A resonator is a device or system that exhibits resonance or resonant behavior, that is, it naturally oscillates at some frequencies, called its resonant frequencies, with greater amplitude than at others. The oscillations in a resonator can be either electromagnetic or mechanical (including acoustic). Resonators are used to either generate waves of specific frequencies or to select specific frequencies from a signal. Musical instruments use acoustic resonators that produce sound waves of specific tones.

A cavity resonator, usually used in reference to electromagnetic resonators, is one in which waves exist in a hollow space inside the device. Acoustic cavity resonators, in which sound is produced by air vibrating in a cavity with one opening, are known as Helmholtz resonators.

A physical system can have as many resonant frequencies as it has degrees of freedom; each degree of freedom can vibrate as a harmonic oscillator. Systems with one degree of freedom, such as a mass on a spring, pendulums, balance wheels, and LC tuned circuits have one resonant frequency. Systems with two degrees of freedom, such as coupled pendulums and resonant transformers can have two resonant frequencies. As the number of coupled harmonic oscillators grows, the time it takes to transfer energy from one to the next becomes significant. The vibrations in them begin to travel through the coupled harmonic oscillators in waves, from one oscillator to the next.

William Rory Gallagher,  /ˈrɔriː ˈɡæləhər/ GAL-ə-hər; 2 March 1948  – 14 June 1995, was an Irish blues-rock multi-instrumentalist, songwriter, and bandleader. Born in Ballyshannon, County Donegal, Ireland, and raised in Cork, Gallagher recorded solo albums throughout the 1970s and 1980s, after forming the band Taste during the late 1960s. A talented guitarist known for his charismatic performances and dedication to his craft, Gallagher's albums have sold in excess of 30 million copies worldwide. Gallagher received a liver transplant in 1995, but died of complications later that year in London, England at the age of 47.

Gallagher was born in Ballyshannon, Co. Donegal; his father, Daniel, was employed by the Irish Electricity Supply Board, who were constructing a hydro-electric power plant on the Erne River above the town. The family moved, first to Derry City, where his younger brother Dónal was born in 1949, and then to Cork, where the two brothers were raised, and where Rory attended the North Monastery School. Their father had played the accordion and sang with the Tir Chonaill Ceile Band whilst in Donegal; their mother Monica was a singer and acted with the Abbey Players in Ballyshannon. The Theatre in Ballyshannon where Monica once acted is now called the Rory Gallagher Theatre.