Science Interviews


Mon, 8th Dec 2014

What is sound?

Dr Rob Toulson, Anglia Ruskin University

Listen Now    Download as mp3 from the show Good Vibrations

Sounds are all around us, all the time, but what exactly is sound? Rob ToulsonSound waves from Anglia Ruskin University has a background in acoustics, electronics and music production, and brought along a variety of musical instruments to help explain the concept to us.

Chris -   Letís get started.  Rob Toulson, you're in the hot seat first, so, youíve got the hard job of actually telling us, what actually is sound?

Rob -   Everything can vibrate.  If we hit something or something has got some kind of energy attached to it, it will vibrate.  When it vibrates, it makes the air molecules around it vibrate as well.  Those air molecules vibrate and they pass on, they knock on to each other, and pass on that vibration onto the next air molecule, onto the next air molecule until it reaches our ears and then our ears vibrate, and our brain turns those signals into things that we know about as sound.

Chris -   So soundwaves quite literally turn into brain waves...

Rob -   Essentially, yeah.  I'm sure Richard will tell you more about that later.

Chris -   When weíre actually listening to a sound, there are lots of different pitches of sound aren't there?  So, what actually is the difference between a note which is really low like Daveís voice and a note thatís really high, like Nicoleís singing.

Rob -   How fast something vibrates determines the pitch.  If something vibrates very slowly, we get a very low sound; but if something vibrates very fast, we get a very high sound.  And so, when we start to manipulate those vibrations mathematically we can organise the sounds into things that sound nice, and thatís generally referred to as "music".

Chris -   But not all music sounds nice.  I mean, James Blunt for example. Somethingís up there, isnít it?

Rob -   He sells a lot of records you know.

Chris -   There's a lot of deaf people!  No, I'm just kidding.  So, youíve brought up the subject of music.  How does a musical instrument work then? 

Rob -   Any musical instrument is generally designed to vibrate and produce some form of sound.  There are a number of different ways we can cause the air around an instrument or any object to vibrate.  I've got some examples.  I donít know if you want me to show you those now.

Chris -   What have you got?

Rob -   Well, there's three very simple ways that we can make sound and one of them is literally by hitting something.  So, if we actually pass some energy onto an object, it vibrates.  What I've got here in front of me is a tuning fork.  If I hit it, (sound), it vibrates.

Chris -   Position number 3 please...

Rob -   When it vibrates, essentially, this tuning fork is the right dimensions, the right material and itís got the right amount of material to vibrate in a very specific frequency.  So, every time I hit it, it always sounds the same.

Chris -   Thatís because of the mass of the material.  Itís moving a certain amount of mass backwards and forwards.

Rob -   Exactly.  And if it were heavier, it would move slower.  If it was a different material, it might move faster, some instruments are made of metal, some instruments are made of wood, some instruments are made of plastics.  They all gave slightly different sounds.

Chris -   If I stuck a bit of Blue Tack or something onto there to damp it or make it heavier.

Rob -   I've got that exactly.

Chris -   But itís not Blu Tack.  Youíve gotÖ

Rob -   No, I've got a little weight here.  So, this is it without the weight... (sound) and if I put a weight on it...(sound) and thatís with the weight.  Now, just before I move on, I want to show you the two at the same time because at the moment, that was one very specific signal.  But if I actually play them both at the same time, now, the air molecules will be all vibrating at different rates.  They will interfere with each other.  So, instead of getting a pure sinewave, we will get something thatís got more interference.  Hopefully, you'll be able to hear thisÖ(sound)

Chris -   Itís almost like a series of peaks and troughs.  Itís a bit louder and a bit quieter.  Can we just hear that again one more time?


Chris -   It's almost like jarring, like when a car goes pass your house and your windows vibrate a bit.  Itís almost doing that.

Rob -   Yeah, itís called beating actually.  What's happening is, the frequencies are very close, but they're not quite the same.  They come into time with each other and then out of time of each other.  As they do, they get louder and quieter.  Actually, we can't really hear two sounds.  We can only hear one, but we hear it wobbling.  If I just change the weight, you'll hear it wobbling at just a slightly different rate.  (sound)

Chris -   Thatís really spooky.  Itís going around in my head.

Rob -   So now, thatís changed the difference in frequency.  So, they're now going slowly in and out, itís called modulation.  They're slowly going in and out of time.  I  work a lot in music studios and recording studios and this can make a real big difference to how well a band or a music ensemble sound because if they're slightly out of tune from each other, we get all these kind of interfering sounds.  So, thatís why some of the best orchestras in the world sounds so good, and that little bit better than all the others is because their instruments are tuned and their ears are better and they can perform purely in harmony.

Chris -   Youíve got some other instruments here.  Have you got some other bits and pieces that you could do with it?

