Sound Waves

Sound Waves


  • Sound is defined as an alteration in pressure which moves through an elastic medium such as air.
  • It is transmitted through the medium as a wave and is therefore called a sound wave.
  • Sound waves are an example of longitudinal waves.

Spherical Waves

  • One of the simplest sound waves is produced from popping a balloon.
  • In the interactive demonstration below, try moving the time slider and see how the pressure changes as the balloon pops.
  • At time 0:
    • The balloon hasn't popped
    • The pressure inside the balloon is higher than the normal atmospheric pressure
  • Once the balloon has popped:
    • The pressure that was inside the balloon radiates outwards as a pulse of pressure
    • If you look at the pressure pulse you will see it is formed of two parts:
      • First, the pressure increases above the normal level. This is called the condensation pulse
      • Then, the pressure drops below the normal level. This is called the rarefaction pulse
    • Also notice that the height of the wave decreases as it moves further away from the center
      • This is because the magnitude of the wave is inversely proportional to the distance from its source.
  • This type of sound wave is called a "spherical wave".
  • Spherical waves propagate outwards in all directions.

Plane Wave

  • Plane waves are different from spherical waves in that they only propagate in a single direction.


  • A cycle is a set of pressure variations that start and end at the same condition.


  • A phase is a specific point in a cycle.
  • In the above example, the red lines represent phases in the cycle.
  • Phases are described as an angle, with the entire cycle spanning 360°.
  • Phase can also be used to describe the shift between two otherwise identical waves:
  • The two waves above are identical apart from the phase. The phase between these two waves is 45°.


  • Technically referred to as the "velocity of the propagation of the wave".
  • There are 3 factors that affect the velocity:
    • γ = the heat capacity ratio of the gas
    • p0 = the static pressure in the gas, measured in dynes per square centimeter
    • ρ = the density of the gas, measured in grams per square centimeter
  • Then the speed, measured in centimeters per second is: c = \sqrt{\frac{\gamma}{p_0 \rho}}
  • As the pressure increases, the density also increases. This means that a change in pressure doesn't affect the speed of the wave.
  • Therefore the speed of the wave can be expressed only in terms of temperature.
  • The speed of a sound wave, in air, measured in centimeters per second is: c = 33,100 \sqrt{1 + 0.00366t} where t = the temperature in degrees centigrade.


  • A sound wave can contain a single cycle, but it is often formed from multiple repeating cycles.
  • The number of cycles in a single second is called the frequency.
  • Frequency is measured in hertz (Hz). 1Hz is one cycle per second.


  • the wavelength is simply the distance a wave travels to complete one cycle.
  • This is related to the frequency of the wave and the speed that it travels: c = \lambda f
    • c = the speed of the wave.
    • λ = the wavelength of the wave.
    • f = the frequency of the wave.


  • A sound wave is a pressure wave.
  • It consists of pressures above and below the normal pressure of the gas.
  • Instantaneous sound pressure:
    • This is the pressure at a given point in the wave, minus the normal pressure of the gas.
    • It is the change in pressure caused by the wave.
  • Effective sound pressure
    • This is the Instantaneous sound pressure over a complete cycle as the wave passes over a given point.
    • It is expressed as a single value by finding the root mean square of the complete cycle.
    • It is measured in dynes per square centimeter.
    • It is often shortened to "sound pressure".