Making music: volume and pitch

How do musical instruments actually work? Katie Haylor found out from science demo expert Dave Ansell...

Dave - Well, they’re producing sound. And if you’ve ever felt your throat while you’re talking you should feel a vibration, and so sound is basically just a vibration in the air. That vibration on a string instrument is happening in the strings and each string has a different pitch at which it likes to vibrate which it resonates at. So if you pluck your different strings they all vibrate at a different pitch. If you change the length of the string it will change that pitch as well, and the tautness of the strings, so when you're turning it you change the tautness of the string. But, as you heard with my beautiful acoustic electric guitar earlier.

So this is basically just the strings vibrating. It’s very quiet and it sounds pretty rubbish. But the difference between this and your cello is that your cello’s got the beautiful wooden box underneath.

凯蒂- Ah, this hollow box right?

Dave - Exactly. Which is there to convert the vibration in the string, which is actually quite quiet because it can’t get out into the air very efficiently, into a vibration of the air. So it’s about making the air vibrate when the string is vibrating.

凯蒂- Ah, so this is the natural amplifier, right, that the electric guitar achieves by being plugged into an amp?

Dave - Yes. The electric guitar gets away from having to have these big boxes by electromagnetically detecting the movements of the strings and then putting that directly into an amplifier and into a speaker.

凯蒂- Okay. So there are pretty important concepts that we need to get our heads around when it comes to playing musical instruments. They include pitch, and volume, and I believe you have a pretty nifty demo in order to illustrate these?

Dave - I have a very nifty demo anyway and it make lots of noise, and I think it’s quite cool.

Dave - To start of we need some producing a vibration so what I’ve got here is a hexagonal nut made out of steel. I’m just going to roll it down something here. You might be able to hear a slight noise. That was just rolling down a magazine in fact, and so that wasn’t very loud but you can hear a bit of a vibration.

If however, you take a balloon.

凯蒂- Oh, thank you very much.

Dave - And another nut. You put the nut in the balloon.

凯蒂- It’s quite hard to do. It’s quite fiddly.

Dave - Then you’re going to inflate it, yeah.

凯蒂- Right, okay. Here goes… Why didn’t I learn the clarinet? I’m not sure my lungs are up to this! Okay.

Georgia - Katie has gone the same colour as the balloon.

凯蒂- Thanks Georgia!

Dave - Which is red rather than green.

凯蒂- So I’ll tie it now.

戴夫-领带。现在而不是滚动nut down the magazine (which obviously finishes) you have a circular thing you can roll it round. So if you swirl the balloon -

凯蒂- Okay. Shall I try this?

Dave - Yeah.

凯蒂- Okay. Here goes…

Dave - So it can start off attempting to roll but it’s just sliding. That’s what you heard then and then it suddenly started rolling and suddenly you heard lots and lots of noise.

If you compare it to my 5p coin… which is much higher but quieter. It’s quieter because the bumps on the 5p are much smaller so you’re getting smaller vibrations, and small vibrations are quiet - make a quieter noise.

凯蒂- Ah. So if you were to draw the vibrations over time, the amplitude of the bumps would be smaller, right?

Dave - Yeah.

凯蒂- In the 5p?

Dave - Yeah.

凯蒂- Because the ridges are smaller?

Dave - Yeah. There’s less changes in pressure so they push on your ear less hard and that’s what you hear is a quieter noise. Which is the reason when you're further away things are quieter because that sound, that energy, is spread out over more area so it can’t push on your ear as hard so it’s quieter.

Georgia - As I’m recording this I’m looking at the waveform going along and it matched what you’re saying for the 5p they were smaller and closer together. So is that pretty much the pattern of the 5p hitting the balloon?

Dave - Yep. Because the 5p hits it more often but more gently.

凯蒂- So the volume we covered. Higher pitch: this is the frequency of bumps, right?

Dave - So how fast the vibration is is the pitch. So because the 5p has got more smaller bumps so to roll the same distance, it’s going to bump a lot more often so it’s going to be a higher pitch noise than the nut. Big nuts make a slightly lower pitch than small nuts as well.

凯蒂- So there you go, we’ve got a demo you can do at home. You just need a balloon, a 5p, and a nut. And there you’ve got frequency or pitch and amplitude or volume.

Dave - And a way of annoying your parents if you’re a child!