Voiceless Plosives

Overview of The Plosives

The plosives are unique among phoneme categories in English in that they involve three distinct phases which are sequential in time, in addition to coarticulation effects on preceding and following segments. We shall see, however, that not all phases occur in all allophonic realizations of the plosives.

The three phases are the following:

1. Silence - The articulators totally block the airstream. There may be some underlying voicing activity during part of this phase. The air pressure increases just behind the obstruction.
2. Plosion - The articulators quickly move away from each other. An explosive burst of air rushes through the opening, involving energy in most or all of the audible spectrum.
3. Aspiration - The articulators are now further apart, and the air pressure at the site of the obstruction has fallen so that the speech sound is no longer a burst with energy in all frequencies, but bands of aspiration which are more narrowly concentrated and which move toward the formant values in the next phoneme.

This multiple personality of the plosives is responsible for the two different names which are traditionally used for this phoneme category: the term stop refers to the first or silence phase, while the term plosive refers to the second or explosive phase. These two phases will also be called closure and release in what follows; this terminology conforms to Worldbet notation in which each plosive is divided into a silent phase and a explosive phase and a separate symbol is used for each phase.

Details of the Plosives

In English there are six plosives: three voiceless and three voiced. It happens that in English the voiceless plosives are normally aspirated, while the voiced plosives are not. Therefore, Worldbet adds the letter `h' to the symbols used for the voiceless plosives, but not for the voiced plosives. This distribution of voicing and aspiration is not universal; the classic example to the contrary is Sanskrit and its modern descendents in India where all four combinations of voiceless/voiced versus aspirated/unaspirated occur. In French and Italian there is much less aspiration in the voiceless plosives than in English. In other languages there are additional plosives at different places of articulation, such as palatal and uvular plosives. Moreover, phonemes equivalent to English plosives may be made at different places of articulation; for example, /th/ is a dental plosive rather than an alveolar plosive in many languages.

  The six English plosives can be subdivided into a voiceless-voiced pair at each of three places of articulation: bilabial, alveolar, and velar. Note that these are the same places as for the nasals in English. Also remember that each plosive has two symbols in Worldbet, so that we have a total of twelve symbols for the plosives.

1. Bilabial - Voiceless /pc/ and /ph/, voiced /bc/ and /b/. Locus: low and greatly dependent on the preceding and succeeding phonemes. Shape to look for: bilabial rounding.
2. Alveolar - Voiceless /tc/ and /th/, voiced /dc/ and /d/. Loci: 1800 and 2800 Hz. Shape to look for: alveolar flags.
3. Velar - Voiceless /kc/ and /kh/, voiced /gc/ and /g/. Loci: Depends on preceding and succeeding phonemes. Shape to look for: velar pinch or triangle where F2 and F3 come together then move apart. Burst may be multiple.

The voiceless stops are usually easier to recognize than the voiced stops, because:

1. There is little or no voicing during the silence phase, while voiced stops by definition are supposed to have some voicing even during their silence phase. This greater silence in the voiceless plosives allows the explosion to stand out more clearly.
2. There is more energy in the explosive phase for the voiceless stops. We can often miss the signs of plosion for voiced stops unless we look at low dB values.
3. There is aspiration after a voiceless stop in some allophones of the voiceless plosives, but almost never in the voiced case. This period of aspiration, if present, again sets the plosion off and allows easy recognition.

Look at Figures 3 through 5 for some good examples of voiceless plosives, along with their voiced counterparts for comparison. See Figure 6 for an example of all three voiceless plosives in different contexts within a single word. Notice the more or less clear division into the three phases of silence, plosion, and aspiration in these voiceless sounds.

Given the proper context, the three contrasting shapes which define the plosives will occur both preceding and following the plosive burst. Remember that the same shapes are also useful in distinguishing the nasals, which are formed at the same three places of articulation.

There are three allophones of the voiceless plosives:

1. The three-phase plosive. This allophone occurs primarily in syllable-initial position in stressed syllables. It includes all three of the regulation phases: silence, burst, and aspiration. We use the closure labels /pc/, /tc/ and /kc/ followed by the release labels /ph/, /th/, and /kh/.
2. The two-phase plosive. This allophone lacks the aspiration component. In voiceless plosives, it occurs after /s/ and in unstressed syllable-initial position. We still use the closure labels /pc/, /tc/, and /kc/ and the release labels /ph/, /th/, and /kh/ if we are doing phonemic labelling. If we are doing phonetic labelling, we use the unaspirated equivalents /p/, /t/, and /k/ for the release.
3. The one-phase plosive. This allophone lacks both the plosive and aspiration components. It thus includes only silence, and is called an unreleased plosive. This allophone occurs in syllable-final position, or in multi-plosive strings as discussed below. We use only the closure labels /pc/, /tc/, and /kc/. See Figure 7 for examples of unreleased voiceless plosives.

      How does the silence of /tc/ differ from that of /pc/ or /kc/? It does not; the phoneme /tc/ is in a sense a fiction, but it corresponds to the reality of coarticulation. In order to distinguish between these three sorts of silence, it is necessary to look at the voiced sound which precedes the closure. If the formants are all moving down toward lower frequencies in a rounded fashion, it is bilabial rounding. If the formants F2 and F3 are at 1800 and 2800 Hz on parallel paths, they constitute the alveolar flags. If the formants F2 and F3 are coming together in a triangular pattern, it is the velar triangle -- or velar pinch. In all three cases, the very end of the preceding voiced sound is often characterized by glottalization, that is, increased time between successive glottal pulses and exaggerated formants. See Figure 8 for an example of glottalization.

English has many examples of two or more contiguous plosives; they may occur in the same word or across word boundaries. An example phrase is talk to, in Worldbet notation /tc th A kc kh tc th u/. In fluent speech we economize on the number of articulatory gestures in order to speak faster; in this case we omit the burst for the first plosive and the silence for the second; we thus get the abbreviated phoneme string /tc th A kc th u/. This is extremely common in English. So be on the lookout for conflicting signals before and after the burst. In such cases the time interval spanned by the two plosives is longer than that normally spanned by a single plosive.

Plosives combine easily with other consonants to form consonant clusters, as in the initial clusters in the following words:

1. /s/ + plosive -- spout, stout, Scout.
2. plosive + /w/ -- tweezer, quorum, square.
3. plosive + /j/ -- pew, cue, skew.
4. [/s/] + plosive + /l/ -- play, splay, clay, sclerosis.
5. [/s/] + plosive + /9r/ -- pray, spray, tray, stray, Cray, scrape.

In cases with a preceding /s/, we have seen that the plosive will be unaspirated. In the aspirated allophones, the aspiration will be used to begin the sketch of the following glide or liquid formants rather than moving toward those of the succeeding vowel. Such approximants can be labelled as devoiced (Worldbet _0).

When a plosive is followed immediately by a fricative, particularly /s/ or /z/, the burst will tend to merge into the frication and thus will not be easily recognizable. Example words are very common in English, since /s/ or /z/ is the mark of the plural. See Figure 9 for some plurals involving voiceless plosives.