Gracey & Associates Hire and Calibration
Sound and Vibration Basics
Background Noise
Background Noise is the noise level at a given location and time, measured in the absence of any alleged noise nuisance or sound sources being studied. It is also referred to as the ambient or residual noise
Background noise levels may be very low, for example in a studio, a library or in the countryside, late at night. It follows therefore that the measurement may call for a very sensitive meter whose internal noise levels are ideally 10 dB lower than the background noise to be measured **.
It is also common practice to measure the background noise, using a sound level meter with a statistical noise level measurement capability. Statistically the LA90 value is often used to describe background noise levels and is defined as the level exceeded for 90% of the measured time.
In other words the background noise level is lower than the LA90 level for 90% of the time. This is sometimes written as L90 = 20 dBA for example.
The A-weighted level is commonly used but it is also 'helpful' to measure the background in octave bands or 1/3-octave bands and calculate the L90 for each frequency band.
We hire a range of meters for recording background noise levels - see also our Environmental Noise section or our quick Sound Level Meter list.
It is also common practice to measure the background noise, using a sound level meter with a statistical noise level measurement capability. Statistically the LA90 value is often used to describe background noise levels and is defined as the level exceeded for 90% of the measured time.
In other words the background noise level is lower than the LA90 level for 90% of the time. This is sometimes written as L90 = 20 dBA for example.
The A-weighted level is commonly used but it is also 'helpful' to measure the background in octave bands or 1/3-octave bands and calculate the L90 for each frequency band.
We hire a range of meters for recording background noise levels - see also our Environmental Noise section or our quick Sound Level Meter list.
** Very low background noise measurements are limited by the internal 'electronic' noise of the meter being used. Manufacturer's normally specify the minimum dBA level as the instrument noise. If this level is 25dBA and you try to measure a background level in a rural area of 25 dBA the meter will show 28 dBA - the addition of the internal level plus the measured level.
As a rule of thumb the difference between the 'meter noise' and the measured noise should be at least 5 dB and ideally greater than 10 dB. This is a grey area, so please feel free to discuss it with our engineers. See also Measuring Extremely Low Levels
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Background Noise Keywords: background noise level, ambient noise, residual noise