hum interesting subjet, so i read this wikipedia page : https://en.wikipedia.org/wiki/Speed_of_ ... eous_media
It seems a non trivial problem, so without some indication of what you want to archive it is not possible to answer.
-form of the sample (rod, 3d form)
-what change between sample (material or just thickness)
-what do you want to make with this mesure (extrapolation to what?)
-how many mesurement (just a few or you need a field instrument)
i think you must think of your final need, then the protocol, then the material, then how to do it cheap. Because if it's just for a few approximate mesure, it could be done in lab with a an oscilloscope and some microphone, but for a field instrument, that's not the same.