The Mayor's Ambient Noise Strategy - Greater London Authority

218 Mayor of London
The Mayor’s Ambient Noise Strategy
A2 what is noise
What is noise
‘Noise’ is typically defined as ‘unwanted sound’. The term ‘noise’
therefore combines the objective physical phenomenon ‘sound’, and its
subjective psychological effects on people. The sound in question may be
unwanted by the recipient for a wide range of reasons, varying by time of
day, place and other factors. The term ‘noise pollution’ may be used to
reinforce negative associations. Subjective responses vary widely between
individuals. Effects on individuals are harder to quantify than the physical
attributes of sound.
‘Ambient’ or ‘environmental’ noise is long term noise from transport
and industry, as distinct from noise caused by neighbours, construction
sites, other local nuisances, and noisy workplaces.
How is sound described
‘Sound’ is a form of energy, normally transmitted through the air.
Typically, a vibrating source sets up pressure variations. The vibrations set
up a pattern of alternate regions of increased and decreased pressure.
These pressure variations travel, as the energy is passed from one particle
to the next, but the particles do not. The vibration can be transmitted
through air, any other gas, water or a solid.
‘Frequency’ is the rate at which the source and the resulting pressure
waves vibrates. It is measured in ‘Hertz’ (Hz). Older publications may use
the equivalent term ‘cycles per second’ (cps). The more rapid the
vibrations, the higher the pitch or tone. ‘Wavelength’ is the distance
between successive pressure waves, during one cycle. Wavelengths of
audible sound vary from a few millimetres to several metres - the higher
the frequency, the shorter the wavelength. The human ear can detect
from around 20 Hz to 20,000 Hz (20 kHz). Humming, throbbing or
droning noises comprise mostly low frequency sounds. Whistling,
squealing and hissing noises contain mainly high frequency sounds. The
musical term ‘octave’ represents a doubling in frequency. For many
practical purposes, a restricted frequency range may be used. It may also
be divided into octave bands and one-third octave bands.
‘Amplitude’ is a measure of the energy of the sound wave and gives a
comparison of the loudness of different sounds. It is the maximum
excursion of the pressure difference of a sound wave. The particles vibrate
back and forth about a given position. The further they move the greater
the sound energy. Amplitude can be measured in audible sound pressure
variations (using units for measuring pressure, Pascals, Pa). These pressure
units are inconvenient to use, so the magnitude of a sound is generally
described by its sound pressure level (SPL). SPL is the logarithmic ratio