Chapter 12: Sound

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Chapter 12 Sound
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Do Now 3/23/09

• Sound waves are longitudinal waves. When sound
  waves occur in air, air molecules press together
  and push apart. Based on what you know about
  longitudinal waves and what they look like, draw
  a picture of what you think the air molecules in
  a sound wave look like.

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Sound Waves are Longitudinal
High Pressure COMPRESSION
Low Pressure RAREFACTION
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Compressions and Rarefactions occur in any
Longitudinal Wave
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Speed of Sound depends on Medium

• Sound waves can travel through gases, liquids,
  and solids.
• Sound travels faster in liquids than in gases,
  and even faster in solids. The reason why sound
  travels the fastest in solids is because the
  particles are already close together they come
  into contact with each other more frequently.
  Therefore, the disturbance can travel through the
  solid faster since the particles are already
  close together.
• Will sound travel faster in air or in water?
  In air or in steel?

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Pitch

• High pitch sounds have high frequency waves
• Low pitch sounds have low frequency waves
• The average human can hear pitches with
  frequencies from about 20 to 20,000 Hz.
• Sounds with frequencies from 20 Hz to 20,000 Hz
  are called audible sounds

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Infrasonic and Ultrasonic Sound

• Sound waves with frequencies below 20 Hz are
  called infrasonic
• Sound waves with frequencies above 20,000 Hz are
  called ultrasonic
• We cannot hear infrasonic of ultrasonic sound
  waves, but many animals can.
• Bats, dogs, dolphins can hear ultrasonic sounds
  Elephants and giraffes can hear infrasonic
  sounds.

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Do Now 3/24/09

• Draw a sound wave with compressions and
  rarefactions. Label the compressions and
  rarefactions. Draw the sine curve (underneath
  your sound wave drawing) that corresponds to the
  sound wave. Label the crests and troughs of the
  wave that is represented by the sine curve.

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Doppler Effect

• When a fire engine approaches you, you experience
  the sound waves of its siren more frequently
  (high pitched sound). But as it moves away from
  you, you encounter the waves less frequently (low
  pitched sound).
• The Doppler Effect occurs whenever there is
  relative motion between the source of waves and
  the observer. Either one can move towards the
  other, or both could be moving at the same time.
  
• A person standing in front of the ambulance will
  hear a higher pitch. A person standing behind
  the ambulance will hear a lower pitch.
• There is only a change in frequency the speed of
  the sound waves does not change (speed of sound
  in air is always 340 m/s)

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Do Now 3/25

• 1.) As the air temperature increases, what
  happens to the speed of sound?
• 2.) Does sound travel faster in water or steel?
• 3.) Why does the sound of a fire engines siren
  get louder as the fire engine approaches you?

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Sound Intensity

• The rate at which wave energy is transferred
  through the area in which the sound wave occurs.
• Intensity ?E / ?t
• area
• Power is also the rate at which energy is
  transferred, so intensity can be described in
  terms of Power.
• Intensity Power
• area
• Units W/m2

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Sound Intensity

• Intensity Power
• 4p (distance from the source)2
• Intensity Power
• 4pr2

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Forced Vibration

• When one vibrating object forces another object
  to vibrate, producing a louder sound.
• Examples Cell phone vibrating on table forces
  the table to vibrate, and an increase in sound
  occurs. A guitar string attached to the body of
  a guitar forces wooden body to vibrate, and an
  increase in sound occurs.

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Natural Frequency and Resonance

• Every object has a natural frequency - the
  frequency at which the object will naturally
  vibrate.
• When an objects natural frequency matches the
  frequency of a forced vibration, an increase in
  amplitude occurs this is called resonance.
• A swing has a natural frequency. When you pump
  your legs back and forth, you are causing a
  forced vibration in the swing. When your
  frequency the frequency at which you pump your
  legs matches the natural frequency of the
  swing, an increase in amplitude occurs.

