The science of noise and
noise terminology
The science of noise
Noise is usually defined as unwanted sound so the question
really is what is Sound? Sound may be defined as any pressure
variation (in air, water or other medium) that the human ear can
detect. The most familiar instrument for measuring pressure
variations in air is the barometer. However, the pressure
variations which occur with changing weather conditions are much
too slow for the human ear to detect - and hence do not meet out
definition of sound. But, if variations in atmospheric pressure
occur more rapidly - at least 20 times a second - they can be heard
and hence are called sound. (A barometer cannot respond quickly
enough and therefore cannot be used to measure sound).
The number of pressure variations per second is called the
frequency of the sound, and it measured in Hertz (Hz). The
frequency of a sound produces its distinctive tone. Thus, the
rumble of distant thunder has a low frequency, while a whistle has
a high frequency. The normal range of hearing for a healthy young
person extends from approximately 20 Hz up to 20 000 Hz (or 20 kHz)
while the range from the lowest to highest note of a piano is 27,5
Hz to 4186 Hz.
These pressure variations travel through any elastic medium
(such as air) from the source of the sound to the listener’s ears.
You probably already have some idea of the speed of sound for the
familiar rule for determining how far away a thunder storm is:
count 3 seconds per kilometre from the time you see the lightning
until you hear the thunder. This time interval corresponds to a
speed of sound in air of 1238 km/hour. For acoustic and sound
measurement purposes, this speed is expressed as 344 meters per
second at room temperature.
Noise terminology
Frequency
The rate of repetition of a sound wave. The subjective
equivalent in music is pitch. The unit of frequency is the Hertz
(Hz), one hertz is equivalent to one wave per second. A thousand
hertz is often denoted kHz, e.g. 2 kHz = 2000 Hz. Human hearing
ranges approximately from 20 Hz to 20 kHz. For design purposes, the
octave bands between 63 Hz to 8 kHz are generally used. The most
commonly used frequency bands are octave bands, in which the mid
frequency of each band is twice that of the band below it. For more
detailed analysis, each octave band may be split into three
one-third octave bands or in some cases, narrow frequency
bands.
The Decibel dB
A sound's intensity, or loudness or volume is measured in bels,
named after Alexander Graham Bell. A bel is a very large
measurement so the loudness scale is shown in decibels, (dB,
one-tenth of a bell) The ratio of sound pressures which we can hear
is a ratio of 106 (one million : one). For convenience,
therefore, a logarithmic measurement scale is used. The resulting
parameter is called the ‘sound pressure level’ (Lp) and
the associated measurement unit is the decibel (dB). As the decibel
is a logarithmic ratio, the laws of logarithmic addition and
subtraction apply.
Equivalent Continuous Sound Level
Another index for assessment for overall noise exposure is the
equivalent continuous sound level, Leq. This is a
notional steady level which would, over a given period of time,
deliver the same sound energy as the actual time-varying sound over
the same period. Hence fluctuating levels can be described in terms
of a single figure level.
Statistical Noise Levels
For levels of noise that vary widely with time, for example road
traffic noise, it is necessary to employ an index which allows for
this variations. The Leq, the equivalent continuous
sound level, has been adopted in this country for the assessment of
road traffic noise. The L90, the level exceeded for
ninety per cent of the time, has been adopted to represent the
background noise level. A-weighted statistical noise levels are
denoted LA10, dBLA90 and such like. The
reference time period (T) is normally included, for example.
dBLA10,5min or dBlA90,sh.
See also: What is a statutory
nuisance.