Claire Russell, of Palmerston North Intermediate Normal School asks :-
When a balloon pops why does it make such a loud bang?
Bill Williams, a biophysicist at Massey University, responded.
We first have to think about how our ears work and what has really happened when we "hear" a loud bang. Sound is transported to our ear as a pressure wave in the medium surrounding us, which is usually air, but it could be water if we are underwater. The outer ear funnels sound to the ear drum, where it is amplified in the middle ear and passed into a pea-sized, snail-shell shaped structure in the inner ear, called the cochlea. Inside the cochlea fluid movements move small hairs and it is these disturbances that generate the signals interpreted by our brains as sounds.
When we blow up a balloon we trap gas molecules and increase the pressure of the gas inside. Indeed, the skin of the balloon is tight because the gas molecules inside are all banging on the inside of the balloon trying to get out. So in some sense, as the balloon contains trapped high pressure gas, it is a noise just waiting to happen! As soon as the balloon is popped the high pressure gas flies out, banging into the air around it and thus transmitting a pressure wave to your ear drum.
Surprisingly the rubber skin of the balloon doesn't explode into pieces after being punctured but quickly shrinks along the balloon surface. I suspect at high pressures it may be retracting faster than the speed of sound in air hence a really loud pop may be due to a sonic boom, as happens with an aeroplane breaking the sound barrier or when "cracking" a whip.