Physics 12: Sound

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Physics 12 Sound

• Christopher Chui

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Characteristics of Sound

• Speed of sound in air at 0oC and 1 atm is 331
  m/s at 20oC, it is 343 m/s
• In general, v (331 0.6T) m/s
• Loudness is related to the energy
• Pitch refers to whether it is high/low
• The lower the pitch, the lower the freq
• Audible range is 20Hz to 20 kHz
• Above 20 kHz freq are ultrasonic
• Dogs can hear up to 50kHz, bats 100kHz
• Infrasonic waves are below 20Hz (earthquakes,
  thunder, volcanoes, etc)

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Intensity of Sound dB

• Intensity level, b 10 log (I/Io)
• Indoor rock music 120dB
• Ordinary conversation 65dB
• Whisper 20dB
• 10 dB 1 bel (after Alexander Bell)

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Vibrating Strings Air Columns

• Note C has a freq of 262 Hz
• Note A has the freq of 440 Hz, B has 494 Hz
• Stringed instrument, v sqrt FT/(m/L)
• High freq standing waves are overtones or
  harmonics
• For open ended tubes, L ½ l1, f1 v/2L
• For close ended tubes, L ¼ l1, f1 v/4L

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Interference of Sound Waves Beats

• Constructive interference occurs when the path
  difference of two sound sources are whole numbers
  of wavelength
• Destructive interference occurs when the path
  difference of two sound sources are ½, 1½, 2½
  wavelengths
• When two sound sources are close in freq but not
  the same, beats will occur
• Beat frequency difference in two wave freq

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Doppler Effect

• When a source of sound is moving toward an
  observer, the pitch is higher than the source at
  rest, f f/( 1 vs / v )
• When the source of sound is moving away from an
  observer, the pitch is lower than at rest, f
  f/( 1 vs / v )
• Observer moving toward stationary source, f
  ( 1 vo / v ) f
• Observer moving away from stationary source,
  f ( 1 vo / v ) f