Explanation of Noise Terms
Noise is unwanted sound. Sound is any variation in atmospheric pressure that can
be detected by the ear. For the ear to detect pressure variations as sounds the
variations must occur 20 times per second. Noise is measured using an instrument
that is known as a sound level meter. The sound level meter displays what is known
as a sound pressure level. Although the sound is actually a variation in pressure
that the ear can detect it is simply too messy to measure the sound in terms of
pressure because the ear can actually respond to a huge range of pressure variations
and the numbers involved would become to unwieldy. To overcome this problem we use
the decibel (dB). The decibel is not an absolute unit of measurement; it is a logarithmic
ratio between a measured quantity and an agreed reference level. The use of a logarithmic
scale is actually very useful because the ear reacts to a logarithmic change in
level, which corresponds to the decibel scale where 1dB is the same relative change
everywhere on the scale.
The sound level meter is an instrument designed to give objective, reproducible
measurements of sound pressure level. There are many different sound measuring systems
available; however, basically each system consists of a microphone, a processing
unit and a display unit.
Sound is made up of different frequencies and because the ear is more sensitive
to some frequencies than to others the sound level meter can apply a weighting to
simulate the response of the ear. This is called the “A” weighting and it is commonly
used in environmental noise measurements.
Because sound varies with time it is necessary to determine suitable metrics to
quantify and compare the sound. Some of the commonly used metrics are listed below.
L10,T : The level of sound exceeded for no more than 10% of the measurement period
(T).
L90,T: The level of sound exceeded for 90% of the measurement period (T). This level
of sound can be used to define the background sound level, and is influenced by
constant sources such as industrial equipment and constant background city sounds,
eg from air handling equipment.
Leq,T: The ‘equivalent continuous noise level’, which is a parameter that calculates
a constant level of noise with the same energy content as the varying acoustic noise
signal being measured. It ay help to look at the figure below.
Lmax: This is the maximum level recorded during the period of measurement.
It is sometimes difficult to appreciate what a change of 1dB will sound like. It
may help to note that doubling or halving of the sound pressure level is equivalent
to a change of approximately 3 dB(A), while a subjective impression of a doubling
of loudness generally corresponds to a 10 dB(A) sound level increase. As noise is
assessed as a logarithmic ratio of pressure levels (i.e. decibels), it is sometimes
helpful to consider the relationship between the subjective evaluation of noise
and the actual objective levels, and examples are therefore provided in Table below.
Typical Noise Levels and Subjective Evaluation
|
Noise Level dB(A) |
Description |
| 120
|
Threshold of pain
|
| 95
|
Pneumatic drill (unsilenced); 7m distance
|
| 83
|
Heavy diesel lorry (40 km/h at 7m distance)
|
| 81
|
Modern twin-engined jet (at take-off at 152m distance)
|
| 70
|
Passenger car (60 km/h at 7m distance)
|
| 60
|
Office environment
|
| 50
|
Ordinary conversation
|
| 40
|
Library
|
| 35
|
Quiet bedroom
|
| 0
|
Threshold of hearing
|
The END and corresponding regulations also refer to other noise descriptors, namely
Lden, Lday and Lnight.
The day-evening-night level Lden in decibels (dB) is defined by the following formula:
in which:
— Lday is the A-weighted long-term average sound level as defined in ISO 1996-2:
1987, determined over all the day periods of a year,
— Levening is the A-weighted long-term average sound level as defined in ISO 1996-2:
1987, determined over all the evening periods of a year,
— Lnight is the A-weighted long-term average sound level as defined in ISO 1996-2:
1987, determined over all the night periods of a year;
the default values day, evening and night are 07.00 to 19.00, 19.00 to 23.00 and
23.00 to 07.00 respectively.