Models 55 and 56 are electric horns that produce sound by means of a diaphragm, which resonates a column of air. With greater output than a vibrating horn, the resonating horn can be used wherever a louder, more powerful alert is needed. These projectors are designed to match the frequency of the vibrating diaphragm to produce a smooth tone. The housings are tapped for 1/2 in conduit. Models 55 and 56 have sufficient output to penetrate through high ambient noise levels and have a duty cycle of 5 min ON, 5 min OFF.