<P> In most wired systems, a button on the outside next to the door, located around the height of the doorknob, activates a signaling device (usually a chime, bell, or buzzer) inside the building . Pressing the doorbell button, a single - pole, single - throw (SPST) pushbutton switch momentarily closes the doorbell circuit . One terminal of this button is wired to a terminal on a transformer . A doorbell transformer steps down the 120 or 240 - volt AC electrical power to a lower voltage, typically 10 to 20 volts . The transformer's other terminal connects to one of three terminals on the signaling device . Another terminal is connected to a wire that travels to the other terminal on the button . Some signaling devices have a third terminal, which produces a different sound . If there is another doorbell button (typically near a back door), it is connected between the transformer and the third terminal . The transformer primary winding, being energized continuously, does consume a small amount (about 1 to 2 W) of standby power constantly; systems with lighted pushbutton switches may consume a similar amount of power per switch . The tradeoff is that the wiring to the button carries only safe, low voltage isolated from earth ground . </P> <P> A common signaling device is a chime unit consisting of two flat metal bar resonators, which are struck by plungers operated by two solenoids . The flat bars are tuned to two pleasing notes . When the doorbell button is pressed, the first solenoid's plunger strikes one bar, and when the button is released, a spring on the plunger pushes the plunger up, causing it to strike the other bar, creating a two - tone sound ("ding - dong"). If a second doorbell button is used, it is wired to the other solenoid, which strikes only one of the bars, to create a single - tone ("ding") sound . </P> <P> More elaborate doorbell chimes play a short musical tune, such as Westminster Quarters . </P> <P> Doorbells for hearing - impaired people use visual signaling devices--typically light bulbs--rather than audible signaling devices . </P>

Where does the sound from doorbells come from