Sound

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Chapter 13

• Sound

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Section 13.1

• Sound Waves

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The Production of Sound Waves

• Sound is a result of vibrations or oscillations.
• How We Hear Video (105)

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• A region of lower density and pressure is
  rarefaction.
• A region of high molecular density and high air
  pressure is called compression.

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Sounds waves are longitudinal waves

• The crests in the curve correspond to
  compressions in the sound wave, and the troughs
  correspond to rarefactions.

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Characteristics of Sound Waves

• Frequency the number of cycles per unit of
  time.
• Infrasonic waves lt 20Hz
• Ultrasonic waves gt 20,000 Hz
• 20 Hz lt audible sound waves (humans) lt 20,000 Hz
• Dogs can hear ultrasonic waves, and
• elephants can hear infrasonic waves.

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Cont.- Frequency determines pitch

• The frequency of an audible sound wave determines
  how high or low we perceive the sound to be,
  known as pitch.
• As the frequency of a sound wave increases, the
  pitch rises.
• As the frequency
• decreases, the pitch falls.

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FYI

• Ultrasonic waves have widespread medical
  applications like to produce images of objects
  inside the human body (Ultrasounds)
• Sound waves are partially reflected when they
  reach a boundary between two materials of
  different densities.
• Ultrasonic waves are clearer and more detailed
  than other those of a lower-frequency sound wave
  because they are easily reflected off small
  objects.
• Audible and infrasonic sound
• waves are not as effective
• because their longer
• wavelengths pass around
• small objects.

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Other medical industrial uses of ultrasounds

• Pierre Curie discovered how to produce ultrasonic
  vibrations in 1890. By World War II their first
  practical application was the detection of
  submarines.
• Ultrasonic energy can heat deep tissues, to treat
  arthritis, bursitis, muscular dystrophy, and
  other diseases.

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Other medical industrial uses of ultrasounds

• High-energy ultrasonic waves can be focused into
  a pinpoint "scalpel" for bloodless brain surgery.
  
• Cleaning, sterilization, machining, cutting
  diamonds, burglar alarms and remote-control
  television tuners. It is also sometimes used in
  welding and soldering metals, mixing liquids, and
  dyeing and bleaching textiles.

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Speed of sound depends on the medium

• Sound waves can travel through solids, liquids,
  and gases.
• Solid particles respond more quickly to
  vibrations than gas particles because those
  molecules are closer together. Therefore, sound
  travels faster in solids.

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Temperature

• In a gas, the disturbance can spread faster at
  higher temperatures than at lower ones because of
  an increase in collisions.
• Sound speed and frequency video (151)

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The Doppler Effect

• Doppler Effect Video 201
• Pitch is higher as the object moves towards you
  and lower as it moves away.

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Relative motion creates a change in frequency

• The frequency of the car horn is constant, but
  because the source is moving towards observer A,
  the wave fronts hit Observer A sooner and sooner.
  As a result, Observer A hears a greater frequency
  sound resulting in
• higher pitch.
• Remember we are discussing
• frequency and not speed.
• The speed of the sound
• waves remains constant.

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Relative motion creates a change in frequency

• The sound waves dont reach point B, behind the
  car as frequently as in front of it. Therefore,
  the frequency heard by the observer is less than
  the source frequency.
• The Doppler effect
• is a shift in frequency
• due to motion between
• a wave source and
• an observer.

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• The Doppler effect will occur when the source or
  the observer is moving.
• Even though it is used mostly with sound waves,
  the Doppler effect can be used with
  electromagnetic waves like visible light.
• The Doppler Effect Applet

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Homework

• P 486 1-4, 6

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Sound

• 13.2

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Relative Intensity

• Relative Intensity is the relationship between
  the intensity of a sound wave (its energy) to the
  intensity at the threshold of hearing. It is
  related to loudness and is measured in decibels
  (db).

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Decibel Levels

• Volume (Loudness) doubles with every 10 db.
• Ex
• 0 db threshold of hearing
• 30 db whisper
• 50 db normal conversation
• 70 db vacuum cleaner
• 90 db lawn mower
• 120 db threshold of pain

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Vibrations

• Sympathetic Vibration- when a vibrating object
  causes another object to vibrate.
• Ex Plucked guitar strings vibrate and cause the
  body of the guitar to vibrate.
• Natural frequency- this is the set frequency at
  which a pendulum (or string) will vibrate and is
  determined by length.

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Resonance

• Resonance is a condition that exists when the
  frequency of a force applied to a system matches
  the natural frequency of vibration of the system.
  
• Shattering Glass Resonance Video 518

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Sound

• 13.3

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Harmonics

• A vibrating string will produce standing waves
  whose frequencies depend upon the length of the
  string.

Harmonics Video 234
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Harmonics

• As the harmonic increases the frequency increases
  and wavelength decreases.
• Standing Waves, Fixed at
• Both Ends Animation

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Why do different instruments sound different?

• Timbre is the quality of a steady musical sound
  that is the result of a mixture of harmonics
  present at different intensities.
• This is why a clarinet and a trumpet can play the
  same pitch but they sound different.
• Harmonics Applet

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Beat

• When two waves of slightly different frequencies
  travel in the same direction they interfere. This
  causes a listener to hear an alternation between
  loudness and softness and is called beat.

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Beat

• Formation of Beats Applet
• The frequency difference between two sounds can
  be found by the number of beats per second.

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Homework

• P 493 1, 3, 4
• P 503 2, 3