The Federal Signal Models 55 and 56 are electric horns that produce sound by means of a diaphragm, which resonates a column of air. Because the output is greater than that of a vibrating horn resonating horns can be used wherever a louder, more powerful signal is required. Both models feature corrosion-resistant cast aluminum housings with a brass projector. These projectors are designed to match the frequency of the vibrating diaphragm to produce a smooth musical tone.