What makes a whip go 'crack'?

It isn't just Concorde that breaks the sound-barrier - whips do too. Scientists originally thought that the crack produced by a whip is because the tip of the whip breaks the sound barrier and makes a minature sonic boom - this is when something travels faster than the speed of sound and overtakes its own sound wave so that the sound waves are all compressed together behind the object, making a very loud sound. But now scientists have found that it is not the tip of the whip that is making the noise - because that they have found that when the crack occurs the tip is actually moving at twice the speed of sound - 1434 miles per hour. So what is breaking the sound barrier and making the noise when the whip actually cracks then ? Physicists Alain Goriely and Tyler McMillen at the University of Arizona in the US have found that it is in fact the loop that travels along the length of the whip that breaks the sound barrier and makes the cracking noise.