What it is
The steepness of the envelope rise at the instant sound starts (from silence to tone). How the glottis coordinates its closure in that instant decides the character of the attack.
Where it comes from
Measure the slope of the waveform envelope’s rising edge, then classify it together with the spectral features of the onset instant.
How to read it
Too steep = hard onset (glottal attack — the folds clamp shut then blow apart, harming the voice); too gentle = breathy onset (air leaks first, vibration follows); the ideal is the balanced onset, where breath and closure arrive together. The Onset Detection chart tags every onset with a three-color label.