Train horns are operated by compressed air, typically 125-140 psi (8.6-9.6 bar), and fed from a locomotive main air reservoir. When the engineer opens the horn valve, air flows through a supply line into the power chamber at the base of the horn "(diagram, right)". It passes through a narrow opening between a nozzle and a circular diaphragm in the power chamber, then out through the flaring horn bell. The flow of air past the diaphragm causes it to vibrate or oscillate against the nozzle, producing sound.