Electricity, Sound and Light

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Electricity, Sound and Light
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Chapter Nine Waves and Sound

• 9.1 Harmonic Motion
• 9.2 Waves
• 9.3 Sound

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9.3 The frequency of sound

• The pitch of a sound is how you hear and
  interpret its frequency.
• A low-frequency sound has a low pitch.
• A high-frequency sound has a high pitch.

Each person is saying Hello.
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9.3 The frequency of sound

• Humans can generally hear frequencies between 20
  Hz and 20,000 Hz.
• Most sound has more than one frequency.
• Almost all the sounds you hear contain many
  frequencies at the same time.

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9.3 The loudness of sound

• The loudness of a sound is measured in decibels
  (dB).
• The decibel is a unit used to express relative
  differences in the loudness of sounds.

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9.3 The loudness of sound

• Most sounds fall between 0 and 100 on the decibel
  scale, making it a very convenient number to
  understand and use.

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9.3 The frequency of sound

• Sounds near 2,000 Hz seem louder than sounds of
  other frequencies, even at the same decibel
  level.
• According to this curve, a 40 dB sound at 2,000
  Hz sounds just as loud as an 80 dB sound at 50 Hz.

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9.3 The speed of sound

• The speed of sound in normal air is 343 meters
  per second (660 miles per hour).
• Sound travels through most liquids and solids
  faster than through air.
• Sound travels about five times faster in water,
  and about 18 times faster in steel.

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9.3 The speed of sound

• Objects that move faster than sound are called
  supersonic.
• If you were on the ground watching a supersonic
  plane fly toward you, there would be silence.
• The sound would be behind the plane, racing to
  catch up.

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9.3 The speed of sound

• A supersonic jet squishes the sound waves so
  that a cone-shaped shock wave forms where the
  waves pile up ahead of the plane.
• In front of the shock wave there is total
  silence.
• Passenger jets are subsonic because they travel
  at speeds from 400 to 500 mi/hr.

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9.3 The Doppler effect

• When the object is moving, the frequency will not
  be the same to all listeners.
• The shift in frequency caused by motion is called
  the Doppler effect.
• You hear the Doppler effect when you hear a
  police or fire siren coming toward you, then
  going away from you.

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9.3 What is a sound wave?

• Sound waves are pressure waves with alternating
  high and low pressure regions.
• When they are pushed by the vibrations, it
  creates a layer of higher pressure which results
  in a traveling vibration of pressure.

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9.3 What is a sound wave?

• At the same temperature, higher pressure contains
  more molecules per unit of volume than lower
  pressure.

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9.3 The wavelength of sound

• The wavelength of sound in air is similar to the
  size of everyday objects.

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9.3 The wavelength of sound

• We usually think about different sounds in terms
  of frequency, but wavelength is also important.
• Musical instruments use the wavelength of a sound
  to create different frequencies.

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9.3 How we hear sound

• The parts of the ear work together

1. When the eardrum vibrates, three small bones
   transmit the vibrations to the cochlea.
2. The vibrations make waves inside the cochlea,
   which vibrates nerves in the spiral.
3. Each part of the spiral is sensitive to a
   different frequency.

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Technology Connection
Sound All Around

• From experience you know that a cafeteria and gym
  are often loud places, and libraries are quiet.
• How should we design these spaces and what types
  of materials should we use in them?

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Activity
Make Your Own Speaker

• Almost all speakers contain magnets and coils of
  wire that interact to create mechanical
  vibrations out of pulsating electric currents.
• You can make your own speaker to hear music.