Sound is a longitudinal wave
Physics 10
Sound
A wave in which the disturbance is parallel to the direction of
travel of the wave.
disturbance
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Sound
direction of travel
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.
⇒ All sound is produced by an
object that oscillates back and
forth.
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Sound
Sound
⇒
A speaker produces sound by a diaphragm which
oscillates back and forth.
compression
compression
⇒
The diaphragm produces successive regions of
increased pressure (compressions) and decreased
pressure (rarefactions).
Regions of increased pressure are Regions of decreased pressure are
called compressions.
called rarefactions.
Sound
Sound
compression
⇒ Your eardrum then detects the differences
in air pressure.
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Sound
Exercise 31
If the handle of a tuning fork is held
solidly against a table, the sound of the
tuning fork will become louder. Why?
⇒ The sound is louder when a struck
tuning fork is held against a table because
the surface of the table is set into
vibration. This results in more air
molecules vibrating which makes the
sound louder.
Exercise 13
Why is the moon described as
the silent “planet”?
⇒ The moon is described as a
silent planet because it has no
atmosphere to transmit sounds.
The Reflection of Sound
⇒ Whenever a sound hits a surface, some of
the sound gets reflected (echo) and some of the
sound gets absorbed.
⇒ How much of the sound gets reflected and
how much gets absorbed depends upon the
surface.
⇒ The more rigid and smooth a surface is, the
more sound it will reflect.
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The Reflection of Sound
Some concert
halls place
highly reflective
surfaces above
the stage to
direct sound out
to the audience.
Exercise 23
Why is an echo weaker than the
original sound?
⇒ Two reasons:
1) Sound spreads out and becomes
weaker with distance.
2) Not all of the sound waves get
reflected. Some of the sound waves get
absorbed.
The Reflection of Sound
⇒ If the surfaces of a room are too reflective,
you can have multiple reflections
(reverberations) taking place and the sound
becomes garbles.
Interference
⇒ Sound waves, like any waves, can interfere
with each other.
Constructive
Interference
Destructive
Interference
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Interference
Beats
⇒ Constructive interference occurs when the
compressions (or the rarefactions) from two sounds waves overlap.
⇒ Destructive interference occurs if the compression from one
⇒ If two sound waves with slightly different frequencies
sound wave overlaps the rarefaction from another sound wave.
overlap, the interference of the two sound waves produce
beats.
Beats
Exercise 28
f = 10 Hz
Why do soldiers break step in marching
over a bridge?
f = 12 Hz
⇒ Soldier’s break step when crossing a
bridge so they will not set the bridge into
resonance. If the frequency of the
soldier’s steps matches the bridge’s
natural frequency, they might cause the
bridge to collapse.
f = 2 Hz
beat frequency = difference in the two frequencies
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The Refraction of Sound
⇒ Sound waves bend (refract) when different parts of the
wave fronts travel at different speeds.
⇒ Sound waves bend away from warmer air and towards
cooler air.
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