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How a Police Siren and the Doppler Effect Work

October 02, 2019

How a Police Siren and the Doppler Effect Work

A staple of police vehicle equipment are sirens, one of the most powerful tools in a first-responder's arsenal to ensure they arrive at a scene promptly.


Tones such as yelp, wail, and air horn afford sirens the capacity to meet the sound requirements for any situation – a truly adaptable piece of police vehicle equipment.


For such a quintessential piece of police vehicle equipment, few have taken the time to understand the basic design principles behind the siren. Today’s piece will provide insight into this technology and how a phenomenon known as the Doppler Effect can explain the oddities a passerby experiences around them:


Electronic Police Siren


The iconic synthetic tone we have all come to recognise can be attributed to the basic components that comprise an electric siren:

  • Amplifiers
  • Circuits
  • Modulators
  • Oscillators


These all combine to provide the capability of producing numerous siren tones, depending on the situation at hand; this is because the effects can be modulated.


Have you ever wondered as to the logic behind why the police siren is so capable of demanding attention in almost any situation? This can be traced back to the tritone – a common interval seen in a siren call, and is well understood in classical music to be used for harmonic and melodic dissonance i.e. to draw attention.


On top of this acoustic design philosophy, the volume of siren can be adjusted via a PA system to meet the demands of a situation. The typical decibel range of an emergency vehicle siren is 110-120 dB and is transmitted by having the speaker system mounted to the front bumper, resulting in less acoustic absorption.

Modern day sirens and police vehicle equipment have to deal with how much more insulated other vehicles are and how unattuned populations are to the world due to sound-muffling headphones. The ingenious solution to get around this involves utilising low frequency tones in sirens, penetrating these more insulated materials and providing a more tactile feel of the siren to everyone around.


What Is the Doppler Effect in Simple Terms?


The way most people come across the phenomenon of the Doppler Effect is through their experiences with sound waves; sound travels through the air as waves. These waves characteristically demonstrate peaks and troughs, with the intervals between subsequent peaks known as the wavelength.


The Doppler Effect comes into play when you consider the interplay between the speed and direction of the sound source, should it be moving, in relation to the listener. This produces an effect where a higher pitch (shorter wavelength) occurs as the sound source and listener approach and a lower pitch (longer wavelength) as they drift apart.


What Is an Everday Example of the Doppler Effect?


Relevantly, the Doppler Effect is most commonly associated with sirens heard from police vehicle equipment. As a police vehicle - with sirens on - approaches, the distance between you and the siren decreases, allowing the sound waves to hit your ears more frequently; the listener perceives an apparent higher pitch. As you may expect, when the distance between you and the siren increases, a lower pitch is perceived.


It should be noted that no real change to the frequency of the sound source occurs; the sound source is constantly outputting the same frequency, only your perception of the sound changes.


We at Darta Fleet Solutions have the capacity to outfit your emergency vehicle with a range of police vehicle equipment. From siren speakers and mount brackets to prisoner partitions, our comprehensive services guide you from consultation to installation. Get in touch to learn more!