# Homework Help: Help on Sound Absorption Coefficient Calculation

1. Jul 18, 2011

### athanor

Hello all,

I come from musical sciences, but I'm trying to learn something about acoustics, so here I am, asking for some help about the impedance tube sound absorption coefficient test.

I have and impedance tube, two microphones technique (ISO 10534-2), B&K microphones connected to the PULSE platform, and the ability to get the transfer function of the signals(H12). According to the referred ISO norm, I need to calculate the following equation in order to get the pressure reflection coefficient (R), and from there the absorption coefficient:

where: L is the distance from sample to the first mic, s the mics separation, k the wave number, f is frequency, and c is speed of sound

From the measurement platform, I can get two columns of numeric data from the transfer function of the two microphone signals(H12), (frequency and sound pressure), but now I don't know how to implement those values in the formula, as it has imaginary numbers. Should I import the air pressure values in real and imaginary form, or could I use the modulus as they come?

I've tried EXCEL, since it has some complex calculation capabilities, so I could make a sheet with the calculations and copy-past the values from the measurement platform to excel and get a graph, but I only get errors or unreal values. Should I use another software, like matlab or something?

I understand that the problem is my lack of knowledge in the adaption of mathematics to the realm of computers.. Unfortunately, I'm just starting to learn everything from scratch, mathematics included (I always get myself fascinated with everything I don't understand.. LOL ).

Could someone please give me some int into the good path?

Many thanks,

Isaac

2. Jul 20, 2011

### mariorabelo

Issac:
Hi, my name is Mario, from Brazil. Did you get any reply for your question? I am new in this subjet too and trying to do the same thing. I found that they use or Matlab or LabView to do complex calculations. With Matlab you can do not only the calculations but also the graphics. Do you know these programs?
Take care.
Mario

3. Jul 20, 2011

### athanor

Hello Mario,

thank you for sharing my question! E olĂˇ de Portugal! I do know Matlab, but I'm not proficient in it. Nevertheless, to the moment, you were the only one to reply to my thread, so I'm still looking for help. In any case, I'll keep the thread up to date, if any development occurs!

Cumprimentos,

Isaac

4. Jul 21, 2011

### mariorabelo

Isaac:
I have just tested and BrOffice Calc (Brazilian OpenOffice) can do all kind of calculations with complex numbers. Just enter the formula and the data from your experiment and do the calculations. You need the microphones pressure in Pa, the frequency, the K, the microphones distance in m (L and s). Download the BrOffice, open the Calc and see the Help menu for "numeros complexos". Are you to planning to use two or one microphone option?
This experiment is not a piece of cake, need some knowledge about acoustics, complex numbers, lab skills, etc.
Take care.
Mario - mario.rabelo2@gmail.com

5. Aug 22, 2011

### vickyeight

Hey,,,, we are at present doing a similar experiment. Did you solve it yet?? the absorption and reflection coefficient can be obtained from the impedance tube results. you get them as text files. in fact, you get the values of both NSAC (Normal Sound Absorption Coefficient) and R (reflection coefficient).

6. Aug 23, 2011

### mariorabelo

vickyeight:
I am working with the University of Brasilia, Mechanical Dep. and they have just returned from winter vacations. We stop one month in July. As far I know, they already did the set up and programed the LabView. I need to return there to see how thinks are going. Do you have any specific question?
Take care - Mario

7. Aug 23, 2011

### vickyeight

Hi Mario,
we got the 'NSAC' and 'R' text files from B&K 29mm impedance tube (2 microphone transfer function method). We don't have a software to derive other non- acoustic properties such as flow resistivity etc. we are trying to use the equations from http://www.mecanum.com/files/ThreeMicMethod.pdf
to obtain the relevant non- acoustic properties. The purpose of obtaining these are to use them as input in AutoSEA2 software and get simulation results. Correlation between experiment and simulation is what we are looking at.

Does deriving the properties from formula sound as simple as it seems? We found real and imaginary parts of wave number ..... but sort of stuck cause we are getting varrying values for flow resistivity; the difference in the frequency range of 400Hz- 5000 Hz is very large(5K-10K in some cases). Is that how the values should be or is it just that we are going wrong with the formula??

8. Sep 3, 2011

### davidm1732

Isaac,

Have you had success with this yet? Your original question asked if you needed to export the values in real & imaginary terms...I think you do. Working with the magnitude won't work. I do this measurement, but I'm using an older HP signal analyser that can do math on the measured traces itself, so it's very easy. You can use complex #'s in Excel however, older versions may require you to turn on a math toolkit option if I remember correctly. If you're using B&K equipment, check out their web site, or find your local B&K technical rep...they should be able to help you figure it out....you may be able to do that calculation using the Pulse system itself, rather than doing it outside on Excel.