The weak fricatives are f v T D; f and v are labio-dental while T and D are dental. Just as with the strong fricatives, the first member of each pair is voiceless, while the second is voiced.
The observations made about voicing for the strong fricatives also apply to the weak fricatives; phonemically voiced fricatives may lack obvious voicing during part or all of their duration, while phonemically voiceless fricatives may show some voicing, particularly in intervocalic contexts.
The weak fricatives are among the most difficult phonemes to recognize. Sometimes their energy is so low that they do not even show up on the spectrogram. However, succeeding voiced phonemes do show the influence of the weak fricatives via coarticulation effects.
The phoneme f has the most energy of the weak fricatives. It can show up on the spectrogram as a triangular region of frication, as a double triangular region, as a plosive-like burst, or as a sporadic collection of spots. In most cases there is a strong locus at or around 1200 Hz.
The phoneme v has several variants: one with strong frication in the region above 4000 Hz, sometimes with a triangular pattern like the f; another with burst-like frication, and a third with no frication whatsoever. Like f, v has a locus at about 1200 Hz. Intervocalically, v may be indicated more by the V-shaped drop and then rise of energy levels than by any marked frication.
The phoneme T can also take two basic forms: a burst-like form more common at syllable-initial position, and a fricated form which is typical of syllable-final forms. It is often accompanied by low-frequency energy. The dental loci are at or around 1500 Hz and 2500 Hz, so they are easily confused with the alveolar plosives as well as the labiodentals. However, frication is usually concentrated above 3000 Hz.
The phoneme D has the weakest pattern and energy of all the fricatives; it can almost disappear in rapid speech, and unfortunately it occurs in many of the most common function words in English (the, that, then, there, their, etc.). When clearly observable, it can contain voicing as well as formants at 1500 and 2500 Hz, as well as some higher-frequency energy.
See Figures 3, 4, 5, and 6 for examples of the weak fricatives.
The English language is rich in fricatives; they can occur in sequences such as in the words ``sphere'', ``slothful'', and ``Seth's'' - in these cases they are even more difficult to recognize than in single occurrences.
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