Transverse Wave
Physics 11
Waves and Sound
disturbance
Types of Waves
Sound
The Doppler Effect
direction of travel
Longitudinal Wave
disturbance
Sound is a longitudinal wave
A wave in which the disturbance is parallel to the direction of
travel of the wave.
direction of travel
disturbance
direction of travel
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Sound
Sound
⇒ In sound waves, air molecules are disturbed in the same
direction as the wave.
⇒ However, an individual air molecule does not travel
with the wave. It only oscillates back and forth.
Sound
⇒ All sound is produced by an
object that oscillates back and
forth.
Sound
⇒
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).
Regions of increased pressure are Regions of decreased pressure are
called condensations.
called rarefactions.
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Sound
Sound
condensation
⇒ Your eardrum then detects the differences
in air pressure.
Sound
A stationary sound source
What happens if the sound source is moving?
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If the sound source is moving toward an observer,
the observer will hear a sound of higher frequency.
I hear a
lower frequency
The Doppler Effect
I hear a
higher frequency
If the sound source is moving away from an observer,
the observer will hear a sound of lower frequency.
The Doppler Effect
⇒ The frequency of sound (pitch) increases when
a source moves towards you.
⇒ The frequency of sound (pitch) decreases when
a source moves away from you.
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).
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.
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An object traveling at the speed of sound
Bow Waves and Shock Waves
bow wave ⇒ V-shaped wave produced by an object
moving on a liquid surface faster than the wave speed.
⇒ A similar thing happens when a plane is traveling
A shock wave is formed!
faster than the speed of sound. A sonic boom is heard
when the shock wave reaches listeners on the ground.
Breaking the Sound Barrier
sin θ =
v
vs
v = speed of sound
vs = speed of object
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We could figure out the speed of the jet by measuring
the angle of the Mach cone!
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