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The second acoustic correlate of the labial flap is the rise of the first three formant
frequencies—F1, F2, and F3—during the transition into the following vowel. These rising
transitions are typical of labial articulations in general (Lieberman & Blumstein, p. 225).
Thus, this property can be employed to distinguish labial flaps from alveolar taps and
flaps. In some cases, F3 appears to be flat with an abrupt transition. Figure 8.1 shows an
example of this case. However, Figure 8.3 shows a spectrogram of the word =MAA =MAA =MAA =MAA
‘catfish’ in which the rise in the transition of F3 is visible.
Third, during the articulation of the labial flap, the transition of the formants into
the following vowel is very brief, as noted by Thelwall (1980: 81). This transition has an
average of 19 ms (SD=4.8, n=25) in Mono. This is slightly shorter than for stop
consonants, which typically have a formant transition duration of 20-40 ms (Lieberman
& Blumstein, p. 224). This property is particularly useful for distinguishing the flap from
semi-vowels, which must have a formant transition duration of at least 40 ms (p. 226).
Figure 8.4 shows a spectrogram of the word =M= =M= ‘road’. The duration of the formant
transitions at the release of the M, about 40 ms, are indicated by the arrows. Even when
the closure of the labial flap is not evident, such as in Figure 8.2, this rapid formant
transition of the labial flap is still present, and so still distinguishes the flap from a semi-
vowel.
A sound can also be distinguished from other sounds by the lack of certain
acoustic properties. During the production of a labial flap, there is no aperiodic noise
such as is characteristic of fricatives, nor is there a burst such as is characteristic of stops.
When a labial flap occurs in word-initial position, it is preceded by a short period
of voicing. This indicates that the onset of the sound may be important in its perception.
Figure 8.5 shows a spectrogram of the word M M M M ‘calf, shin’. The period of voicing
preceding the labial flap is indicated by the arrows. F2 is visible during this period,
dropping from a value of about 1140 Hz to 780 Hz immediately before closure.
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