Part of the show New Planets, Harmonics and the science of Sound.
Andy in North Carolina asked:
If you take a two-litre plastic soda bottle and blow across the lid, you get a very low note. If you squeeze the bottle and make it flat, you get a higher note. What's going on?
I have a bottle hear so I can demonstrate this for you. First of all, I'm going to blow a note. Now we all know that if we put a bit of water in the bottle, we expect the note to go up because there is less air in the bottle to vibrate. If there is less to vibrate, then you get a higher note. Now the question is, if I squeeze the bottle and make it smaller, surely the note should also go up. But what happens is that it goes down. If I take the squeezed bottle and lower it into a tub of water, it goes up again, and this gives us a clue. When the bottle is perfectly round, we know that round things are stiff and solid. That's why pressure vessels are round and why a coke bottle with pressure in it is round. That's the strongest shape. But when you squeeze it you make the sides flat and that makes them weak. So actually you can tell when you put your finger on the side of the bottle, you'll actually be able to feel the flat bits vibrating a lot. That means that the bottle has an effective size that is bigger than it really is because it's moving some of the space around it. So the note goes down.