Chapter 26 Sound

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Chapter 26 Sound
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I. Origin of sound

• A. All sounds are produced by vibrations of
  material objects

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I. Origin of sound

• A. All sounds are produced by vibrations of
  material objects
• 1. In instruments and voices the original
  vibration stimulates the vibration in something
  else.

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• 2. Vibrating material sends disturbance through
  surrounding medium usually air

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• 2. Vibrating material sends disturbance through
  surrounding medium usually air
• 3. The frequency of the vibrating source equals
  the frequency of the sound produced

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• B. Pitch is how high or low sound frequencies
  appear to be
• C. People can hear 20- 20,000 Hertz
• Below 20 infrasonic
• Above 20,000 ultrasonic

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II. Sound in Air

• A. Sound travels in longitudinal waves

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II. Sound in Air

• A. Sound travels in longitudinal waves
• 1. compression pulse of compressed air

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II. Sound in Air

• A. Sound travels in longitudinal waves
• 1. compression pulse of compressed air
• 2. rarefaction a disturbance in air in which
  the pressure is lowered

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II. Sound in Air

• A. Sound travels in longitudinal waves
• 1. compression pulse of compressed air
• 2. rarefaction a disturbance in air in which
  the pressure is lowered
• Example door opening and closing

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• B. The pulse travels NOT the medium.

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III. Media that transmit sound

• A. Solids and liquids are better conductors than
  air
• B. Speed of sound varies between different
  materials
• C. No medium no sound
• Nothing to compress and expand

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Concept Quiz

• There is a lot of bad physics in movies. For
  example, most space movies have really noisy
  space battles. What is wrong with that?

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IV. Speed of Sound

• A. Much slower than light
• Light travels one million times faster
• B. Speed depends on
• 1. the medium
• 4 x faster in water than air
• 15 x faster in steel than air

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• 2. Temperature
• A. 0 degrees C dry air 330 m/s
• 20 degrees C dry air 340 m/s

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• 2. Temperature
• a. 0 degrees C dry air 330 m/s
• 20 degrees C dry air 340 m/s
• b. Higher temperature has faster moving molecules

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• 3. elasticity
• The ability to change shape in response to an
  applied force and then resume shape

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• C. Sound moves faster in moist air than dry
  because water molecules move faster than
  molecules in the air.
• Water (hydrogen and oxygen) weighs less than air
  (nitrogen and oxygen)

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How fast is that storm?

• dvt
• 340 m/s t
• Sound travels at 4.7 sec/mile

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Concept Quiz

• How far away is a storm if it takes three seconds
  for the thunder to reach you?
• 1. 340 m
• 2. 1020 m
• 3. 3 m
• 4. 680 m

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Concept quiz

• The Factory Whistle
• QUESTION
• Does the wind affect the pitch of the factory
  whistle you hear on a windy day? If so, why? If
  not, why not?
• 1. yes
• 2.no

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• ANSWER NO
• The wind does not affect the pitch. The wind does
  affect the speed of sound because the medium that
  carries the sound moves. But the wavelength of
  the sound changes accordingly, which results in
  no change of frequency or pitch.

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• This can be seen by analogy
• Suppose a friend is placing packages on a
  conveyor belt, say at a frequency of one each
  second. Then you, at the other end of the belt,
  take off one package each second. Suppose the
  speed of the belt increases while your friend
  still places one package per second on the belt.
  Can you see that the packages (farther apart now)
  will still arrive to you at the rate of one per
  second?

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V. Loudness

• A. Intensity of sound is proportional to the
  square of the amplitude
• 1.Loudness is related to the intensity of the
  sound but it is subjective

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Fromhttp//cnx.org/content/m12372/latest/fig1b
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• B. measured in decibels (dB)
• Starts at zero
• Each increase of 10 dB is an increase of sound
  intensity by a factor of 10
• 10 20 dB 10x intense
• 10 30 dB 100x intense

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Concept quiz

• A sound increases from 20 dB to 50 dB. What is
  the increase in the intensity of the sound?
• 1. 10x
• 2. 30x
• 3. 100x
• 4. 1000x

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• Decibel Exposure Time Guidelines
• Accepted standards for recommended permissible
  exposure time for continuous time weighted
  average noise, according to NIOSH and CDC, 2002.
• For every 3 dBs over 85dB, the permissible
  exposure time before possible damage can occur is
  cut in half.

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• Continuous dB Permissible Exposure Time
• 85 db 8 hours
• 88 dB 4 hours
• 91 db 2 hours
• 94 db 1 hour
• 97 db 30 minutes
• 100 db 15 minutes
• 103 db 7.5 minutes
• 106 dB 3.75 min (lt 4min)
• 109 dB 1.875 min (lt 2min)
• 112 dB .9375 min (1 min)
• 115 dB .46875 min (30 sec)

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VI. Forced vibration

• A. the vibration of an object that is made to
  vibrate by another vibrating object that is
  nearby
• B. Sounding boards in musical instruments amplify
  the sound
• C. Demo with tuning fork and box

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VII. Natural Frequency

• A. Different objects vibrate differently
  depends on elasticity and shape of objects
• B. Natural frequency minimum energy required to
  produce a forced vibration
• P. 395

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VIII. Resonance

• A. when the frequency of a forced vibration on an
  object matches the objects natural frequency
• B. Tacoma Narrows Bridge
• tacoma bridge from nova
• tacoma bridge black and white

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• http//www.madsci.org/posts/archives/may98/8926785
  04.Eg.r.html
• Why it happened

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• C. Tuning a radio adjusts the natural frequency
  of the electronics to match the incoming signal

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• C. Tuning a radio adjusts the natural frequency
  of the electronics to match the incoming signal
• Resonates one station at a time

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IX. Interference

• A. Sound waves can be made to interfere like
  transverse waves

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IX. Interference

• A. Sound waves can be made to interfere like
  transverse waves
• 1. crests overlap constructive interference
• 2. compressions overlap same thing
• 3. compression and rarefaction destructive
  interference

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• B. Affects the loudness of sounds
• 1. Equally distant from two speakers that
  simultaneously trigger identical sound waves

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• B. Affects the loudness of sounds
• 1. Equally distant from two speakers that
  simultaneously trigger identical sound waves
• A. sound is louder because they are in phase
• Constructive interference

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• 2. Move to the side of the speakers so that the
  sounds differ by half a wavelength.
• Rarefactions arrive at the same time as
  compressions
• Destructive interference

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• 3. Speakers emit many frequencies so not all
  wavelengths destructively interfere

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• 3. Speakers emit many frequencies so not all
  wavelengths destructively interfere
• 4. Sound usually reflects from other places to
  fill in cancelled spots but sometimes there are
  dead spots

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• 3. Speakers emit many frequencies so not all
  wavelengths destructively interfere
• 4. Sound usually reflects from other places to
  fill in cancelled spots but sometimes there are
  dead spots
• 5. Antinoise technology
• Noise canceling headphones for pilots and jack
  hammer users

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X. Beats

• A. periodic variation in the loudness of sound
• B. occurs when two tones of slightly different
  frequency are sounded together

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• First tone
• Second tone
• Combined

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• C. Throbs between maximum and minimum loudness
• demo with different tones

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• D. There are times where in phase and times when
  not
• another demo with tones
• graphic representation

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Examples

• police radar
• Hum into a fan
• Often you will hear the voice of a singer
  fluctuate with a vibrato. Singing two notes very
  close to each other cause this beat frequency.

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• piano tuner counts the beats
• When you fly in a passenger plane, you may often
  hear a fluctuating drone. That is a beat
  frequency caused by engine vibrations at two
  close frequencies.