Claire Russell, of Intermediate Normal School, asks :-

When a ballon pops why does it make such a loud bang?"

Bill Williams, a physicist at Massey University, responded.

To answer it we first have to think about how our ears work and what has really happened when we "hear" a loud bang. The outer ear funnels sound to the ear drum, where it is amplified in the middle ear and passed onto 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.

So far, so good - we know something of how we detect sounds when our ear drum vibrates, but how exactly are sounds transmitted to us? To understand this we need the idea of pressure and a description of air itself. Air on Earth is largely a mixture of the gases nitrogen and oxygen, and gases most simply can be thought of as many small particles (molecules) moving around largely independently of each other. Because the gas molecules are moving around, if we try and squash them up in a container they bang into its walls and give a force that resists making the volume smaller. This is the pressure of the gas and it turns out that sound travels as pressure waves. In detail the science of acoustics is a rich and complex subject but simplistically, banging hammers, vocal chords or screaming electric guitars all generate disturbances in the pressure of the surrounding gas in which we live and this is how we hear them. In fact, if you put an alarm clock in a jar with all the air removed you won't hear it at all, and I'm afraid that Master Luke's futuristic weaponary would also be silent in space.

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 passing on the pressure until it collides with your ear drum and you hear the bang.