Voice onset time

In this figure, the top line shows the closure (single line) and opening (double line) of the lips. Each of the three other lines shows when voicing begins relative to the opening of the lips (the dashed vertical line) for three different stops, [p^h], as in English pie; [p], as in Spanish pie; and [b], as in English buy and Spanish vaya.

For voiceless plosive consonants, vocal fold vibration is stopped for a period that is a little longer than the hold phase, so there is still no vocal fold vibration around the moment of release (when the articulators part and the plosion is heard) and posibly for a further brief time afterwards. This delay, measured from the start of the explosion to the point where vocal fold vibration begins, is called voice onset time (VOT). Though so short that it is best expressed in milliseconds, it is very important for the listener.

If the VOT is longer than 30 ms, a plosive doesn't just sound voiceless. The VOT can actually be heard as a brief [h]-like segment following the explosion and the plosive is said to be aspirated.

There can be a zero VOT (where the onset of the vocal fold vibration coincides with the plosive release), positive VOT (where there is a delay in the onset of the vocal fold vibration after the plosive release) and negative VOT (where the onset of the vocal fold vibration preceeds the plosive release).