the "crack" of a whip?

[Robert Dyer]

Pardon me if I am posting this in an inappropiate group.

	What causes the sound that a whip (i.e bullwhip) makes when you
"crack" it?

	A friend and I are trying to settle a bet.  My friend contends
that the sound is caused by the tip of the whip moving at such a spped
that it breaks the "sound barrier" and therefore couses a small "sonic
boom."  I disagree.
	The trouble is that neither of us have been able to find any
information to support our positions.  Can anyone point me in a direction
where information on the subject may be found.

Thanks,

Bob