Squillo is the resonant, trumpet-like sound in the voices of opera singers. It is also commonly called "singer's formant", "ring", "ping", "core", and other terms. Squillo enables an essentially lyric tone to be heard over thick orchestrations, e.g. in late Verdi, Puccini and Strauss operas. Achieving a proper amount of squillo in any performing context is imperative: too much and the tone veers towards the shrill; too little and the purpose of the squillo cannot be achieved.
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. Voices with squillo are also easier to record.
Uses of the squillo include:
Singers known for their mastery of this technique have included Maria Callas, Kirsten Flagstad, Renata Tebaldi, Giuseppe di Stefano, Jussi Björling and Luciano Pavarotti. Some dramatic singers may also employ squillo rather than volume in the course of a performance, for example Birgit Nilsson.