PHYS 012A Lecture Notes - Lecture 3: Sound, P-Wave, Modulus Guitars
12 Jun 2018
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Chapter 17 – Sound Waves 9/11/17
Intro
- Sound waves are longitudinal and travel through material medium
Categories
- Audible – we hear them
- Infrasonic – too low
- Ultrasonic – too high
Producing a Periodic Sound
- A 1D periodic sound wave can be made by causing the piston to move in simple harmonic
motion.
- Darker parts of the areas represent areas where the gas is compressed and the density and
pressure are above equilibrium values.
- The compressed region is called a compression.
- Low pressure regions are rarefactions and they follow compressions and move at the speed
of sound in the medium.
Periodic Sound Waves, Displacement
- The harmonic position function is s (x, t) = smax cos (kx – wt)
- S max is the displacement amplitude of the wave.
- Pressure wave is 90 degrees (quarter cycle) out of phase with the displacement wave.
o The pressure is a maximum when the displacement is zero.
o Delta P(x,t) = delta Pmaxsin(kx-wt)
Speed of Sound Waves
- The speed of sound waves in a medium depends on the compressibility and the density of the
medium.
- The speed of all mechanical waves follows a general form ->
elastic property
inertial property
v=
- For waves on a string: v = sqrt(T/mew)
- For sound waves: v = sqrt(B/row) = Bulk Modulus (how hard to compress medium/ Mass
Density)
Speed of Sound in Air
- The speed of sound depends on temp of the medium.
- Relationship between speed and temp ->
T
vC
(331 m/s) 1 273
=+
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Document Summary
Sound waves are longitudinal and travel through material medium. A 1d periodic sound wave can be made by causing the piston to move in simple harmonic motion. Darker parts of the areas represent areas where the gas is compressed and the density and pressure are above equilibrium values. Low pressure regions are rarefactions and they follow compressions and move at the speed of sound in the medium. The harmonic position function is s (x, t) = smax cos (kx wt) S max is the displacement amplitude of the wave. Pressure wave is 90 degrees (quarter cycle) out of phase with the displacement wave: the pressure is a maximum when the displacement is zero, delta p(x,t) = delta pmaxsin(kx-wt) The speed of sound waves in a medium depends on the compressibility and the density of the medium. The speed of all mechanical waves follows a general form ->
