3.7 Noise characteristics and measures

The annoyance characteristics of noise are subjective. Whether or not a noise causes annoyance mostly depends upon its reception by a person, the environment in which it is heard, the type of activity and mood of the person who hears it and how acclimatised that person is to the sound.

Sound is measured in decibels (dB). When measuring environmental noise, a weighting network is used which filters the frequency of the sound so that it better corresponds to the response of the human ear. Noise measurements made using this weighting network are expressed as dBA.

To manage the enormous range of sound pressures able to be detected by the human ear, the decibel scale is logarithmic, which often leads to confusion. For instance, if two machines emit exactly the same noise level of 80 dBA, the total noise level is not 160 dBA, but 83 dBA, a doubling of intensity which is barely noticeable from one day to the next. Also, while a 10 dBA increase in sound level, is a tenfold increase in intensity, it represents only a doubling of loudness. Typical examples illustrating the decibel scale are shown below.

Examples of dBA ratings of noise events
​els dBA​
​Threshold of pain ​140
​130 ​Jet engine
​Typical airblast limit ​120
​110 ​Production blast at 100 m
​100 ​Rock hammer at 2 m
​90 ​Dump truck at 10 m
​80
​70 ​Conveyor at 5 m
​60 ​Conversation at 2 m
​50
​Typical license condition ​40 ​Quiet living room
​30
​20 ​Night time, rural area
​10
​Threshold of hearing ​0
 

Noise emissions are measured using sound level meters which detect and record changes in sound pressure. More expensive models can also include frequency information. For surveys of background noise, environmental noise loggers are generally used. These are basically sound level meters, in robust, weatherproof cases, that can be set up and left unattended to monitor at suitable locations.

Airblast is measured in decibels, but is not weighted as for typical environmental noise, so is expressed as ‘dB(linear)’. Special equipment is normally used to measure airblast; it is designed to be left unattended and set to trigger when an emission levels exceeds a predetermined level. Waveforms of the event should also be recorded.

To describe the overall noise environment, a number of noise descriptors have been developed. These involve statistical and other analysis of the varying noise over sampling periods, typically taken as 15 minutes. The four most commonly used descriptors, which are demonstrated in the graph below, are:

  • Maximum noise level (LAmax)—The maximum noise level over a sample period is the maximum level, measured on fast response, during the sample period.
  • LA10—The LA10 level is the noise level which is exceeded for 10 per cent of the sample period. During the sample period, the noise level is below the LA10 level for 90 per cent of the time. The LA10 is a common noise descriptor for environmental noise and road traffi c noise.
  • LAeq—The equivalent continuous sound level (LAeq) is the energy average of the varying noise over the sample period and is equivalent to the level of a constant noise which contains the same energy as the varying noise environment. This measure is also a common measure of environmental noise and road traffic noise.
  • LA90—The LA90 level is the noise level which is exceeded for 90 per cent of the sample period. During the sample period, the noise level is below the LA90 level for 10 per cent of the time. This measure is commonly referred to as the ‘background noise level’.

image3-7.jpg
A noise survey sample demonstrating the main noise level measures.

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