Why is the acoustic absorption coefficient of hexane in air less than air? 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?
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.