The Physics of Sound

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The Physics of Sound
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• Sound a series of disturbances of molecules
  within, and propagated through, an elastic
  medium or
• Sound is an alteration in the pressure,
  particle displacement propagated in an elastic
  medium
• Elasticity capacity of a medium to return to
  its original position after being altered
• Brownian motion heat effects particle velocity
  heat velocity

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Sound Wave
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Sine Wave
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Compression\ Rarefaction
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Degrees of Wave
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Wave Length
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Velocity

• Acceleration increase in velocity
• Deceleration decrease in velocity
• Increase density increase V. (fastest in
  solids, then liquids, then gases)

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Degrees of Sine wave
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Sound Characteristics
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Damping

• Free vibration - when no outside force is added
  to vibration
• Lightly damping - when vibrations of a mass decay
  over time
• Heavy damping -
  rapidly
• Critically -
  before 1 Hz.
• Forced vibration - when outside force is added to
  vibration. When removed free vibration

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Vibrator Length, Mass and Stiffness

• If length decreases frequency increases
• If length increases frequency decreases
• If increase in mass decrease in frequency
• If increase in stiffness increase in frequency

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Vibration

• Periodic - a waveform that repeats over time
  (i.e. speech)
• Aperiodic - a waveform that doesnt repeat over
  time (noise)

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Fourier Analysis
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Frequency Pitch Intensity Loudness

• Frequency is the number of complete oscillations
  of a vibrating
• body per unit of time (in audiology we use
  Hertz Hz)
• Pitch is the subjective impression of highness or
  lowness of a
• sound. The psychological correlate of
  frequency.
• Intensity is the amount of sound energy per unit
  of area (dB)
• Loudness is the subjective impression of the
  power of a sound.
• It is the psychological correlate of
  intensity (sones)

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Resonance

• Resonance - the ability of a mass to vibrate at a
  particular frequency with a minimum application
  of external force. Also the ability of a vibrator
  to put another vibrator into vibration.
• Resonant frequency - the frequency at which a
  mass vibrates with the least amount of external
  force the natural frequency of vibration of a
  mass

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Fundamental frequency/overtones/harmonics

• Fundamental frequency - the lowest frequency of
  vibration in a complex wave
• Overtones - any whole number multiple of the
  fundamental frequency of a complex wave
• Harmonic - same as overtones except in numbering
  system
• (i.e. 1st overtone is to 2nd harmonic)

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Formant
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Impedance

• Opposition to the path of sound. Denser the
  object the more the impedance

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Sound Measurement

• Audiometer
• air conduction
• bone conduction
• speech (sound field, free field and under phones)

• Sound level meter
• A scale
• B scale
• C scale

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Intensity/Decibel

• It involves a ratio
• It involves a logarithm
• It is therefore non-linear
• It is expressed in terms of various reference
  levels which must be specified
• It is a relative unit of measure
• 3 dB increase, if intensity levels are doubled

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dB Levels
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Force and Pressure

• Force - the impetus required to institute or
  alter the velocity of a body. A Newton is also a
  measure of force.
• Pressure - force over an area of surface (i.e.
  Pascals .0002 dynes per cm2 the smallest
  pressure variation for hearing.
• Sound Pressure Level (SPL) .0002 dynes per
  centimeter squared 20 micropascals 0 dB SPL

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Thresholds

• The softest level an individual can hear a sound
  50 of the time.
• Threshold of pain is about 140 dB (SPL)
• Sensation Level (SL) dB above threshold

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Power

• the rate at which work is done. Units of
  measurement are watts, or ergs per/sec.

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Audiometric Zero

• Standards
• ASA 1951
• ISO 1964
• ANSI 1996