Why Do Fire Truck Sirens Change Frequency From A Bystander’s Perspective?

Emergency vehicles such as police cruisers, ambulances or fire trucks all have their special lighting and sound system. It is the norm to hear the wail of a fire truck siren before the vehicle is visible. However, many bystanders also witness a peculiar phenomenon when a fire truck passes through. In this blog, this sound-related phenomenon will be addressed in detail.

Doppler effect and fire truck sirens.

What is the doppler effect?

When the emergency vehicles approach a person on the street, they hear the pitch of the siren get louder as it passes them by. However, as it speeds away, the pitch returns to its normal volume. This variation in pitch with respect to the bystander is known as the doppler effect. 

What causes the doppler effect?

To understand the underlying cause of the doppler effect, it is first crucial to learn about the propagation of sound waves. Waves travelling in a three-dimensional space are known as wavefronts. A stationary sound source will emit sound at a constant frequency, meaning the wavefronts are equidistant from one another. Thus, a person will hear the same frequency, irrespective of their distance from the source. 

However, when the source is moving, in this case, an emergency vehicle, the wavefront accumulates together in the front of the moving object. Thus, a bystander standing in front of the fire truck will hear the siren on a higher frequency. Conversely, the sound waves/wavefronts in the back of the vehicle will be spaced farther apart. Hence, a person standing behind the moving vehicle will hear a lower frequency sound. Therefore, as the vehicle continues to move, bystanders will keep hearing the siren on varying frequencies, depending on their position with respect to the emergency vehicle.

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