What a decibel number needs beside it

A decibel value becomes comparable only when its physical quantity, reference, frequency support, averaging interval, and calibration are known. This methods note synthesises laboratory guidance and acoustic-methods research to distinguish sound pressure level, peak measures, sound exposure, and spectral density. Hypothetical calculations explain the conventional reference difference between air and water, the effect of duration on exposure, and why averaging logarithmic levels can differ from an energy-equivalent average. The discussion also separates received levels from source levels and pressure measurements from particle motion. It does not establish current regulatory thresholds or biological equivalence between equal exposures. The practical result is a reporting minimum that prevents an apparently precise acoustic comparison from concealing different quantities, processing choices, or measurement limits.