How noise is created

Defining noise and sound and factors that affect noise levels

Noise is often defined as ‘unwanted sound’. Put another way, sound only becomes noise when it exists in the wrong place or at the wrong time – and so causes annoyance, sleep disturbance or other effects. 

How sound is produced
Sound is produced when a rapidly vibrating object causes the air around it to vibrate. As a result, waves of rapidly changing air pressure travel away from the object.

These are measured in terms of:

  • Frequency – ie how fast the sound source is vibrating – using units of hertz (Hz), which describe the number of vibrations per second, and
  • Amplitude - the amount by which the air pressure changes. The greater the amplitude, the louder the sound.

Factors that affect noise levels

  • Distance - noise levels decrease the further you are from the sound source. That is one reason why planes in mid-flight are less of a noise issue than planes at take-off or landing.
  • Barriers -  noise screens, buildings and natural features like hills and grass can help block and absorb the sound to stop it from spreading over a wide area. On the other hand, large hard surfaces (e.g. buildings, paved areas and even lakes) can reflect sound, helping it to travel further.
  • Meteorological factors - wind speed and direction, and even the temperature, can affect how sound is carried and so affect how loud a noise is and how far its impact reaches.

For additional information, you may wish to find out how aircraft create noise.