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.