The presence of strongly emphasized s, sh, ch, z, j sounds in speech called sibilants. These CONSONANTs are created by air moving through the vocal tract and being constricted by the position of the tongue and lips.
The SPECTRUM of sibilants is that of NARROW BAND NOISE in the high frequency range (5-10 kHz) and therefore their perception is the first to be affected by hearing loss with age (PRESBYCUSIS). Sibilants may be unvoiced (i.e. without pitch) or voiced (i.e. with an added vibration of the vocal cords).
Compare: HISS, TREBLE, VOWEL, WHITE NOISE.