Sound is due to pressure variations travelling through the air from the source (for example an aircraft) to the receiver (for example a human ear). The pressure variations are caused by air molecules vibrating back and forth which cause a sound wave to travel through the air in the direction of the vibration.

Sound is measured on a logarithmic scale using decibels (dB).

When sound is measured by acoustic equipment a correction factor is applied to reflect the sensitivity of the human ear. This factor is referred to as being A-weight corrected and is indicated by the letter ‘A’ next to the unit measure ‘dB’. Therefore noise levels are usually shown in dB(A) units.

The sound level of typical daytime urban-based activities can vary between 40dB and 85dB. The sounds in a nightclub often exceed 90dB.

The typical aircraft noise levels measured by our noise monitors are between 65dB(A) and 95dB(A).

The frequency of a sound is what gives it a distinctive pitch or tone. The rumble of distant thunder has a low frequency, while a whistle has a high frequency. The ear is more sensitive to high frequency noise events than low frequency ones.

Most environmental sounds contain a broad range of frequencies. However, it is middle to high frequency sounds that tend to annoy most people.

Sound waves travel out equally in all directions from their source. This is similar to the way ripples travel when a rock is thrown into a calm pond.

As soundwaves travel away from a source they become less intense as the energy is spread out over an ever increasing area and absorbed by the atmosphere.

Higher frequencies are absorbed at shorter distances, while lower frequencies can travel further before they are absorbed.

As a result an aircraft can sound different depending on how far away it is flying. For example a distant jet aircraft is often heard as a low frequency rumble.