vital to understanding this post, could someone explain the meaning of "Modal soundfield" or "soundfield" all together?

Sigert Do a search on "room modes" and interference here in the forum and as well on the net and you find lots. And starting with this and the links included can be helpful as well. http://www.johnlsayers.com/phpBB2/viewtopic.php?t=6210

Eric basically put this one to bed. However, I thought I'd make a few assumptions and illustrate mathematically: Assume instead of NRC, the 0.20 and 1.00 numbers are absorption coefficients (α) at some frequency. Assume also, to make the math easy, that the room volume (V) is 125 m³ and the surface area of the room (S) is 150 m² (5 m x 5 m x 5 m). Simply using the Sabine equation (and thereby assuming its relevance), T60 = 0.161*V/A, where A = Σ(S*α), and assuming that the absorption of the room surfaces without treatment is 0.05, we can calculate for the arbitrary frequency in question: 100% coverage; α = 0.20 T60 = 0.161 * (125) / (150 * 0.20) T60 = 0.67 s 20% coverage; α = 1.00 T60 = 0.161 * (125) / [(150 * 0.20 * 1.00) + (150 * 0.80 * 0.05)] T60 = 0.56 s The only way these two cases can wind up being the same is if the α for walls without treatment is the same as the α for walls with treatment, which would give you a modified 100% coverage case! :) All the best,

thank you very much Desert, Lovecow and Singert. one more question: So... *modal activity it's reduced with absorption. * the two room will sound different * room #1 will sound more uniform, in either way: 20% coverage-NRC 1.00 or 100% coverage- NRC 0.20 won't be enough to reduce modal problems in a small music room. NRC 0.35, 100/% coverage , will be enough instead?

BDR, Do you have more information about the room? Blindly and universally applying percentages and NRCs is not a good approach. One live room may need 20% coverage, another 43.6% coverage. There are many other factors to consider. That's why we ask for room dimensions, furnishings, layout, wall finish materials, and other information. What I supplied above was only in response to the academic exercise of the 100/0.20 vs. 20/1.00 question. To get recommendations for your room, please provide more information about it. If you don't have a room to treat and you are only looking for a better understanding of the general concepts involved, I can tell you that there are no straight answers to your most recent questions. One can only reply: "It depends." :?

lovecow wrote:

BDR, Do you have more information about the room? Blindly and universally applying percentages and NRCs is not a good approach. One live room may need 20% coverage, another 43.6% coverage. There are many other factors to consider. That's why we ask for room dimensions, furnishings, layout, wall finish materials, and other information. What I supplied above was only in response to the academic exercise of the 100/0.20 vs. 20/1.00 question. To get recommendations for your room, please provide more information about it. If you don't have a room to treat and you are only looking for a better understanding of the general concepts involved, I can tell you that there are no straight answers to your most recent questions. One can only reply: "It depends." :?

thank you Lovecow, Just learning, I was only looking for a better understanding of the general concepts involved, thanks. But yes, I do have a little room under development : http://www.johnlsayers.com/phpBB2/viewtopic.php?t=9383 So maybe if you have time... can you please use my room as an example for this topic?? :oops: :oops: :oops: