Sounds are vibrations through the air or other medium that are received and interpreted or ‘heard’ by a person or animal. Whereas ‘noise’ can be simply defined as unwanted or unpleasant sound.
Sound is measured in decibels (dB) and is represented on a non-linear (logarithmic) scale. This means that a person would have difficulty noticing a change in 1 or 2 dB while a 10 dB change in noise levels effectively reflects a doubling or halving of loudness.
Sound sources that have similar intensities will generally sound equally loud. For example, it would be difficult for a person to distinguish between the sound levels of a loud cafe (70 dB) from that of a passing car (70 dB).
The human ear is less sensitive to low audio frequencies so instrument measured sound levels are typically ‘A-weighted’ to mimic the response of the human ear to sound. This is indicated by adding (A) to the dB unit and expressed as dB(A).
The majority of aircraft noise is generated by the engines and from aerodynamic drag on the airframe and wings. The amount of noise produced by an aircraft depends on a range of factors including:

• aircraft size and weight,
• number and type of engines,
• thrust setting,
• speed, and
• altitude and distance.

Atmospheric conditions can also have a significant impact on the distance and intensity that sound can be transmitted through the air. Passenger aircraft are typically loudest on take-off when weight and thrust settings are high and during landing when thrust settings are varied and landing gear and other drag producing components are extended.