why does ambulance siren sound high pitched on approaching but suddenly changes when it drives past?

Answer 1

'observer' is right -- this is due to the Doppler effect. The Doppler effect refers to the perceived change in sound wave frequency based on the velocity of the source. Here's an explanation, hopefully in clear language:

As the ambulance moves toward you, it emits the siren sound at a constant frequency; this actual frequency never changes, even though it sounds like the frequency is higher than normal (higher pitch = higher frequency). The frequency is the same, but the wavelength is compressed in your direction due to the ambulance's movement toward you. To your ears, the compressed wavelength is equivalent to a higher frequency pitch. As soon as the ambulance passes you, its relative velocity away from you causes the siren wavelength in your direction to expand, which sounds like a sudden shift to a lower frequency pitch. Note that the effect also holds if you are moving and the ambulance is standing still; it's the relative velocity that counts.

Answer 2

Have you heard of the term "Doppler Effect" ? thats what it is called.

As the ambulance approaches, the sound waves from its siren are compressed towards the observer. The intervals between waves diminish, which translates into an increase in frequency or pitch. As the ambulance recedes, the sound waves are stretched relative to the observer, causing the siren's pitch to decrease. By the change in pitch of the siren, you can determine if the ambulance is coming nearer or speeding away. If you could measure the rate of change of pitch, you could also estimate the ambulance's speed.

Answer 3

Thats explained by The Dopplers effect.
This happens due to the relative motion between the object and the observer

As the ambulance approaches, the sound waves from its siren are compressed towards the observer. The intervals between waves diminish, which translates into an increase in frequency or pitch.
Wavelength is easily influenced due to relative motion.
(Here amubance is moving with some speed relative to person who is steady)
Wavelength get apparently decreased during approach
but v is constant
Also, v=freq * wavalength
=>Frequency increases

But,As the ambulance recedes, the sound waves are stretched relative to the observer, causing the siren's pitch to decrease. By the change in pitch of the siren, you can determine if the ambulance is coming nearer or speeding away.


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Answer 4

The Doppler shift due to compression of the sound waves as the ambulance approaches (higher frequency sound waves giving a higher pitch) and the reverse situation when it is going away from you. Imagine you holding one end of a concertinaed strip of cardboard and someone else the othe end, as they rund towards you the strip "folds up" the concertinaed sections getting closer; as they run away the strp unfolds and flattens out, the concetinaed sections stretch out, this is what happens to the sound waves.
The white noise someone mentioned is there to make the sound more directional (it means you here the siren more clearly when its behind you than when the ambulance is to one side of you due to the way they time the burst of white noise, its a bit more technical and I'm not 100% sure as to how they work out the timing and how it effects the siren sound but I know thats the purpose, it was introduced to stop drivers getting confused when there was an ambulance in the vicinity and pulling over even though the ambulance wasn't coming up behind them).

Answer 5

Ambulance Noise

Answer 6

It's called the "Doppler Effect."

As the vehicle approaches you the sound waves are compressed by the motion, which raises the pitch. As it goes away the sound waves are dragged out, which lowers the pitch.

The same effect makes a difference in colors of the stars in space as they approach our corner of the universe at speeds which approach that of light.

Only its with light waves instead of sound waves.

Answer 7

It is called the Doppler effect, basically as the siren approaches the wavelength gets smaller; then as it passes the wavelength is longer causing the change in pitch! I am not real good in explaining it, so look up Doppler effect on the old www for a better definition...but my answer is right on though!!!

Answer 8

Ambulance Sound

Answer 9

Doppler Effect.
On approaching the sound waves are compressed - high pitch.
On moving away the sound waves are stretched - low pitch

Answer 10

this is called the Doppler effect

when it is moving towards you,the sound Waves stack because the source is moving closer,causing a higher pitch