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?

  • $\begingroup$ 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  • $\begingroup$ The only reference is my experimental work, currently unpublished.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

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.

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