Waves
Physics 4A
Waves - II
Sound is a longitudinal wave
A wave in which the disturbance is parallel to the direction of
travel of the wave.
disturbance
Waves
Interference
Longitudinal Standing Waves
Beats
Doppler Effect
Shock Waves
direction of travel
Waves
⇒ Remember that a transverse wave is one in which the
disturbance is perpendicular to the direction of travel of the wave
Waves
⇒ In sound waves, air molecules are disturbed in the
same direction as the wave.
disturbance
direction of travel
⇒ However, an individual air molecule does not travel
with the wave. It only oscillates back and forth.
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Waves
Waves
⇒ All sound is produced by an object that
oscillates back and forth.
⇒
The frequency of the sound waves is the
same as the frequency of the oscillating source.
Waves
Regions of increased pressure are Regions of decreased pressure are
called condensations.
called rarefactions.
Waves
⇒
A speaker produces sound by a diaphragm
which oscillates back and forth.
⇒ The diaphragm produces successive regions of
increased pressure (condensations) and decreased
pressure (rarefactions).
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Waves
Waves
⇒ Your eardrum then detects the differences in
air pressure.
Interference
Interference
⇒ Sound waves, like any waves, can interfere
with each other.
Constructive
Interference
Destructive
Interference
Constructive Interference
Destructive Interference
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Interference
Longitudinal Standing Waves: tube open at both ends
There must be an antinode at
both open ends since the air
molecules there are free to
move
⎛ v ⎞
fn = n ⎜
⎟
⎝ 2L ⎠
n = 1, 2,3, 4,...
⇒ For sound waves, constructive interference occurs when the
condensations and the rarefactions from two sounds waves overlap.
If the condensation from one sound wave overlaps the rarefaction
from another sound wave, destructive interference occurs.
Longitudinal Standing Waves: tube open at only one end
There must be an antinode at the
open end since the air molecules
there are free to move but there
must be a node at the closed end.
⎛ v ⎞
fn = n ⎜
⎟
⎝ 4L ⎠
n = 1,3,5,...
Beats
f = 10 Hz
f = 12 Hz
f = 2 Hz
beat frequency = difference in the two frequencies
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Doppler Effect
Beats
A stationary sound source:
⇒ If two sound waves with slightly different frequencies overlap,
the interference of the two sound waves produce beats
Doppler Effect
I hear a
lower frequency
What happens if the sound source is moving?
Doppler Effect
I hear a
higher frequency
⇒ If the sound source is moving away from an observer, the
⇒ When the source of sound is moving toward an observer, the
wavelength is smaller so the frequency is greater.
observer will hear a sound of lower frequency.
⇒ If the sound source is moving toward an observer,
the
⇒ When the source of sound is moving away from an observer,
the wavelength is larger so the frequency is less.
observer will hear a sound of higher frequency.
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Doppler Effect
The Doppler Effect:
Formal definition: the change in frequency or
pitch of the sound detected by an observer
because the sound source and the observer have
different velocities with respect to the medium of
sound propagation.
⇒ When an observer is moving towards a stationary source, the
frequency is greater (and the wavelength is smaller)
⇒ When an observer is moving away from a stationary source, the
frequency is smaller (and the wavelength is bigger)
Doppler Effect
What it means: if either you or the source of
sound is moving, you will hear a sound of a
different frequency than is being emitted.
Shock Waves
An object traveling at the speed of sound:
NEXt Generation Weather RADar
A shock wave is formed!
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Shock Waves
Shock Waves
An object traveling faster than the speed of sound:
A Mach cone is formed!
Breaking the sound barrier
Shock Waves
sin θ =
Shock Waves
v
vs
v = speed of sound
vs = speed of object
We can figure out the speed of the jet by measuring the
angle of the Mach cone!
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