Releasing nasals into vowels

In coming out of a nasal into a vowel, the oral closure must be released, and the velum must be raised to block airflow through the nasal cavity. These two articulations are generally simultaneous, so that when the oral closure is released, the airflow becomes oral. This produces a discontinuity in volume, which can be seen in waveforms and spectrograms as a rapid change in amplitude. Sometimes, movements of the velum can be seen in a spectrogram.

Figure 9.1 gives a schematic representation of a nasal followed by a vowel. The upper line represents the action of the velum, and the lower line represents the action of the lips; the two articulations are temporally aligned so that they co-occur. The transition in time from [m] to [a] is quite fast (a few tens of milliseconds).

Figure 9.2 gives a spectrogram and a waveform of the start of the word ‘map’, as produced by a male speaker of RP. The nasal and vowel portions are marked [m] and [a] respectively: notice the rather abrupt change in amplitude as the nasality ends.

What can also be seen is that during the spectrogram of the portion labelled [m], there are some areas of low amplitude, such as around 1600 Hz. This is because the nasal cavities absorb some of the acoustic energy, and gaps like this (called zeroes) are often evident on spectrograms. Although there are formants visible during the nasal portion, they are less distinct than in the vocalic portion; this is because the formant peaks are wider, which makes them also quieter and less visually prominent. This is particularly noticeable for F1.