This article needs additional citations for verification. Please help improve this article by adding citations to reliable sources. Unsourced material may be challenged and removed. (November 2010)

This article's factual accuracy is disputed. Please help to ensure that disputed statements are reliably sourced. See the relevant discussion on the talk page. (November 2010)

Squillo is a technical term attached to the resonant, trumpet-like sound in the voice of opera singers. 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 is imperative: too much and the tone veers towards the sharp; too little and the purpose of the squillo cannot be achieved.

Training the squillo involves increasing the intensity of the higher formants of the voice without altering the fundamental i.e. without altering the pitch of the note. This is done mainly by using the head resonance, which provides very little volume but ample opportunity for projection. Among methods employable to achieve this include messa di voce. 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, homogeneous registers etc. even in dramatic singing. Voices which have properly trained squillo are also easier to record.

Squillo is easily recognizable by a certain piercing quality in the basic timbre of the voice, especially in the high tessitura. The difference between a true dramatic singer and a spinto singer - apart from capability to sustain a large volume over the course of an entire opera - is the spinto voice itself may not be produced very loudly, but due to this squillo it is able to be heard over a full orchestra. It is a known fact that squillo can only be achieved after a voice is fully warmed up, which is why most demanding passages in an opera tend to appear later in each acts, e.g. a forte note over a chorus. However some famous arias in which squillo may be used to an advantage may appear comparatively early, e.g. Laggiù nel Soledad from La fanciulla del West.

The sonic effects of squillo includes:

• a sung high note sounds higher than it is - this is attributed to the singer's formant phenomenon.
• an impression that the singer's voice is coasting over the waves of sound produced by the orchestra - an effect best experienced in a live un-amplified performance.

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 breathing technique) 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 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.

In reviews squillo is usually referred to as laser, ping, shimmer, siren, spear (as in pointed spear with a thick core behind it, a contemporary description of Franco Corelli's voice), and steel.

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
• when feminine (una squillo) a slang term for prostitute