Fire safety

Alarm Sound Level Calculator

Sound falls 6 dB every time distance doubles - a 100 dB sounder is only 82 dB eight metres away, before a single closed door.

Calculator
Target

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How it works

Free-field level = rated dB(A) at 1 m minus 20 x log10(distance). Compare with the target: BS 5839-1 wants 65 dB(A) generally, or 75 dB(A) at the bedhead to wake sleepers - and a closed door costs roughly 20-30 dB.

Frequently asked questions

What sound level must a fire alarm reach?

BS 5839-1: 65 dB(A) in general areas (or 5 dB above a persistent background), and 75 dB(A) at the bedhead in sleeping risk - measured with doors shut.

Why does my loud sounder fail in bedrooms?

Distance and doors. 100 dB at 1 m is ~82 dB at 8 m, and a closed fire door can take off another 25 dB - hence sounders or bases in the bedrooms themselves.

Related free tools

Smoke & Heat Detector Coverage Floor area to the number of detectors (BS 5839-1). Detector Spacing Check Test detector grid spacing against BS 5839-1 limits.