Sound

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Sound
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• Sounds good, eh?
• Ch 12

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Sound
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• Sound is a longitudinal and mechanical wave.

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Sound Frequencies
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• Human audible frequency range 20 Hz 20,000 Hz
• Ultrasonic Frequencies gt20,000Hz
• Infrasonic Frequencies lt 20 Hz

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Perceptual vs. Physical
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• Pitch
• Loudness (volume)
• Tonal Quality (Timbre)

• Frequency
• Intensity, Amplitude, Energy
• Harmonics

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Speed of Sound Close approximation
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• Depends on Temperature and Material

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Various Speeds of Sound
We will use 343 m/s as our default speed of sound
in 20C air
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Sound Intensity
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• Audible sounds depend on both frequency and
  intensity
• Intensity the rate at which energy is
  transferred from one molecule to another

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Intensity in decibels
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Threshold of
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• Hearing 1000 Hz _at_ 1.0x10-12 W/m2
• Pain Any intensity at 1.0 W/m2

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Common Sound Intensities
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Forced Vibrations
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• When a system is set in motion it vibrates at its
  natural frequency, but if an external force is
  attached a forced vibration will occur.Example
  A guitar string rigidly attached to a guitar will
  make the entire guitar vibrate at the frequency
  of the strings vibration

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Resonance

• When the frequency of a force applied to a
  non-vibrating system is the same as the natural
  (resonant) frequency of vibration of a different
  vibrating system the original system will vibrate
  at the resonant frequency.Example Tacoma
  Narrows Bridge, Tuning a radio

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Beats

• The interference of waves (sound) at slightly
  different frequencies traveling in the same
  direction
• Beats are perceived as a variation of intensity
  (volume)

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Beats
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Harmonics

• The fundamental frequency (lowest frequency of
  vibration for a standing wave) and integral
  multiples of the fundamental frequency
• 3 Situations we will analyze
• Strings example guitar
• Open tube example flute
• Closed tube example trumpet
• Great Sound Link for in depth music theory with
  Physics

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Harmonics for Strings
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Harmonics for Strings
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Law of Strings
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Harmonics for Tubes

• Tubes Rules At open ends an antinode must be
  present. At a closed end a node must be present.
  Displacement Picture

Open Tube Picture
Closed Tube Picture
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Open Tube Harmonics
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Open Tube Harmonics
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Closed Tube Harmonics
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Closed Tube Harmonics
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Harmonic Series
Clarinet f1 233 Hz
Alto Recorder f1 523 Hz
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Doppler Effect

• If a sound source or and observer of a sound is
  moving towards or away from the other there is a
  shift in the perceived frequency.
• Approach? frequency shifted higher
• Recede? frequency shifted lower

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Doppler Equation
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Doppler Effect for Light
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Doppler Effect
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Homework and Reading

• Read Ch 12
• pp 407 - 433

• HW due on test day
• p 413 1 7
• p 415 1 5
• p 420 1 5
• p 427 1 4
• p 431 1 5
• Extra Practice
• pp 434 435, 1 35