The police (and other emergency services) use sirens as a warning to let motorists know they are coming.<\/p>\n
Modern sirens oscillate (i.e. the pitch goes up and down), usually in a band that sits between 1-3kHz (this is the pitch of the siren), at a volume of around 100-110dB. Our ears are the most sensitive at around 2kHz, which means that less power is required to create the perception of a higher volume.<\/p>\n
To give some perspective, 1kHz is just over an octave higher than middle A on a piano, and 3kHz is around 1.5 octaves higher than that. 110dB is the threshold of pain and is 16 times louder than a vacuum cleaner – check out some sound comparisons here<\/a>. This is one of the reasons why a lot of motorbike traffic officers wear ear plugs (along with the fact that their bikes are loud, too); even the volume in police cars is loud with the windows up<\/a>. Long term exposure to loud sounds causes hearing damage.<\/p>\n You can hear an example siren in the video below. It’s a dual siren – two oscillators.<\/p>\n