• Sound is a vibration that typically propagates as an audible wave of pressure, through a transmission medium such as a gas, liquid or solid.

Speed of Sound

  • The speed of the sound depends on the medium
  • In air, sound travels about 340 m/s
  • In liquid, sound travels fast
  • In solid, sound travels fastest


  • Energy from sound is spread over the surface area of a sphere
  • Intensity of sound drops with the square of distance
  • At double the distance, the sound is a quarter as loud


  • The decibel is a unit of measurement used to express the ratio of one value of a power or field quantity to another, on a logarithmic scale, the logarithmic quantity being called the power level or field level, respectively. It can be used to express a change in value or an absolute value.


  • Aside from spreading out, the energy from sound waves can be lost to friction in the medium. This is called damping or attenuation.


  • Beat is an interference pattern between two sounds of slightly different frequencies, perceived as a periodic variation in volume whose rate is the difference of the two frequencies. When tuning instruments that can produce sustained tones, beats can be readily recognized.
  • In areas of constructive interference, the waves add together
  • In areas of destructive interference, the waves oppose each other and cancel
  • The beat frequency is the frequency of the big oscillations

Standing Waves

  • A standing wave, also known as a stationary wave, is a wave which oscillates in time but whose peak amplitude profile does not move in space. The peak amplitude of the wave oscillations at any point in space is constant with time, and the oscillations at different points throughout the wave are in phase.
  • They represent sound-pressure waves with a graph of air pressure

Waves in a pipe

  • The largest wavelength, smallest frequency wave that can fit in a pipe has adjacent extrema
  • The smallest frequency is the fundamental
  • Maximum pressure is the antinode
  • Minimum pressure is the node

The Doppler Effect

  • The Doppler effect is the change in frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source. It is named after the Austrian physicist Christian Doppler, who described the phenomenon in 1842.