0
$\begingroup$

Why is the acoustic absorption coefficient of hexane in air less than air? A 120 kHz pulse train exhibits a larger amplitude after traveling in a hexane/air mix than in pure air.

My hypothesis is that the hexane molecule has fewer degree of freedom than that of $\ce{N2/O2}$. Is this plausible?

| improve this question | |
$\endgroup$
  • $\begingroup$ I was taught that linear molecules have 3N-5 DOF while nonlinear ones have 3N-6, where N is the number of atoms. Using that, N2/O2 have 1 DOF each, while hexane has 54 DOF, so it seems that "hexane molecule has fewer degree of freedom" is incorrect. ----- The core question here, however, is quite an interesting one. Do you have a reference for the claim about the 120kHz pulse train? $\endgroup$ – chipbuster Aug 26 '15 at 5:14
  • $\begingroup$ The only reference is my experimental work, currently unpublished. This observation arose when checking amplitudes. I had set the receiver amplitude to max in air and assumed it would be less in the hexane/air mix. However, the signal saturated the receiver. have not tried it on any other alkanes yet, because I was not looking for the effect. I do know that stuff like dichloromethane really kills the signal. Maybe 120kHz is some kind of window in its absorption spectrum, for some unknown reason. $\endgroup$ – Dirk Bruere Aug 26 '15 at 7:56
1
$\begingroup$

Best hypothesis so far is that as gas density increases the transducer couples more efficiently to the medium, giving the impression of a decrease in absorption coefficient due to the increased amplitude at the receiver.

| improve this answer | |
$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.