Sound

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Sound

• Mechanical Vibrations in Gas, Liquid or Solid

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California State Standards

• Students know sound is a longitudinal wave whose
  speed depends on the properties of the medium in
  which it propagates.
• Students know how to identify the characteristic
  properties of waves interference (beats) ,
  diffraction, refraction, Doppler effect, and
  polarization.

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Anatomy of a Sound wave

• Sound waves consist of alternating compressions
  and rarefactions in a longitudinal wave

Courtesy University of Wisconsin
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Sound Waves Passes By Us
Courtesy University of Sydney
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What Causes Sound

• Vibrations of matter produce sounds
• Sound could be a wave in air, or another medium,
  solid, liquid or gas.
• The original vibration makes something more
  massive vibrate, such as a sounding board

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Frequency and Pitch

• Frequency is the technical term of the number of
  vibrations per second in a sound wave going past
  us
• Pitch is how we hear frequency
• Range of human hearing is about 20 to 20,000 Hz
• As we age, we can no longer hear the highest
  pitch sounds
• Listen to Different Frequencies

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Speed of Sound

• About 331 m/s at 00 C
• Increases by 0.6 m/s for every degree
• 1200 km/hr
• 1100 feet per second
• Depends on properties of medium such as
• Density
• Elasticity(ease of changing shape)

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Experiencing Speed of Sound

• Name two experiences you have had caused by the
  relatively slow speed of sound
• You see runners in motion before you hear
  starting gun
• You see ball off bat before hearing it
• Echos reflections off canyon walls
• Delay between seeing lightning and hearing thunder

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Thunder Delay

• You hear thunder five seconds after you see
  lightning. How far away from you did the
  lightning strike?
• D s x t
• D 1100 feet/sec x 5 s
• 5500 ft or about one mile
• Metric 340 m/s x 5 s 1700 m 1.7 km

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Sounds in Liquids and Solids

• Sound travels much faster in liquids than in air
• About 1500 m/s in water
• Much faster still in solids
• About 4500 m/s in steel
• Can you think of a scene in a movie based on the
  high speed of sound in solids?
• Can you think of a scene in a movie that shows
  wrong sound physics?

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Wave Properties Review

• What are the characteristic properties of all
  waves?
• Frequency, wavelength, speed, amplitude
• Intensity is proportional to the square of
  amplitude
• The sensation we experience due to the intensity
  of a sound wave is called loudness

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Analogy

• Complete Loudness is to intensity as pitch is
  to ________________

frequency
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Intensity in decibels

• Energy in a sound wave in a certain area
• dB 10 log10 (I/I0) dB is decibels

Logarithms Review Log10 (10) 1 Log10 (100)
2 Log10 (1000) 3 Log10 (10,000) ? To find the
logarithm of a number to a certain base is to
find the exponent to which the base needs to be
raised to obtain the original number
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(No Transcript)
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Questions

• Use dB 10 log10 (I/I0) to answer
• (1) If the sound of a siren is twenty times
  more intense than that of a person speaking, how
  many decibels more is this?

Answer 13 dB more since log 20 1.3
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Questions

• Use dB 10 log10 (I/I0) to answer
• (2) If the sound of a rock concert is fifty
  times more intense than the sound of street
  traffic, how many decibels more is this?

Answer 17 dB more
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Forced Vibration, Natural Frequency and Resonance

• All objects have frequencies they vibrate at
  naturally natural frequencies
• Any object can be forced to vibrate
• When object is forced to vibrate at its natural
  frequency, the result is called resonance

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Examples of Resonance

• How do you need to push the person on the swing
  to get the maximum amplitude?
• At natural frequency!

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The Tacoma Narrows Bridge

• What do you think happened next?
• What was the cause?
• Wind induced vibrations

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• Resonance, Tacoma Narrows bridge failure, and
  undergraduate physics textbooks by Yusuf Billah
  and Bob Scanlan, (Am. J. Phys. 59 (2), February
  1991)
• " . . . in many undergraduate physics texts the
  (1940 Tacoma Narrows bridge) disaster is
  presented as an example of elementary forced
  resonance . . . Engineers, on the other hand,
  have studied the phenomenon . . . and their
  current understanding differs fundamentally from
  the viewpoint expressed in most physics texts. In
  the present article the engineers' viewpoint is
  presented . . . It is then demonstrated that the
  ultimate failure of the bridge was in fact
  related to an aerodynamically induced condition
  of self-excitation or "negative damping" . . .
  This paper emphasizes the fact that. physically
  as well as mathematically, forced resonance and
  self- excitation are fundamentally different
  phenomena.

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Wave Interference

• Complete constructive
• Complete Destructive
• Applets
• http//www. mysite.verizon.net/vzeoacw1/wave_inter
  ference.html

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Describe Each of These
Answers from top down complete destructive,
partial destructive, constructive
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Anti-noise Technology

• Example of destructive interference
• Use microphone, amplifier and speaker to produce
  opposite sound
• This combines with the original sound by
  destructive interference to produce
• No sound!
• Sometimes called active noise cancellation
• Buy headphones

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Speakers Out of Phase

• Face speakers toward each other
• Change one wire so they are connected wrong (out
  of phase)
• What do you predict will happen

Two speakers sound less loud than only one
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Beats

• Another example of sound wave interference
• When two tuning forks close in frequency are
  played you hear a third sound, whose frequency is
  the difference between the two tuning fork
  frequencies
• Called beat frequency
• Beats simulation applet

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Unequal Comb Spacing Produces Moire Pattern like
Beats
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Beats Questions

• Tuning forks of 254 and 256 Hz are played. What
  is the beat frequency?
• 2 Hz
• Tuning forks of 514 and 518 Hz are played. What
  is the beat frequency?
• 4 Hz

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Another Example of Beats

• How could you use the phenomenon of beats to get
  these engines to run at the same speed?
  (synchronize them)

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Doppler Effect in Sound

• Source or observer moving towards pitch
  increases
• Source or observer moving away pitch decreases
• Fun applets
• http//www.ndt-ed.org/EducationResources/HighSchoo
  l/Sound/dopplereffect.htm
• http//library.thinkquest.org/19537/java/Doppler.h
  tml
• http//www.lon-capa.org/mmp/applist/doppler/d.htm
  

Wavelength increases
Wavelength decreases