Download - Chapter 13 Sound
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Chapter 13Sound
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Sound is a compressional wave created by a disturbance or vibration
that compresses molecules.
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When the tuning fork pushes toward the direction of wave motion, the air molecules are compressed, when it pushes away, the air molecules are spread out.
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Frequency is how many compressions are created in a second (Hertz), and is also called pitch.
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The speed of sound depends on the density of the medium. Molecules are closer together in denser materials, so there is less distance the disturbance needs to move through to reach the next molecule.
vmedium (m/s)air (0°) 331water 1490iron 5130
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Sound waves propagate in 3d spherically
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If a sound source is moving waves get pushed together and spread apart parallel to the motion.
This causes a change in frequency
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Sound Intensity
Sound intensity is the rate at which sound energy flows through a unit of area.
Intensity =E/tarea
P
= area = P
4r2
Units = W/m2
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Loudness is a function of the log of the intensity. For something to sound twice as loud, it would have to be 10x as intense.
Loudness is measured in decibels (dB), with sound being twice as loud every 10 dB.
vacuum 70 dBalarm clock 80 dB
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All things have natural vibrating frequencies
If one tuning fork is struck and the other tuning fork is the same frequency, it will start to vibrate. This is called resonance.
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HarmonicsStanding waves can be created in a vibrating string.
,...3,2,12
nL
vnfn
fn = frequency of harmonic numbern = harmonic number
v = wave speed
L = length of vibrating string
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Standing waves in an air column
Open at both ends
,...3,2,12
nL
vnfn
Closed at one end
,...5,3,14
nL
vnfn