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# Physics of an acoustic guitar

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Hi all,

I have an interview with Lancaster University and have been tipped off that because I have put in my personal statement that I play guitar I will very likely be asked to explain what the physics are behind an acoustic guitar, but to be honest I'm not completely sure what it is. I have an idea it is to do with resonance and the natural frequency of the sound waves but could anyone help explain what goes on inside the box?

Many thanks,

Sion

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Think first of all what goes on outside the box. A vibrating string pushes air, hence creates a sound wave. The musical pitch depends on the frequency of the wave. The frequency depends on the length, tension, and mass per unit length of the string. If you make the string shorter (by stopping a fret) the frequency is higher and you hear a higher note: if you half the length the note goes up by an octave, and other musical intervals such as a fourth or fifth arise from simple ratios (as the ancient Greeks discovered). Increase the tension (by turning a peg) and you likewise increase the frequency of vibration, hence raise the pitch.

The vibrations cause the whole instrument to vibrate. Sound waves inside a cavity can create constructive interference (resonance), hence sound louder. This is what the soundbox does.

For an instrument to sound nice you want it to approximate to a simple harmonic oscillator (so that the overtones on the note are natural harmonics). Instruments of irregular shape (e.g. bells) produce overtones that are not in the harmonic series. In the case of something like a guitar or piano this means you want the strings to be as effectively one-dimensional as possible, i.e. long, thin and regular.

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Think first of all what goes on outside the box. A vibrating string pushes air, hence creates a sound wave. The musical pitch depends on the frequency of the wave. The frequency depends on the length, tension, and mass per unit length of the string. If you make the string shorter (by stopping a fret) the frequency is higher and you hear a higher note: if you half the length the note goes up by an octave, and other musical intervals such as a fourth or fifth arise from simple ratios (as the ancient Greeks discovered). Increase the tension (by turning a peg) and you likewise increase the frequency of vibration, hence raise the pitch.

The vibrations cause the whole instrument to vibrate. Sound waves inside a cavity can create constructive interference (resonance), hence sound louder. This is what the soundbox does.

For an instrument to sound nice you want it to approximate to a simple harmonic oscillator (so that the overtones on the note are natural harmonics). Instruments of irregular shape (e.g. bells) produce overtones that are not in the harmonic series. In the case of something like a guitar or piano this means you want the strings to be as effectively one-dimensional as possible, i.e. long, thin and regular.

We are glad to offer you an unconditional place, Mr Acey.

Lancaster University.

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Think first of all what goes on outside the box. A vibrating string pushes air, hence creates a sound wave. The musical pitch depends on the frequency of the wave. The frequency depends on the length, tension, and mass per unit length of the string. If you make the string shorter (by stopping a fret) the frequency is higher and you hear a higher note: if you half the length the note goes up by an octave, and other musical intervals such as a fourth or fifth arise from simple ratios (as the ancient Greeks discovered). Increase the tension (by turning a peg) and you likewise increase the frequency of vibration, hence raise the pitch.

The vibrations cause the whole instrument to vibrate. Sound waves inside a cavity can create constructive interference (resonance), hence sound louder. This is what the soundbox does.

For an instrument to sound nice you want it to approximate to a simple harmonic oscillator (so that the overtones on the note are natural harmonics). Instruments of irregular shape (e.g. bells) produce overtones that are not in the harmonic series. In the case of something like a guitar or piano this means you want the strings to be as effectively one-dimensional as possible, i.e. long, thin and regular.

Thanks for the answer . Seems nice and simple.

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We are glad to offer you an unconditional place, Mr Acey.

Lancaster University.

Ha ha, you beat me to it Olly, very good.

@Sion: Maybe you could fit Acey's response onto the back of a fag packet and take it with you?

("fag packet" that sounds so dated now, doesn't it? )

Can you sing? Maybe you could take your guitar and sing them these verses, if you know the song?:

What can this strange device be?

When I touch it, it gives forth a sound

It's got wires that vibrate and give music

What can this thing be that I found?

See how it sings like a sad heart

And joyously screams out it's pain

Sounds that build high like a mountain

Or notes that fall gently like rain

The people will all see it's light

Let them all make their own music

The Priests praise my name on this night...

'2112' by Rush, from the album of the same name.

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Olly - that made me chuckle!

Sion - best of luck with the interview.

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Let us know how it goes

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Yes, good luck. I believe that the first time mathematics was applied to 'reality' was in describing the musical scale. SInce mathematics is now the underpinning of physics it might be worth reading up on this. Also maybe read up on Gallileo's observation that nature was written in the language of mathematics.

I think science should be taught in this sort of way and that the jackasses behind the national curriculum should be sent into low earth orbit so that we could prepare accurately angled pointed sticks to welcome them on their return.

Olly

(I was a renegade English teacher so imagine my delight when the first national curriculum for English went out to schools informing them that 'Students should have an opportunity to - have an opportunity to read a wide range of literature.' Somewhere else it extolled the virtue of careful proof reading. Result!!! Physician, heal thyself.)

Edited by ollypenrice

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Let us know how it goes

Had the interview today and got an offer . They didn't ask me about guitars though. I had a question on calculating distance of horizon, why you aren't pulled to the floor of the ISS and what happens to a sealed room if a fridge is left open inside it.

Congratulations!

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what happens to a sealed room if a fridge is left open inside it

If I'm not in the room: the room gets warmer.

If I'm in the room: the room gets warmer & most of what was in the fridge mysteriously disappears.

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