[Tell us about the issues that you had while developing impedance tubes]
[The issue that I had has been solved, but I didn't manage to fully understand why, since I change my measurement system and my data analysis script also.
[Nevertheless, I would like you to tell us about the issues that arose while developing an impedance tube since it could provide reference information for other researchers]
I developed an impedance tube to measure the sound absorption coefficient with ISO 10534-2 method. While processing reflection coefficient and absorption coefficient from transfer function data or audio files obtained using (Arta or audio recordings of white noise, sine sweep or MLS inside the tube) (transfer function is in dB), I obtained negative reflection coefficients or data out of common bounds (see images) Some ideas on possible sources of error or necessary preprocessing of the signals or the transfer function?
Complex Reflection Coefficient R = [ {H-e(-j*k*s)} / {e(j*k*s)-H} ] * e(2*j*k(L+s))
absorption coefficient alpha = 1-|R|^2
[The issue that I had has been solved, but I didn't manage to fully understand why, since I change my measurement system and my data analysis script also.
[Nevertheless, I would like you to tell us about the issues that arose while developing an impedance tube since it could provide reference information for other researchers]
Hey Tomas,
I build a modified impedance tube myself some time ago, and got similar results first. Key things that fixed my issues, where 1) microphone calibration and 2) confirming the microphone distances.
You can apply the mic calibration/correction before computing the R. For microphone distance measurements, I used hard termination at the end of the tube and checked the peak locations in the impulse response data.
Maybe you've done this already, but for my case it improved the results considerably.
Miikka Tikander thanks for your insight, I have already tried correcting microphone distances, but the output does not seem to have sensibility to the distance parameter. On the other hand, by microphone calibration do you mean phase and amplitude mismatch correction by the transfer function?
kind regards
Tomás
Yes, I meant phase and magnitude. Phase error is equivalent to moving the microphone. As for magnitude, especially when measuring hard materials (abs close to zero), the results get really noisy without calibration.
I did calibration by swithcing the mic positions and repeating a measurement with same conditions.
According to Uno Ingard a really stiff and hard end piece is needed. I think it was minimum 50mm of solid alumninium. But look it up in his book on absorption.
I'd converted a standing wave ratio impedance tube to a two microphone setup. The only issue I had arose at higher frequencies where the imaginary part of the cross spectral data switched from positive to negative (or visa versa I can't recall). I concluded that this was due to the two microphones being too close to each other causing this inversion of sign. Too close despite a fair bit of lit review on my part and analysis to arrive at an optimal spacing for a given tube diameter and cutoff frequency.
You should have mentioned your dimensions of the impedance tube and the corresponding upper and lower limit of frequencies. The dimensions such as length of the tube(plane wave condition), the distance between the microphone and diameter of microphones. The source of error could be either because of the (1)non-plane wave generation(length of impedance tube is less), range of frequency measurement is out of range of your designed impedance tube, and (2) or it may be in signal processing and mathematics behind it, such as calibration and in methematics part it could the G11,G12 and G22(Auto and cross function) G12=/G21,so the transfer function change with reference to the microphone assigned as 1 and 2 connected to the DAC input 1 and 2, what i mean to say is that the sign convention or notations should properly matched with formulas you are using. Rigid backing is must and leakage of air from holes should be avoided using the wax. The material u are testing it should also posses the ability to absorb sound profoundly, multiple reflections may be a cause of this. The improper distance between the microphone is not able detect the phase change or the leakge around it causing the air noise because of restrictions. I hope this will help you.
I have developed impedance based on the ASTM-E 1050
Design parameter and MatLab code attached in the form of jpg. Hope this will help you.
I have done a fair amount of work on the development of an impedance tube starting from scratch. The only known information I had before starting the work was the color of the commercial impedance tube. All my experiences and ups and downs are summarized in:
Article Development of an experimental set-up to measure sound absor...
Conference Paper DEVELOPMENT OF AN ECONOMICAL SET UP TO MEASURE SOUND ABSORPT...
For any further queries, feel free to contact me.!
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