 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"").