This problem is usually caused by the actual vocal formant, but can also be exaggerated by microphone placement and technique. Sibilance is often centered between 5kHz to 8kHz, but can occur well above that frequency range. Vocal sibilance is an unpleasant tonal harshness that can happen during consonant syllables (like S, T, and Z), caused by disproportionate audio dynamics in upper midrange frequencies.
