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Sound. Sound Waves. Sound waves are longitudinal waves. The source of a sound wave is a vibrating object. Only certain wavelengths of longitudinal waves in the air can be detected as sound. Sound Waves. Sound waves require matter in order to propagate. They cannot travel in a vacuum

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sound waves
Sound Waves
  • Sound waves are longitudinal waves.
  • The source of a sound wave is a vibrating object.
  • Only certain wavelengths of longitudinal waves in the air can be detected as sound.
sound waves1
Sound Waves
  • Sound waves require matter in order to propagate.
    • They cannot travel in a vacuum
  • The speed of sound is different in different materials.
  • The speed of sound is also somewhat dependent on temperature.
speed of sound
Speed of Sound
  • Air (room temp) – 343 m/s
  • Air (0° C) – 331 m/s
  • Helium – 1005 m/s
  • Water – 1440 m/s
  • Steel – about 5000 m/s
  • Concrete – about 3000 m/s
loudness
Loudness
  • Loudness is related to intensity.
  • Loudness itself is not a measurable physical quantity.
    • It depends on the listener.
  • Intensity is measurable, however.
loudness1
Loudness
  • Sound intensity levels are usually given on a logarithmic scale.
  • The unit on this scale is the decibel (dB)
  • β = 10 log (I/I₀)
    • β = sound level (in dB)
    • I = sound intensity
    • I₀ = base sound level
loudness2
Loudness
  • The base sound level used (I₀) is usually what is know as the “threshold of hearing”
  • This is the minimum intensity that is audible to a good ear.
  • 1.0 x 10ֿ¹² W/m²
sound intensities
Sound Intensities
  • Threshold of hearing – 0 dB
  • Whisper – 20 dB
  • Talk, at 50 cm – 65 dB
  • Busy street traffic – 70 dB
  • Loud rock concert – 120 dB
  • Jet plane at 30 m – 140 dB
pitch
Pitch
  • Pitch is related to frequency of a sound wave.
  • A high pitch corresponds to a high frequency.
  • The audible range is the range of frequencies that a normal person can hear.
pitch1
Pitch
  • Frequencies between 20 Hz and 20,000 Hz are audible.
  • The threshold of hearing varies for different frequencies.
  • It is easiest to hear sounds between 2000 and 5000 Hz.
pitch2
Pitch
  • Frequencies too high to hear are ultrasonic.
    • Some animals can perceive sounds up to 100,000 Hz (bats)
  • Frequencies too low to hear are infrasonic.
beats1
Beats
  • Occurs due to interference between two sound waves that are close in frequency.
  • The sound level rises and falls as time passes.
  • This regular change in intensity is a beat.
doppler effect
Doppler Effect
  • The Doppler Effect is the apparent change in frequency of a wave due to relative motion between the observer and the sound source.
  • Has many uses.