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Sound Mechanical Vibrations in Gas, Liquid or Solid California State Standards Students know sound is a longitudinal wave whose speed depends on the properties of the ... – PowerPoint PPT presentation

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Title: Sound

  • Mechanical Vibrations in Gas, Liquid or Solid

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

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

Courtesy University of Wisconsin
Sound Waves Passes By Us
Courtesy University of Sydney
What Causes Sound
  • Vibrations of matter produce sounds
  • Sound could be a wave in air, or another medium
  • The original vibration makes something more
    massive vibrate, such as a sounding board

Sound-Harmonic Applets
Frequency and Pitch
  • Frequency is the technical term of the number of
    vibrations per second in a sound wave going past
  • 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

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)

Experiencing Speed of Sound
  • Name two experiences you have had caused by the
    relatively slow speed of sound
  • Echoes
  • Delay between seeing lightning and hearing thunder

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

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?

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

  • Complete Loudness is to intensity as pitch is
    to ________________

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
dB(SPL) Source (with distance)
194 Theoretical limit for a sound wave at 1 atmosphere environmental pressure
180 Rocket engine at 30 mKrakatoa explosion at 100 miles in air(160 km)1
150 Jet engine at 30 m
140 Rifle being fired at 1 m
130 Threshold of pain train horn at 10 m
120 Rock concert jet aircraft taking off at 100 m
110 Accelerating motorcycle at 5 m chainsaw at 1 m
100 Jackhammer at 2 m inside disco
90 Loud factory, heavy truck at 1 m
80 Vacuum cleaner at 1 m, curbside of busy street
70 Busy traffic at 5 m
60 Office or restaurant inside
50 Quiet restaurant inside
40 Residential area at night
30 Theatre, no talking
10 Human breathing at 3 m
0 Threshold of human hearing (with healthy ears)
  • 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
  • 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
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

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

The Tacoma Narrows Bridge
  • What do you think happened next?
  • What was the cause?
  • Wind induced vibrations

Resonance Applets from New Zealand
  • Resonance, Tacoma Narrows bridge failure, and
    undergraduate physics textbooks by Yusuf Billah
    and Bob Scanlan, (Am. J. Phys. 59 (2), February
  • " . . . 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

Wave Interference
  • Complete constructive
  • Complete Destructive
  • Applets
  • http//www.

Describe Each of These
Answers from top down complete destructive,
partial destructive, constructive
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

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
  • 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
  • Called beat frequency
  • Beats simulation applet

Unequal Comb Spacing Produces Moire Pattern like
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

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

Doppler Effect in Sound
  • Source or observer moving towards pitch
  • Source or observer moving away pitch decreases
  • Airplane Flyby

Wavelength Increases Frequency Decreases
Wavelength Decreases Frequency Increases
Doppler from New Zealand

Opposing Waves
Standing Waves
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