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Do Now 3/31/09

• 1.) What are the nodes of a standing wave? What
  are the antinodes?
• 2.) How can you find the speed of a wave?
• 3.) Solve the following sequence for n 1, 2,
  and 3
• Fn 2n 5

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Harmonic Series

• The series of frequencies of a vibrating string

L ?1/2 ?1 2L
string length and half the wavelength
V (wave speed) f? f v/? By substituting in ?1
2L we get f v/?1 v/2L (fundamental
frequency, or first harmonic)
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String length and wavelength
?2 L. f v/? v/?2 v/L 2(v/2L) or 2
times the fundamental frequency f2 (2nd
frequency) 2f1 (2 times the fundamental
frequency). This is called the second harmonic
of the vibrating string, because it is the second
possible vibration for the string.
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• The third harmonic frequency is equal to three
  times the fundamental frequency or the first
  harmonic
• f3 3f1
• The fourth harmonic frequency is equal to four
  times the fundamental frequency
• f4 4f1
• Each harmonic is a multiple of the first harmonic
  (the fundamental frequency). Therefore, fn nf1
  where f1 is the fundamental frequency (f1 v/2L)
  and fn is the frequency of the nth harmonic.

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Harmonic Series

• The series of frequencies, including the
  fundamental frequency and the multiples of the
  fundamental frequency
• Harmonic series of a vibrating string
• fn nv
• 2L n 1, 2, 3,
  
• frequency
• harmonic number x (speed of the waves on the
  string)
• 2(length of the vibrating string)

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Harmonic Series of a pipe open at both ends

• fn nv
• 2L n 1, 2, 3,
• frequency
• harmonic number x (speed of sound in pipe)
• 2(length of vibrating air column)

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Harmonic Series of a pipe closed at one end

• If one end of the pipe is closed, only the odd
  harmonics are present.
• The simplest vibration occurs when the length of
  the pipe is ¼ of the wavelength
• (L ¼ ?1)
• ?1 4L (4 times the length of the pipe)
• v (wave speed) f?
• f v/? v/?1 v/4L
• f1 (fundamental frequency) v/4L

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• The second type of vibration occurs when the
  length of the pipe is ¾ the wavelength
• L 3/4? ? ? 4/3L
• f3 3f1 The second frequency has been skipped,
  moving right to the third frequency.
• Only the odd harmonics are present when one end
  of the pipe is closed.

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Harmonic Series of a pipe closed at one end

• Fn nv
• 4L n 1, 3, 5,
• Frequency
• harmonic number x (speed of sound in pipe)
• 4(length of vibrating air column)

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Do Now 4/2/09

• 1.) The following is a diagram of the sound waves
  produced by a moving police car. In which
  direction is the police car moving?
• If the police car remains stationary,
• in which direction is the observer
• moving?
• 2.) What are the compressions of a sound wave?
  What are the rarefactions?
• 3.) How can infrasonic sounds be used? How can
  ultrasonic sounds be used?
• 4.) As the decibel level is increased by 10 dB,
  what happens to the intensity of a sound?

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Beats

• When two compressions of a sound wave meet, the
  sound is at a maximum (at its loudest).
• When a rarefaction and a compression meet, the
  sound is at a minimum.
• These variations in loudness are called beats.
• Two sound waves that are in phase then out of
  phase then in phase again and so on, produce
  beats.

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Beat Frequency

• The beat frequency will tell you how many beats
  are heard each second.
• The beat frequency is the difference between the
  two frequencies.
• If one sound has a frequency of 200 Hz and
  another sound has a frequency of 204 Hz, the
  number of beats heard each second is 4.
  Therefore, the beat frequency 4 Hz (beats per
  second)

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• A piano tuner using a 392 Hz tuning fork to tune
  the wire for G-natural hears four beats per
  second. What are the two possible frequencies of
  vibration of this piano wire?
• Two notes are sounding, one of which is 440 Hz.
  If a beat frequency of 5 Hz is heard, what is the
  other notes frequency? How many beats are heard
  each second?
• Suppose three tuning forks of frequencies 120 Hz,
  125 Hz, and 127 Hz are available. What beat
  frequencies are possible for these forks sounded
  together?