Fire alarm systems audible appliances ratings are usually stated as a sound pressure level (SPL)...

Fire alarm systems audible appliances ratings are usually stated as a sound pressure level (SPL) at 10 ft (3.05 m) from the appliance. The SPL is measured in decibels (dB) and indicates the loudness of the sound produced by the appliance. The SPL at 10 ft (3.05 m) is used as a standard reference point for comparing different audible appliances and for designing fire alarm systems to meet the required audibility levels.References:

NFPA 72: National Fire Alarm and Signaling Code, 2019 Edition, Section 18.4.3.11

Fire Protection Handbook, 20th Edition, Volume 1, Chapter 7, Section 7.8.2.12