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13 nature of-sound
 

13 nature of-sound

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  • Compressions The close together part of the waveRarefactions The spread-out parts of a wave
  • 1831 collapse of the Broughton Suspension Bridge and the 1850 collapse of Angers Bridge

13 nature of-sound 13 nature of-sound Presentation Transcript

  • The Nature of Sound
  • What is Sound? In human experience: That which we hear. In Physics: Oscillation of pressure transmitted through a medium at frequencies between 20Hz and 20 kHz.
  • Sound travels in longitudinal waves — vibrating compressions and rarefactions through air
  • Forced Vibrations setting up of vibrations in an object by a vibrating force
  • Natural Frequency The frequency at which an object tends to vibrate when hit, struck, plucked, strummed or somehow disturbed. Depends on – elasticity – shape
  • Harmonic frequencies are multiples of the fundamental natural frequency.
  • Resonance occurs whenever successive impulses are applied to a vibrating object in rhythm with its natural frequency.
  • Mechanical Resonance Following the collapse several bridges led to notices being placed warning troops to break step when crossing the bridge.
  • Shake-table crash testing of a building model.
  • Interference combined effect of two or more overlapping waves
  • Waves overlap crest-to- crest, resulting in a wave of increased amplitude.
  • Waves overlap trough-to- crest, resulting in a wave of decreased amplitude.
  • Destructive interference is the principle behind active noise-cancelling technology.
  • Beats are periodic variations in the loudness of sound due to interference.
  • Standing waves
  • This standing wave is a sum of two waves travelling in opposite directions.
  • Nodes of standing wave
  • Reflection
  • Sound waves refract when air near the ground is warmer than air above. Refraction
  • Interference combined effect of two or more overlapping waves
  • Beats are periodic variations in the loudness of sound due to interference.
  • Doppler Effect the change in frequency as measured by an observer due to the motion of the source or listener. Named after Austrian physicist and mathematician, Christian Johann Doppler
  • Water Bug Doppler Effect Top view of water waves made by a stationary bug jiggling (up and down) in still water. Water waves made by a bug swimming in still water toward point B. A & B receive different wave frequencies.
  • Doppler Effect Example of Doppler Effect: Frequency of waves received by an observer increases as a sound source approaches. Wave frequency decreases as the source recedes.
  • Stages of Wave Speeds Bug swims at successively greater speeds. Overlapping at the edges occurs only when the bug swims faster than wave speed.
  • Shock Waves and the Sonic Boom Shock wave pattern of overlapping spheres that form a cone from objects traveling faster than the speed of sound
  • Shockwave The shockwave actually consists of two cones. – A high pressure cone with its apex at the bow – A low pressure cone with its apex at the tail. – A graph of the air pressure at ground level between the cone takes the shape of the letter N.
  • Shockwave The shock wave has not yet reached listener A, but it is now reaching listener B, and it has already reached listener C.
  • Musical Sound Graphical representations of noise and music. (a) Noise has no clear repeatable pattern. (b) Music has a frequency (repeatable wave), wavelength, and speed.
  • Music and Standing Waves Each harmonic of a guitar string is a standing wave. Image shows the first four harmonics on the string.
  • Variations in Tone Images of a piano and clarinet playing note C Each has the same frequency, but with different extra vibrations. These differences produce tone.
  • end