Mechanical Waves and Sound

Wave Concepts

and some facts and terms

  • Mechanical waves can be thought of as a disturbance in a medium. This disturbance transfers energy from one place to another, although the medium itself does not travel.
  • There are two basic kinds of waves, transverse and longitudinal.
  • A wave pulse is a single disturbance.
  • A traveling wave is produced by repeated disturbances in some pattern (aka simple harmonic motion).
  • Period and frequency of a wave are inversely related, if one goes up the other must go down.
  • Wavelength and frequency of a wave are inversely related, if one goes up the other must go down.
  • Velocity of a wave is its wavelength multiplied by its frequency.
  • A large amplitude wave transfers more energy than a small amplitude wave, all other things being equal.
  • Two waves can be in one place at the same time (not true for matter). When they are, they add (or subtract) to produce a "combination wave."
  • Constructive interference is the term for two (or more) waves that add their crests together. Destructive interference is the term for two (or more) waves that add crests to troughs, effectively canceling part of the wave.
  • In the ideal case, two waves can pass through each other without undergoing change.
  • A wave that passes into a different medium is partially reflected and partially transmitted.
  • A wave that passes from a less dense to a more dense medium produces an inverted reflected wave. (The opposite produces an upright reflected wave.)
  • A standing wave results when identical waves travel in opposite directions. A wave on a string is an example. A standing wave has nodes and antinodes.
  • The law of reflection is true for waves.
  • Waves refract, or change direction (bend) when they meet a change in their medium at an angle.
  • Waves refract because one part of the wave slows down before another.
  • Waves pass through a small aperture (hole) in a barrier by spreading out through the hole (diffraction). Waves can also bend around an obstacle (also diffraction). When diffraction occurs because of nearby holes, a pattern of alternating constructive and destructive interference may be set up.
  • A wave front can be thought of as infinitely many miniature wave fronts (Huygens' Principle).
  • Procedure for solving a standing wave problem:
    • Decide whether each end acts like a fixed or free end (closed or open tube).
    • A node occurs at a fixed end, an antinode at a free end.
    • Draw the wave pattern that you think fits
    • Decide what fraction of a wave you have drawn
    • Use the wave fraction and length to solve for lambda, the wavelength
    • Use the wave equation to solve for whatever else is need

--useful links--

PhET Wave on a String

Standing Wave

Acoustics Animations