Re: Dispersion of sound waves??

[nguilber@peddie.k12.nj.us (nguilber)]

> For the past few days there has been a stack of big steel pipes in the road
> outside where they are installing a new gas main. These are steel pipes (I
> guess) about 25cm in diameter and 12 metres long as measured by pacing them.
>
> Great, I thought, we can observe reflection from the open end. This is done
> by going "boh" into the pipe and listening to the sound bouncing back and
> forth. Works well. Today I chanced to click my tongue instead of going "boh"
> and observed an interesting effect. What I heard back was a sort of high
> frequency "pew" sound, just as if the click had been dispersed, with the
> higher frequencies arriving first. Snapping my fingers produced the same 
> effect.
>
> Now I thought that sound in air is non-dispersive. Any ideas?
>
> Mark.
>
> Mark Sylvester
> UWCAd, Duino, Trieste, Italy.

    It's not the air - it's the pipe.  The 'click' can make any number (i.e., 1 
to n where n is very large) of reflections off the inside of the pipe on its 
way down and back.  The delay in arrival times for different n values gives a 
closely-spaced (in time) series of clicks which makes the sound you hear.  The 
general phenomenon is known as 'chirping', and what you heard is known as a 
'culvert whistler'.  

    Frank Crawford had a nice article on this phenomenon in an AJP article 
(August '88, pp. 752-754) that, among other things, related culvert whistlers 
to ionospherically-reflected radio 'tweeks' after a lightning bolt to (when the 
chirp is time-reversed) the generation of sub-picosecond laser pulses.  A good 
read!


    Nick

    Nick Guilbert
    Peddie School

    nguilber@peddie.k12.nj.us