the plunger pushes the plunger back, causing it to strike the other bar, creating a two-tone sound ("ding-dong"). If a second doorbell button is used, it might be wired to a second solenoid, which strikes only one of the bars, to create a single-tone sound ("ding"). Alternatively, the second button might feed the single solenoid via an oscillating switch (often a mercury tilt switch), to give a "warbling" sound ("ding-dong-ding-dong-ding-dong"). The Edwards Sylvan C-26 had both additional features, suiting three doors. Some chimes have tubular bells instead of bars. More elaborate doorbell chimes play a short musical tune, such as Westminster Quarters. Doorbells for hearing-impaired people use visual signaling devices — typically light bulbs — rather than audible signaling devices. Fully battery-powered wired models are also common, either using a two-bar design or an electric bell. These do not consume standby power, but require the user to change the batteries, which are usually large primary cells located in the bell box. Wireless doorbells In recent decades, wireless doorbells have become popular, to avoid the expense of running wires through the building walls. The doorbell button contains a built-in radio transmitter powered by a battery. When the button is pushed, the transmitter sends a radio signal to the receiver unit, which is plugged into a wall outlet inside the building. When the radio signal is detected by the receiver, it activates a sound chip that plays the sound of gongs through a loudspeaker—either a two-note "ding-dong" sound, or a longer chime sequence such as Westminster Quarters. Frequencies in the 2.4 GHz ISM band are usually used. To avoid interference by nearby wireless doorbells on the same radio frequency, the units can usually be set by the owner to different radio channels. In larger metropolitan cities, a trend has developed over the past decade that uses telephone technology to wirelessly signal doorbells, as well as to answer the doors and remotely release electric strikes. In many cities throughout the world, this is the predominant form of doorbell signalling. Musical and continuous power doorbells As with wireless doorbells, musical doorbells have also become more common. Musical and continuous power doorbells serve as an attempt to bridge the gap between newer digital circuitry and older doorbell wiring schemes. A major difference between the standard setup of a wired doorbell and a musical doorbell is that the musical doorbell must maintain power after the doorbell