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Sound
Notes
Sound Waves
• Sound is a disturbance that travels
through a medium as a longitudinal
wave.
• Sound waves are longitudinal waves
that begin with a back and forth
vibration.
• If you pluck a guitar string, it vibrates
back and forth. The vibrating string
sends a longitudinal wave through the
air.
Sound Waves
• Sound waves need a medium to travel
through - gases such as air, liquids
such as water, and solids such as
wood.
Interactions of Sound
Waves
• As discussed before, sound waves
reflect off objects, diffract through
narrow openings and around barriers,
and interfere with each other.
The Speed of Sound
• The speed of sound depends on the
elasticity, density, and temperature of
the medium through which it travels.
• Sound travels at different speeds in
different mediums. For example, sound
usually travels more quickly in solids
than in gases.
• At room temperature (20°C), sound
travels through air at about 343 m/s.
The Speed of Sound
• Sound travels faster in mediums that
are elastic, or stretchy. A rubber band is
very stretchy. Therefore, sound travels
quickly in a rubber band.
The Speed of Sound
• Sound travels more slowly in denser
mediums. Something that is dense feels
heavy for its size. Lead is a very dense
solid. Therefore, sound travels more
slowly in lead than in most other solids.
The Speed of Sound
• Sound travels more quickly when the
medium is warm. For example, sound
travels more quickly in air that is 20°C
than in air that is 0°C.
Loudness
• The loudness of sound depends on two
factors: the amount of energy it takes
to make the sound and the distance
from the source of the sound.
• Loudness is how loud or soft a sound
seems to the person who hears it.
Loudness
• The more energy it takes to make a
sound, the louder the sound. For
example, pounding hard on a door with
your fist makes a louder sound than
tapping gently on a door with your
fingers.
• The amount of energy a sound wave
carries per second through a unit of
area is its intensity.
Loudness
• The closer you are to the source of the
sound the louder the sound. The
whisper of a person next to you is
louder than the whisper of a person
across the room.
• The loudness of a sound is measured in
decibels (dB). The loudness of a
whisper is about 20dB. The loudness of
a rock concert is about 120dB.
Pitch
• The pitch of a sound that you hear
depends on the frequency of the sound
wave.
• Pitch is how high or low a sound seems
to a person who hears it. The sound of
a whistle has a high pitch. The sound of
thunder has a low pitch.
Pitch
• The pitch of a sound depends on the
frequency of the sound waves. A sound
wave with a higher frequency makes a
sound with a higher pitch. Conversely, a
sound wave with a lower frequency has
a lower pitch.
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Pitch
Humans hear only a certain range of
pitches. A sound that is too high for
humans to hear is called ultrasonic
(ultra- means above). A sound that is too
low for humans to hear is called
infrasonic (infra- means below).
In music, different pitches are usually
referred to as notes. Singing, an
individual can change your pitch by
stretching or relaxing your larynx, or
vocal chords.
The Doppler Effect
• When a sound source moves, the
frequency of the waves changes
because the motion of the source adds
to the motion of the waves.
• The Doppler effect is a change in pitch
that happens when a sound source is
moving. For example, when a
motorcycle races by you, the sound of
the engine changes pitch.
The
Doppler
Effect
• When a motorcycle is coming toward
you, the bike is moving in the same
direction as the sound waves you are
hearing. This makes the waves closer
together. The waves have a higher
frequency, so the sound has a higher
pitch.
• When the motorcycle is going away
from you, the bike is moving in the
opposite direction from the sound
waves you are hearing. This makes the
waves farther apart. The waves have a
lower frequency, so the sound has a
http://en.wikipedia.org/wiki/Doppler_effect