Rob -   The other things weíve got is strings.  So, this was hitting a piece of metal, an object, but strings vibrate as well.  So, this is a small ukulele.  When I pluck this string, it vibrates.  There's different strings which vibrate.  The reason they vibrate differently is because some are tighter than others and some have got different thicknesses than others.  So, depending on the thickness of the string and also, by putting my finger on the instrument here, I can effectively make this string shorter.  So, length, mass and tension all determine the sound.  The other thing is, we can play lots of different sounds at the same time.  If I play something where they're in harmony with each other, it sounds good and if I play something random, they donít sound so good.  So there are all these musical rules which give us an idea of when we might make certain sounds and when we might make others.

Chris -   Thatís the string section if we think about orchestra.  What about the wood wind or wind instruments?  How do they make sound?

Rob -   Okay, so we can make sound directly without actually hitting anything or plucking anything.  We can actually vibrate the air directly by creating some kind of wind power and thatís obviously the types of instruments, the woodwind instruments.  What I've got here is a pipe out of a church organ.  An old church organ blows wind down certain pipes and pipes of different diameters and different lengths all make different sounds.  So, if I blow into this one?  (sound)  It just makes a noise and thatís purely because of its size.  A different size pipe makes a different noise.  If you go into a church, you'll see all different sizes of pipes in the church.  What actually happens is, this mouth here in the pipe, this slotÖ

Chris -   Thatís the little slot.  Youíve got aboutÖI donít know, 20 centimetres along the tube.

Rob -   Yeah.  Essentially, it causes what we call turbulence in the air which mixes all the air up really randomly.  And then because this pipe is such a straight and round structure, the air all falls into some kind of organisation and gives us a pure tone.

Chris -   So, the air is blown in, thatís the energy coming in, but itís the turbulence created by that little slot that then creates the vibrations that go up and down the pipe and they become maximum or minimum according to how long the tube is.

Rob -   They fall into the frequency they want to vibrate in.  but there's another interesting thing with this, which if I blow it gently, (sound) it makes one noise.  But if I excite that air lots and I blow it harder, (sound) it makes a completely different noise.  And thatís because there's more energy at higher frequencies and then it falls into a different order.

Chris -   Any questions for Rob?

Ben -   I'm Ben and I'm from Barrington.  Is that why when you get like a whistle and you put your hand over the small tube, and you put the hand over all of the holes, it sounds really strange?

Rob -   Yeah, thatís because when you put your fingers over everything, that makes it go to its highest possible frequency and thatís really quite piercing.  But yeah, thatís exactly how recorders and woodwind instruments work.  by putting our fingers on the holes, weíre effectively are changing the mechanical, sort of, properties of that instrument.

Chris -   Thank you very much, Rob.  GinnyÖ

Ginny -   So Rob, you also brought along another little demo for us.  You said you were going to change our voices somehow.  How are you going to do that?

Rob -   Okay, so what I'm going to do is show you some sound effects which basically are taking some sound, i.e. a voice and manipulating it, doing some clever mathematical processing.  Itís all built-in to the computer and thatís some of the things I research these days are, how we can manipulate sound and change sound.  So, let me just switch my speaker on.  This is an example of just clear voice Ė no effect and this is the same thing with a load of reverb, which makes it sound like itís in a cave or in a really big church.  Something else I can do is add delay.  Hello, hello, hello.  Is anyone there?  Is anyone there?  Or I can add the two together (echoing sound) and it sounds really spooky, ha ha ha ha!

Ginny -   Anyone else want to have a go at changing their voice?  I thought that might be a yes.  What's your name and where are you from?

Rowan -   I'm called Rowan and  I'm from Cottenham.

Rob -   So, what are you going to say, Rowan?

Rowan -   I donít know really.

Rob -   Okay, here you go.

Rowan -   (high pitched, distorted voice) Hello

Ginny -   What's your name?

Rowan -   (high pitched, distorted voice)  Rowan. I've been at school...

Ginny -   And what's your favourite lesson?

Rowan -   (high pitched, distorted voice)  Probably maths or history

Ginny -   What have you done to Rowanís voice there?

Rob -   So, weíve just added a process effect which takes his voice and it makes it much higher pitch and it adds extra harmonics and it modulates them so they come in and out, and a bit of distortion and it sounds like a robot alien-type sound.

Ginny -   Made you sound very silly, didnít it?

Rowan -   Yeah.

Ginny -   What's your name?

Ben -   Ben and I'm from Barrington.

Ginny -   What are we going to do to Benís voice?

Ben -   (low pitched voice) Hello.  Oh come on!

Ginny -   So, what have you been doing today, Ben?

Ben -   (low pitched voice) I've been at school.  It's just unfair.

Rob -   Well, thatís a very similar thing except instead of going up in pitch, it's going down and thatís the kind of sound effect thatís used if you want to disguise your voice.  (low pitched voice) One, Two, this makes you sound very different.  Computers are so fast now, it can do it in what's called real-time which is essentially, as you're talking.


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