[1] Squillo enables an essentially lyric tone to be heard over thick orchestrations (e.g., in late Verdi, Puccini and Strauss operas).
[2] Squillo is recognizable by a distinctive brilliant, ringing quality in the timbre of the voice.
This perception is caused by the presence of a peak in the 2–5 kHz frequency range, to which the human ear is particularly sensitive.
The amplification of these particular harmonics is believed to be a result of a narrowing of the aryepiglottic fold just above the larynx.
Voices with naturally acquired squillo (i.e., having naturally strong higher formants) are especially prized in opera because they allow a singer to sustain lyric qualities such as limpid high notes and consistency of tone throughout the vocal range, even in dramatic singing.