Sunday, 15 June 2008

How to Play String Instrument using Physic terms

How to play a string instrument using physics terms?

Homework question please help!
Describe the physical process that occurs as your instrument is played and produces sound. Begin with force and finish with detection by the human ear. Include types of waves produced and any and all technical (physics) terms possible. Extra points may be given for terms from 1st semester. (pressure, force, newtons laws)

Answer

You pluck or pull or bang on a string that is stretched between two fixed points. Because it's fixed at two points, that limits the wavelengths that are possible--the endpoints have to be nodes of the wave, so the wavelength must be 2, 2/3, 2/5, 2/7, 2/9, etc times the length of the string. These are the harmonics of the string.

Waves on a string travel at a fixed speed--the square root of tension over density. So if you know the speed and the harmonic wavelengths, you know the harmonic frequencies.

So when you deform the string, it will vibrate with all its harmonic frequencies. The ratio of the frequencies depends on the shape of the deformation. You could use Fourier analysis to figure this out. If you create a sharp deformation by plucking the string, you get a lot of the higher harmonics, so the sound is very twangy. If you strum it gently, you get mostly the principal frequency and the sound is smoother.

The sound is produced when the vibrating string bangs into the surrounding air and causes pressure disturbances. These pressure disturbances propagate into the surrounding air (which is what sound is).

If you want more basic physics on how a wave propagates through a string, read this wiki:
http://en.wikipedia.org/wiki/Vibrating_string
basics of sound:
http://en.wikipedia.org/wiki/Sound#Physi...

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