Knowledge Centre · Understanding Noise
What is LA90?
LA90 can be thought of as the background noise level over a period of time — one of the most commonly used descriptors in environmental acoustics.
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2 minutes
Relevant to
Anyone reading a noise report or condition
What LA90 stands for
LA90 stands for: A-weighted Noise Level Exceeded for 90% of the Measurement Period.
Fortunately, it is much simpler than it sounds.
LA90 can be thought of as the background noise level — one of the most commonly used descriptors for assessing the typical ambient noise in an area.
A changing sound level, representing the background noise
Most environmental noise is constantly changing. A truck passes, then a quieter car, then perhaps a motorcycle. The noise level rises and falls throughout the measurement period.
LA90 identifies the noise level that is exceeded for 90% of the time. Because it excludes most short-term noise events, LA90 is widely used to represent the underlying background noise level of an environment.
If you measured traffic noise over one hour and recorded an LA90,1hour of 48 dB(A), this means the noise level remained above 48 dB(A) for 90% of that hour—approximately 54 minutes. It represents the typical background noise level, largely unaffected by occasional louder events such as passing trucks or motorcycles.
Where LA90 is used
LA90 is commonly used where the background or ambient noise level is important.
Typical applications include:
- Environmental noise assessments – establishing the existing background noise level before assessing a new development.
- Environmental Protection (Noise) Regulations – supporting the assessment of intrusive noise where background levels are relevant.
- Planning and development – establishing existing acoustic conditions for residential, commercial and industrial developments.
- Wind farm assessments – measuring existing background noise for comparison with predicted turbine noise.
- Community noise investigations – identifying the typical ambient noise environment experienced by nearby residents.
Related reading
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See how multiple sound levels combine logarithmically, not by ordinary addition.
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