Today I received a Tweet with a question regarding a principle of acoustics, which I aim to answer in this post.
The effect that causes this pitch change is known as the Doppler Effect and regards the change in frequency of a sound source for an observer moving relative to its source. To understand how it works, a few basic principles must be underlined.
Sound is created by pressure changes in the air, which is caused via vibrations of an object (e.g. a speaker). As the speaker vibrates outward, pressure is increased in front of it and as it moves back inwards pressure is decreased again because it is compressing and decompressing the air particles in front of it. These pressure changes travel through air, away from the speaker, at a fixed speed of 344 metres per second. To play a lower pitch (or frequency) sound, the speaker vibrates slower (less vibrations per second). As the pressure variations (sound waves) move away from the speaker at the same speed no matter what frequency is made, there is a greater distance between the sound waves at lower frequencies (as the sound wave has had more time to travel further between the speaker vibrations).
Now back to that siren… As the fire engine travels, the sound it is creating from the siren, travels away from it in all directions. This next step best illustrated with a diagram:
The lines around the fire engine represent the sound waves travelling away from it. As the engine moves forward, the speed at which the sound waves travel away from it (in the same direction the engine is traveling) decreases because it is like the engine is ‘chasing’ the sound waves. This causes the distance between sound waves to decrease. This also means that the engine is ‘driving away’ from the sound waves travelling away behind it and, like said previously, a greater distance between sound waves relates to a lower frequency.
This means that if you are standing on the side of the road and an ambulance drives past with its sirens on, as it comes toward you, it sounds higher as the distance between sound waves is short, but as it has gone past the distance between sound waves increases and it has a lower pitch. This effect creates the classic sound we are used to when we want to recreate the sound of a car going fast, because the greater the speed of the vehicle, the closer its speed is to the speed of sound, the shorter the distance between the wavelengths in front of the vehicle and the more noticeable this effect becomes.
If you are in the vehicle however, this effect is unnoticeable because you are at a constant distance from the sound source (e.g. the siren/engine) meaning that the sound does not change relative to you. Also, if you are standing on a path and a vehicle goes past very slowly, it is not noticed as the difference between the speed of sound and the speed of the vehicle is so great that there is no noticeable effect.
As an aside, you may wonder what would be to happen if a vehicle did go faster than the speed of sound. This can happen, but only in planes as 344 metres per second is the same as 770mph. A sonic boom is created when this happens and is the sound of the shock waves as the object passes this speed.
Of course as some of you may be aware, the Doppler affect has been mentioned on TV before in the Big Bang Theory and I couldn’t really write a blog post on it without including the clip…