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RE: State of the art in the measurement of sound absorption in a reverberation room (longish)

184.5.92.115

In Reply to: RE: State of the art in the measurement of sound absorption in a reverberation room (longish) posted by KlausR. on February 14, 2013 at 04:40:55

What is your question?

Follow Ups:

RE: What is your question? - KlausR. 04:34:39 02/15/13 (6)
No question, just the message that you cannot trust absorption coefficients determined by reverberation room methods as found in product data sheets.

Klaus
- RE: What is your question? - Ethan Winer 12:49:43 02/15/13 (2)
  Agreed, reverb room tests are not totally reliable, though they're okay for midrange frequencies. A better method uses the impedance tube. This article from Sound & Vibration magazine explains a lot about the limitations of reverb room testing as relates to absorbers meant for use in home-sized spaces:
  
  Test Methods for Acoustic Treatment Products
  
  --Ethan
  - RE: though they're okay for midrange frequencies - KlausR. 00:50:54 02/19/13 (1)
    When you look at the results from round robin tests, e.g. fig. 3.2 in D'Antionio's paper, it becomes clear that reverb room methods are not reliable at any frequency.
    
    http://www.rpginc.com/docs%5CTechnology%5CWhite%20Papers%5CState%20of%20the%20Art%20in%20Measuring%20Acoustical%20Coefficients.pdf
    
    Everest does not mention these large uncertainties and discrepancies between labs, neither in the 4th nor in the 5th edition.
    
    Impedance tube methods are valid for perpendicular incidence, whereas in rooms you also have incidence at an angle and random incidence. From the few stuff I�ve read I understood that measurements with incidence at an angle is done in the free field.
    
    You are right when saying that reverb room measurements (probably) do not mimic real use conditions. But I suppose that you cannot do measurements for each and every possible mounting configuration at reasonable costs.
    
    Everest shows a graph to demonstrate the effect of airspace behind the absorber, so maybe there is a way to simply convert the absorption data from surface mounted to spaced away condition.
    
    Your measurements at low frequencies using waterfall plots: I suppose that the results are very much room dependant, meaning that you�d get very different plots/decay times in an empty room and in a fully furnished room, in rooms with hard walls and few openings and in rooms with flexible walls and many openings. Did you check whether or not the data are consistent in different rooms? For comparison purposes your method might work well, but does it also provide absolute data?
    
    In 1996 the Fraunhofer Institute of constructional physics proposed a method similar to yours, i.e. in the frequency range with less than 5 eigenfrequencies/3rd octave band these individual frequencies are driven by a speaker in the room�s corner and the decay time is determined. Sabine�s equation is then used for computing an effective absorption coefficient (which is actually not correct because the sound field is not diffuse).
    
    Klaus
    - RE: though they're okay for midrange frequencies - Ethan Winer 11:43:50 02/19/13 (0)
      The main reason using "waterfalls" to assess effectiveness of bass traps varies from rooms is the variable percent change in the total amount of absorption. In other words, adding four traps to an empty room adds infinite percent more sabins than was there previously. Versus adding the same number of traps to a room that has a bunch of soft furniture. In that case the increase might be only 50 percent.
      
      --Ethan
- RE: What is your question? - jkloganjr 09:34:44 02/15/13 (2)
  I dont find that surprising.
  
  Have you found a method that is trust worthy based on flow resistivity?
  
  Have you found any value to any online calculators such as : http://www.stanleyhallstudios.co.uk/pacalc/pacalc.php?s11=2&d11=52&v11=27000&s12=1&d12=153
  - RE: I dont find that surprising - KlausR. 01:08:40 02/16/13 (1)
    Well, this issue has been never mentioned in acoustics forums, so when I discovered it a few weeks ago it came as a surprise to me.
    
    Apart from an acoustic ceiling I don't have any absorbers, and since I did not read any technical papers on this issue other than relating to ISO 354, I don't have enough knowledge to have an opinion on flow resistivity based methods or simulators.
    
    Bosmans, "Sound absorption of stretched ceilings with an impervious synthetic membrane", J. Acoust. Soc. Am. 106, 233 (1999)
    
    Klaus
    - RE: I dont find that surprising - jkloganjr 06:59:42 02/16/13 (0)
      I would acknowledge that I havent ever seen the burden of proof illustrated as you have presented it in acoustic forums, no. Good job by the way :) But those of us that treat rooms and have used different materials have made room measurements and compared our results to manufacturers claims and found them to be inconsistent. At least I have.

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