Assessment and Rating

Environmental noise assessment compares the noise from a source against the existing background, with corrections for character. Two sites at the same measured level can produce completely different outcomes.

Key terms

• Background sound level — the underlying level in the absence of the source, typically expressed as an exceeded-for-90-percent-of-the-time value.
• Specific sound level — the level from the source under assessment alone.
• Rating level — the specific level plus corrections for acoustic character.

Character corrections

Corrections are added for features that make noise more noticeable and more annoying at the same level:

• Tonal — a distinguishable hum or whine, such as a fan or transformer.
• Impulsive — bangs, clatter, reversing alarms, dropping skips.
• Intermittent — starting and stopping, which draws attention repeatedly.
• Other distinctive characteristics — music, speech, or anything readily identifiable as coming from your site.

Why the comparison matters

The greater the rating level exceeds the background, the greater the likely adverse impact. A site in a quiet rural location faces a far lower permissible level than an identical site beside a motorway, and night-time backgrounds are much lower than daytime ones — which is why a plant that is unremarkable during the day generates complaints at 3 a.m.