Using REW to measure sound absorption in an impedance tube

[JamesHig]

Hello, I've been using an impedance tube to measure sound absorption. Following standard ISO/CD 10534-2, I've been using an old 'Soundbook' with Samurai software, and using a periodic noise signal it measures the complex transfer function between two mics in the the tube. To account for phase differences between the mics, they are then swapped, and another measurement is made. The resulting transfer function can then be found combing the two measurements:
h12=sqrt(H12_swapped./H12_normal).*H12_normal
and the absorption is calculated by:
k=2*pi*f/c;
k0=2*pi*f/c-1i*(0.0194*sqrt(f)/(c*dia)); %tube losses
r=((h12-exp(-i*k0*s))./(exp(i*k0*s)-h12)).*exp(2*i*k0*x1);
abscoef=1-abs(r).^2;

Going forward I want to ditch the soundbook/samurai setup and use REW and two measurements with just one mic (in two positions). Is the frequency sweep measurement function suitable for this application? Do I need to worry about phase between measurements? (It should be matched because I'm using the same mic/preamp right?) When I export the REW measurement data as text - is that response the transfer function? I'm assuming that I can turn the spl/phase output into the complex pressure transfer functions, and then use those two to compute the transfer function h12=Hx2./Hx1 --- is this correct?

Any help is much appreciated!!

 

[John Mulcahy]

It should be OK to make an REW measurement at each position and use the All SPL trace arithmetic A/B function to generate the transfer function between the two positions.

 

[luegotelodigo]

I attach a brief with an explanation of the theory, it's in Spanish but I guess maths are maths.

H12 is the transfer function between 1 and 2, using this method you avoid uncertainties caused by the differences between the mics but repeteability should be checked. Please comment your results.