Sound Waves

A sound wave is a longitudinal pressure vibration set in motion by any event that generates energy, such as a vibrating object.

If a sound wave is moving from left to right through air, then the particles of air will be displaced both rightward and leftward as the energy of the sound wave passes through it. Vibrating air molecules cause the human eardrum to vibrate, which the brain interprets as sound.

It's important to remember that air molecules do not actually travel from the noise source to the ear. Each individual molecule only moves a small distance as it vibrates, which causes the adjacent molecules to vibrate in a rippling effect all the way to the ear.

longitudinal wave
Fig 1 - Longitudinal sound wave

Remember that longitudinal waves, for example, sound waves, are not transverse waves. Most waves are actually transverse, including light and the ripples we see on water.

Longitudinal Wave Simulator


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Use random colors   Show displacement from equilibrium

In transverse waves the vibrations are at 90 degrees to the direction of the wave. In contrast, longitudinal waves have vibrations along the same axis as the direction in which the wave is travelling. Think of the way a slinky behaves if two people are holding each end and one person quickly sends a number of vibrations down it. Compression (squeezing-high pressure) and rarefaction (spreading-low pressure) occurs in the slinky until no more energy is left.

transverse wave
Fig 2 - Transverse wave

All waves carry energy, without transferring matter.

Waveforms

A waveform is a graphical representation of a sound wave (or voltage) as it moves through a medium over time.  Remember not to get confused by an oscilloscope which displays a transverse wave when displaying sounds.  The real sound is longitudinal.

Any waveform has several fundamental characteristics; wavelength, amplitude, frequency and velocity.

waveform
Fig 3 - Primary characteristics of a waveform

Wavelength

The wavelength of a wave is the length in metres from the start to the end of one full cycle of the waveform e.g. from crest to crest.

Amplitude

The amplitude is the maximum displacement of a periodic wave from the middle line to the peak, NOT from peak to peak. The greater the distance from the center line of a wave form, the more intense the pressure variation will be within a medium, hence the louder it is perceived.

Amplitude is measured in two ways:

  1. Zero to peak value which measures the maximum positive or negative signal level.
  2. Root-mean-square (RMS) value which measures a more meaningful average level, like that at which humans hear.

Frequency

Frequency is how many complete waves there are per second passing a certain point. In other words the frequency essentially indicates the rate of pressure variations or cycles per second of a wave. Frequency is measured in Hertz.

The frequency of a sound determines the pitch (the sensation of how low or how high a sound is).

  • Lower frequency sound waves have longer wavelengths and a lower pitch.
  • Higher frequency sound waves have shorter wavelengths and a higher pitch.

The frequency range in which humans can hear is 20Hz to 20,000Hz and is called the audible range or audio spectrum.

Velocity

The velocity is the speed and direction of a sound wave. Sound waves travel at different speeds through various mediums. Through air, sound travels at 344 metres per second. Generally speaking, the denser the medium the faster sound travels through it.

To find the velocity of a wave the following equation is used:

Velocity (V) = Frequency (f) x Wavelength (λ)

Remember not to confuse a waveform, which displays a transverse wave, with the true nature of sound. A real sound-wave is longitudinal. The waveform which represents the longitudinal wave is usually shown in audio diagrams as a transverse wave to help us better visualize what is actually going on.