Reverberation decay is measured from the third signal peak to the noise floor.

Evidence

FIG. 7. Comparison of waveforms and decay curves for the shortest (site 2, summer) and longest (site 3, summer) values of T30. (Top) Normalized pressure squared (Pa2) reverberation waveforms for site 2, summer (gray) and site 3, summer (black). Waveforms are excerpted from the full recording, beginning at the third peak and ending when the signal reaches the noise floor. This excerpt represents the reverberation portion of the signal. — Donald G. Albert, Michelle E. Swearingen, Frank E. Perron, and David L. Carbee, Low Frequency Acoustic Pulse…, 94

Categorization

• XREF: Methodological detail for anyone reading acoustics papers or replicating T30 measurements in room-acoustics work.