Naked Scientists
Dr Hugh Hunt

21 May 2006 - Transcript of the live radio show

music: "science of music, harmonics and making music in a tea cup"

THE SCIENCE OF SOUND - Dr Hugh Hunt, Department of Engineering, University of Cambridge

Dave - Today we've got Hugh Hunt from the Engineering Department at Cambridge University in the studio and he's now holding a normal kitchen knife. What are you going to do with that?

Hugh - We've just heard from Ely about harmonics which you can get from this tube. Well actually if you listen around pretty much anywhere you can find harmonics all over the place. I've got an ordinary eating knife, not one of those dangerous ones you don't want to be holding by the blade, and if I tap it like this I can get a nice note. I'm holding it roughly half way along. But if I hold it somewhere else I can get a different note. So depending on where you hold it, you get different notes. If you hold it right at the end, you don't a note at all.

Kat - So that's holding it at the very tip of the blade.

Hugh - Yeah, holding it at either end you don't get any note at all. But it's quite interesting to explore where you have to hold a knife to get these notes.

Dave - I see you've got a big aluminium tube behind you. Is that a bigger version of the same thing?

Hugh - It is. As ever, it's always much easier to demonstrate these things cleanly if you have a contrived perfect experiment. So I've got a tube here that's six foot long.

Kat - It's like a scaffolding post isn't it?

Hugh - It is. It's hollow inside so you can play it like a digeridoo as well but I won't do that now. If I hold it at the end it just goes thwack. It doesn't do anything. But if I hold it in special places I can get really nice notes. I can get this one….

Kat - So you're holding it about half way up.

Hugh - There are lots of different places. I've marked this tube with different coloured strips so I know exactly where to hold it. Now this is getting the fifth harmonic, so it's starting from the very bottom and going up the harmonic series, a bit like that pipe we heard before with its harmonic series. If I hold it somewhere different, I can get the sixth one up, which is a bit higher. And I can get the seventh one holding it even higher. I can get the fourth one here.

Dave - So what's actually happening to the tube when you hit it?

Hugh - Well the tube is vibrating. If you imagine you've got a long piece of spaghetti, not cooked spaghetti, and you could bend it into the shape of a letter C. Well if you imagine that that piece of uncooked spaghetti was floating around in space, it could vibrate backwards and forwards like that letter C. If you used three fingers, you could bend it into the shape of a letter S and it would vibrate backwards and forwards in the shape of the letter S. You could go one step further and bend it into the shape of a squiggly letter or number three with another bit in it and you could make it really serpentine. It turns out that every time you introduce more bends, there's more energy in it and a higher frequency. That's the same with light. You might know that energy goes up with frequency.

Dave - So the pitch gets higher the more wiggles you have in the rod.

Hugh - That's right. So all I'm doing with these marks on the tube here is noticing that I can pick out these different harmonics.

Dave - So basically where you're holding the rod, the rod can't be wobbling.

Hugh - That's right. It's called the nodal point. And so the fifth harmonic on this rid has six nodal points and the sixth harmonic has seven nodal points and so on. If you draw a letter S then it has three spots that go through the metal and the C has got two spots.

Kat - So you can do that, for example, on a guitar. If you very gently put your finger against the middle of the string. Is that the same principle when you get a high pingy sound?

Hugh - That's exactly the same. When you're playing a stringed instrument such as a guitar, a cello or a violin, you can play the note and then just touch it in the middle and you'll get an octave higher. If you touch it a third of the way along, you get an octave and a half higher. This is all the harmonic series. The reason you heard The Last Post being played before in Kitchen Science is because it uses the harmonic series.

Dave - So what Wendy was doing with the tube was driving different harmonics at different times to make the tune.

Mandy - And Hugh, interestingly enough if you go into a guitar shop to buy a new guitar, the first thing you do is test the harmonics because that will tell you whether the bridge is true or not.

Hugh - Absolutely right. Harmonics are so wonderful that composers throughout the centuries have used them in all sorts of ways. There's a wonderful bit in Brahms' first symphony that just uses harmonics played on the horns. It's just magic.

Dave - Is this because it's hard to play anything but the harmonic series on a horn?

Hugh - Well yeah. If you want to tell your kid not to play any instrument, don't tell then to play the horn because they're the ones that get the blame when things go wrong in the concert.


Q: Andy in North Carolina - 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?

A: 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.

Q: David in Ipswich - Could sound waves be heard on Mars?

A: There are two questions here. One is if we had a very loud screaming baby on the Earth, would somebody on Mars in theory hear the baby screaming, or would you need to have some interplanetary baby monitor? The answer is no, you wouldn't be able to hear the sound from Earth on Mars because the space in between Earth and Mars is a vacuum. There's a wonderful experiment you can do where you put an alarm clock under a bell jar and you start a vacuum pump going. You then stop hearing the bell even though the bell is still going. But then the question is, well what if your baby was in the bedroom in your Mars planetary home? Would you be able to hear the baby crying from downstairs in your bedsit? The answer would depend very much on the atmosphere on Mars and my understanding is that there's not very much atmosphere: about 1% of the atmosphere on Earth, so it's getting close to a vacuum. So there are two things: the amount of energy that you can pump into the atmosphere is less; and also the speed of sound would be a lot faster.

Q: Steph via email - When I play my flute, sometimes it gets a really screechy noise. What is it? It only happens when I play medium E natural.

A: Instruments are quirky things. It's a bit like cars; you might be driving along and when you go 57.5 miles per hour, the steering wheel starts to wobble. That's just a particular characteristic of your car. Some things you can fix and other things you can't. Some of you who play the cello or others stringed instruments might know about the wolf note: there's a certain note that just does funny things. It could be that it's an interaction between the acoustic mode or the note you're playing which is all to do with the air, and the actual structure of the flute such as it being made of metal. That's very important. You can think of a saxophone and a clarinet: a clarinet is made of wood and a saxophone is made of metal. That's essentially what makes the instruments sound quite different. So there is an interaction between the sound, the air and the metal and maybe that's why you pay an extra £500 for an ultra good flute.

Q: John in New York - Is white noise something to do with gravity or the big bang?

A: If I play a nice pure tone, then that's got a particular frequency. But if I were to play all notes together, then that starts to become what you might call noise. Let's imagine that you played every single note that you can possibly imagine, which would not only be the white and black notes on the piano, but all the notes in between. In acoustics we would call that white noise. It's not just sound where you find vibration. You find vibration and waves in the surface of water, light is a wave, you'll find waves in slinky springs, and all over the place. You may have heard of things like the cosmic background radiation. Actually these are all electromagnetic waves of various kinds and light can be as much white noise as anything. So the reason it's called white noise is because we know that white light is made up of all the colours of the rainbow; lots of frequencies all at once. So white noise is taken from white light. When there was the big bang all sorts of frequencies were emitted. The noise from the big bang had one peak frequency so it's not all the frequencies at once and so it probably isn't really white noise.

Q: Suzie in Norwich - What is ultrasound? Is it actually sound?

A: Ultrasound is sound but it's got such a high frequency that you can't hear it. It's so high and the wavelength's so small that you just can't hear it.

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