High-speed videos of the lips

Movement and its aerodynamic consequences

Lip movements were filmed at 200 frames per second using a special high-speed video system.
Here we show examples of the simple VCV sequences /apa/, /upu/ and /ifi/
(extracted from longer repetitive sequences like /apapapapa/).
The videos are set to run (without sound) at 5 frames/s, i.e. 1/40th of the original speed.
An additional link plays the audio at normal speed.
The following notes refer to the millisecond timer at the top of the image.

1. /apa/

After lip closure is achieved (at around 14345ms), the lips continue to move, i.e tissue is compressed.
Thus when the lower-lip starts to move down (14425ms) the lips do not immediately open, but rather start to decompress.
This means that by the time the upper and lower-lip part (14475ms) the lips are moving at close to their maximum velocity.
This in turn means that the lip-opening area - which is the aerodynamically relevant feature of the movement -
changes from zero to a large proportion of its maximum area within the space of just a couple of frames (i.e about 15ms).

These remarks may seem so obvious that one could have based them on introspection, rather than on an expensive video system.
However, this simple example may help to reveal a fundamental point about the efficiency of speech communication:
Since the speech organs like the lips are flexible rather than rigid, smooth movement (i.e with low accelerations, and thus low force input)
can nonetheless result in abrupt modulations of the aerodynamics and the resulting acoustics.
Clear modulations such as these probably have particularly high signalling value for the listener.

2. /upu/

This sequence makes the same point as for /apa/.
Here the pattern of a sudden change from zero lip opening to maximum area within a couple of frames (17740 to 17750ms)
is actually even clearer:
Once again the underlying movement is smooth and continuous.

3. /ifi/

By way of contrast, with a fricative as consonant a different movement pattern results.
As the fricative requires a precise constriction it cannot take advantage of compression of the lip tissue to permit continuous movement through the consonant,
so there is a period from about 40410ms to 40480ms where movement more or less ceases.
This illustrates why fricatives are often considered more difficult to articulate.