Since the introduction of the disc siren in North America, the principal change has been in the volume of sound output. We have measured the siren on a 1912 vintage vehicle at 88 to 96 dBA at 3½ meters. By 1960 siren intensity had risen to 102 dBA at 5 meters. In recent years a new type of yelping siren has been introduced for emergency vehicles, measuring 114 dBA at the same distance. The United States is now manufacturing a yelping siren for police car use which measures 122 dBA at 3½ meters.

Schafer, Murray R, The Soundscape- Our Sonic E…, loc. 561