How to measure Sound Absorption Coefficient? Software?

[Lord Victor]

Hey, I’m currently working on a university project where I’ll need to test the absorption ability (SAC) of various material samples.

I found this paper which is good, but it isn’t clear to me if I can do this without an audio analyzer? Or if there is another simpler way to get results good enough for my project (doesn’t need to be perfect).

I own a single UMIK and would need to buy a second one to build an impedance tube like the one in the paper - but I have no idea how to do the test even if I had the second mic - can REW do it somehow?

Or can I rest it with a single mic somehow? Please help.

This is for an industrial design project, so it will not need to be as perfect tests as for actual acoustic engineering papers etc (though it’d be nice), just good enough to indicate results.

 

[Lord Victor]

Might it make sense to replicate the room scale test at a smaller scale? Making a sealed box, with one wall covered with the test material, placing a speaker at one end and a Umik at the other. Maybe pointing them at the material at 45 degree angles so most of the direct/beaming frequencies are forced to bounce off it to reach the mic?

Maybe building the whole test chamber/tube as a bend, to force the sound to reflect off the material… or would it be best to just point the mic and speaker directly at each other and covering one side wall in test material?

 

[ClassG33]

You'll want a pink noise emitter, and you'll want to measure the RT60 of the room at this frequency band. Then, using Sabines Equation, work backwards with that RT60 and you should be able work backward if you're great at math lol. But there are many tables out there with estimates of coefficients for different materials.

 

[Lord Victor]

You'll want a pink noise emitter, and you'll want to measure the RT60 of the room at this frequency band. Then, using Sabines Equation, work backwards with that RT60 and you should be able work backward if you're great at math lol. But there are many tables out there with estimates of coefficients for different materials.

Thank you for the response! Tables won’t do me any good sadly, since I’m trying to develop new materials, and need to test them.

The question with the RT60 method is wether this would work with smaller scale experiments?

Making a large enough sample to use it in a full size room isn’t really feasible since I’m working with concrete and need to do a lot of samples.

Could I fx use these old Dali 505’s to test with, by turning one of them (it’s broken anyway) into a test chamber, sealing it off, and having the other speaker essentially play into the sealed cabinet by coupling them face to face with the drivers removed on the test chamber speaker, and then placing the sample inside that cabinet along with the measurement microphone?

 

[ClassG33]

Thank you for the response! Tables won’t do me any good sadly, since I’m trying to develop new materials, and need to test them.

The question with the RT60 method is wether this would work with smaller scale experiments?

Making a large enough sample to use it in a full size room isn’t really feasible since I’m working with concrete and need to do a lot of samples.

Could I fx use these old Dali 505’s to test with, by turning one of them (it’s broken anyway) into a test chamber, sealing it off, and having the other speaker essentially play into the sealed cabinet by coupling them face to face with the drivers removed on the test chamber speaker, and then placing the sample inside that cabinet along with the measurement microphone?

I don't see why not. You'll have to get creative to find a way to effectively estimate the Coefficient without a full size test environment, in a way that can give you reasonable conclusions on a research paper.

I'm guessing your school would accept the experiment of a scale down testing chamber

 

[Lord Victor]

I don't see why not. You'll have to get creative to find a way to effectively estimate the Coefficient without a full size test environment, in a way that can give you reasonable conclusions on a research paper.

I'm guessing your school would accept the experiment of a scale down testing chamber

Yea, it’s not too crucial. Ideally it’d be directly translatable. But not a must.
I found a paper detailing how to make a small scale reverb chamber. The question is how small would work. Or if I could still make something along the impedance tube principle that might work.

I worry that with a too small test chamber the tested material becomes so big relative to the overall volume of the chamber that something must be wrong? But that’s just a hunch.