sound absorption of woven fabrics

[parham]

Hello. I am a textile engineer and have a partial information in acoustics. I have studied the effect of fabric structural parameters on sound absorption coefficient of woven fabrics. the results show that, for all the samples the maximum sound absorption occurs at frequency of 1000 Hz and the minimum takes place at frequencies of 250 and 2000 Hz. my question is that, whats the reason?why for all samples the maximum absorption just takes place at frequency of 1000 Hz?why for all samples the minimum takes place at frequencies of 250 and 2000 Hz and not at the other frequencies? why for all the samples an identical trend has been observed for all the samples. I should say that the values of maximum and minimum is different for all the sample, but the trend is identical.
Thanks in advance

[Scott R. Foster]

if we measured the impedance to air flow of all the samples would they vary appreciably?

[parham]

I have measured the sound absorption of woven fabrics. all the fabrics follow an identical trend, as i previously said, for all the samples the maximum sound absorption occurs at frequency of 1000 Hz and the minimum takes place at frequencies of 250 and 2000 Hz. moreover, the absorption of the samples does not differ significantly. in addition we determined the absorption via impedance tube. but we did not measured the impedance to airflow. thank you very much

[Tubamark]

I have measured the sound absorption of woven fabrics. all the fabrics follow an identical trend, as i previously said, for all the samples the maximum sound absorption occurs at frequency of 1000 Hz and the minimum takes place at frequencies of 250 and 2000 Hz. moreover, the absorption of the samples does not differ significantly. in addition we determined the absorption via impedance tube. but we did not measured the impedance to airflow. thank you very much

What you describe suggests that some shortcoming in the test method is dominating the results at those frequencies.
I doubt the airflow impedance is an issue, unless all the fabrics tested have identical density and porosity, and were tested under identical tension (tuned membrane absorption)--extremely unlikely. With differing fabrics, you should get differing absorption profiles, only broadband similarities at best, nothing so narrowly specific.

What you're describing sounds like a resonance based on either fixed distances in the setup, or a physical resonance of some component(s) of the test rig -- like a common frame that the differing fabrics are attached to, or something like that. Is the absorption you are seeing great enough to matter (be audible)?

Please provide specifics about the test setup you've been using, and how all the variables have been accounted for, such that your comparitive data represents only the fabrics, and nothing else. I'll bet we can figure this thing out.

-- Mark