Resonance is the tendency of a mechanical or electrical system to vibrate or oscillate at a certain frequency when excited by an external source, and to keep oscillating after the source is removed. If something tends to have resonance it is said to be resonant. Resonate is the verb form – to resonate. A bell is a good example of a mechanical resonator. When exited into vibration by being struck a bell will oscillate at its many resonant frequencies and thus produce its unique sound. All mechanical structures have some resonance at some frequencies. Resonance is a particular concern with loudspeaker manufacturers because speakers, speaker enclosures, and the listening areas they are ultimately placed in all have resonances that can cause inaccuracies in sound reproduction. An example of electrical resonance would be the oscillator in a synthesizer that is used to produce sound. A good example of both electrical and mechanical resonance is feedback in a PA system.