Now I thought that sound in air is non-dispersive. Any ideas?
Air is nondispersive, but you are hearing guide modes from
sound reflected from the sides of the pipe. Those produce
the dispersion you hear.
A really impressive demonstration of sound dispersion which
cannot be taken into the lecture hall can be performed on a
frozen lake. If a hockey stick is slapped down hard on the
surface a "pew" sound will be heard at a distance, the same
sort of chirp descending in pitch that Mark hears echoing
in his pipe. I had always assumed that this sound was the
result of dispersive transmission in the ice, but this has
made me rethink that idea, and I will have to check it out
this winter. I live on the shore of a small lake in Burnaby
which freezes most years to a depth of fifteen centimeters
or so. It could be that I am also hearing waveguide modes.
If you have never heard this sound you should avail yourself
of the first opportunity to do so this winter. Frozen lakes
produce the most awesome sounds in nature, including the
fantastic sound of an ice sheet cracking as the underlying
supporting water drains out of the lake during an otherwise
perfectly quiet night. Nature provides a myriad of examples
we can use in physics teaching. Every physics teacher should
be attuned to them as Mark is.