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Squillo is a technical term for 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. The purpose of the squillo is to enable 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 easily recognizable by a certain piercing, ringing quality in the basic 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 maintain certain lyric qualities e.g. limpid high notes, and consistency of tone throughout the range etc. even in dramatic singing. Voices with squillo are also easier to record.
Uses of the squillo includes:
- projecting a small timbre e.g. Alfredo Kraus, Juan Diego Flórez
- underscoring a dramatically important passage e.g. No, non voglio morir in Sola, perduta abbandonata from Puccini's Manon Lescaut
- singing through a thickly textured orchestration, e.g. the final bars of Libera me from Verdi's Requiem, in which a soprano has to compete against a tutti orchestra and full chorus
- supporting a pianissimo note floated over an orchestra (which also demands a secure breath control) e.g. Montserrat Caballé
- supporting a long trill
- simulating a scream without compromising the timbre, especially in a verismic opera; however it is not unheard that a bona fide scream be used in operatic setting, e.g. Tosca's final jump in Tosca
- giving an impression of 'youth' to an aged voice, mainly via a cultivation of the head register, ref. Section IX Meine Gesangskunst, by Lilli Lehmann; best exemplified vocally by Mirella Freni
Famous singers who personify this technique include Mariella Devia, Renata Tebaldi, Giuseppe di Stefano, Jussi Björling and Luciano Pavarotti. Certain dramatic singers may also employ squillo as opposed to volume over the course of a performance, for example Birgit Nilsson.
Squillo may also refer, in current Italian:
- when the grammatical gender is masculine (uno squillo) word for "ring" (as in "telephone ring"), currently a slang term for missed call
- to form a verb a word "squillare" which means "to ring" or "to blare" 
- Starker, Leonard Bonn. "From physics to music: an analysis of the role of overtones in the improvement of choral tone". dspace.nmmu.ac.za. Retrieved 9 July 2014.
- Carter, Brian Barker. "An Acoustic Comparison of Voice Use in Solo and Choral Singing in Undergraduate and Graduate Student Singers". The University of Texas at Austin. ProQuest. Retrieved 9 July 2014.
- Frisell, Anthony (2007). The Baritone Voice: A Personal Guide to Acquiring a Superior Singing Technique. Branden Books. p. 132. ISBN 0828321817. Retrieved 9 July 2014.
- Jamie Squillare. "Interpretation of the family name Squillare". Scribd.com. Retrieved 23 July 2014.