What is sound?

Sound is an invisible vibration. It travels in waves, spreading outwards from the source of the sound. Sounds are different both in loudness and pitch, often known as frequency.


  • Loudness is measured in decibels (dB).
  • Pitch/frequency is measured in Hertz (Hz).

All sounds are made up of different frequencies. We can describe this as the pitch of a sound. The frequency of a sound affects the pitch that it is heard at. The sound a whistle makes when it’s blown is an example of a high frequency pitch whereas the noise made when banging on a drum is an example of a low frequency pitch.

Speech consists of sounds of mixed frequencies. Consonants are generally higher in frequency than vowels. Some children’s deafness is the same across all the frequencies. For other children, their deafness occurs at different levels, depending on the frequency.



Chart showing frequency levels © IDCS

Levels of deafness
The four categories below are used to indicate the level of an individual’s hearing loss:

  • Mild deafness: 20 – 40 dB
  • Moderate deafness: 41 – 70 dB
  • Severe deafness: 71 – 95dB
  • Profound deafness: 95+ dB
School girl from Iraq © Richard Mills