Usually we evaluate sound absorption coefficient using two microphone impedance test tube. And sound transmission loss using four microphone impedance test tube.
Is it possible to evaluate sound absorption coefficient using four microphone impedance test tube.?
In case of four microphone method,
alpha= 1- (reflection coefficient)^2- (transmission coefficient)^2
In case of two microphone impedance test tube,
alpha= 1- (reflection coefficient)^2
Is it correct?
I guess you can look this up in any of the textbooks on the theme. There may also be standards available. You hcan of course also do tests and see what you get.
I would recommend doing reverberation room measurements in addition, as they are closer to the real room acoustics applications. Not perfect, but better than tube meausrements only giving the perpendicular sound absorption and not the integreated over the whole 180 degrees directions.
yes sir i agree with you but reverbent room method is expensive and not available for me in my locality my question is related to principle if i use rigid boundary in place of anechoic termination in four microphone method then my Will the first formula be correct or not?
You can probaly use a reverberant room in your institution, like a bathroom or similar. It has some limitations due to limited modal density, but it is possible to get some useable data even in a room as small as 10m3.
In normal rooms the absorption will be lower than found in a tube test or reverberation room anyway. So a possible route is to use a reverberant normal room.
Thanks for your response.
Right now, we are using B and K impedance test tubes as we are working at prototype stage. Can you recommend me a good book on acoustic metamaterials?
I guess it would be helpful for you to learn more about the theory of the impedance tube measurements and about the absorption coefficient itself - please consult your nearest standard text book on acoustics.
The key difference of the 2 and 4 mic measurement as mentioned by you here is that in the one case you want to qualify a material probe mounted in front of the rigid end of the tube, i.e. assuming that the probe itself will be used that way in a real application (or does not transmit any sound). With thr 4 mic measurement, you assume that there is a (significant) transmission through the material and that this will also be the case in the later application - and in this case, there is sound power that is neither absorbed nor reflected (and thus measured by the 2 mics). To capture this power, you need to apply the additional 2 mics on the "backside" of the sample.
So, please try to understand what you want to measure (and why), as this helps to find the best way to measure the quantity you are really looking for. Just taking some receipt, say, 2 mic method, and evaluating some data gives you some result - not necessary a meaningful one.
This will also help to understand whether a impedance tube measurement will or will not be helpful for your application (i.e. normal incidence vs. arbitrary incidence of the sound wavers, etc.).
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