The strong fricatives are s z S Z; s and z are alveolar and S and Z are palato-alveolar. The primary difference between the two alveolars, as between the two palato-alveolars, is level of voicing. The weak fricatives show more differences between voiceless and voiced cognates, as discussed below.
Voicing may not be evident throughout an entire phoneme perceived as a voiced fricative. Voicing in fricatives will be present in an intervocalic environment whether or not the phoneme is voiced, because of the influence of the surrounding vowels. On the other hand, when completely surrounded by voiceless segments, voiced fricatives may become entirely devoiced. When this happens, rely on length information.
Often in a voiced context, you may see ``left over'' voicing at the beginning of a phonemically voiceless fricative. This generally will not continue throughout the entire segment. Thus, a partially voiced (but phonemically voiceless) fricative can be distinguished from the partially devoiced (but phonemically voiced) fricative by whether or not the end of the segment is voiced. Generally even if a voiced fricative is partially devoiced, the voicing will pick back up at the end of the segment when followed by a vowel, whereas a voiceless fricative will continue to be voiceless once it has begun in that state, even when a vowel follows. This is a general tendency only.
The alveolar fricatives have energy focused higher than the palato-alveolars, usually not extending below 4000 Hz, although sometimes the alveolars do extend somewhat lower but with a strong energy rise around 4000 Hz. High energy for S and Z reach lower frequency levels, down to 2500 Hz.
Voiced fricatives tend to be shorter than voiceless fricatives in continuous speech.
The alveolar phonemes s and z will have maximum energy above 4000 Hz. The phoneme z will sometimes show periodicity above 4000 Hz; it will also have a periodic F1 area when it is truly voiced. The phoneme s will rarely show any glottal pulses either in the area of F1 or above 4000 Hz.
For the phonemes S and Z, the energy is centered slightly lower than for s and z, and strong energy will usually begin at around 2500 Hz. The phoneme Z will have a periodic F1 if it is truly voiced; sometimes it will show periodicity in the frication itself.
See Figures 1 and 2 for examples of the strong fricatives with their characteristic rectangular shape. The phonemes s and S show the highest energy level of all of the fricatives, but all the strong fricatives are relatively easy to recognize.