# Change in a siren's volume question

In summary, an ambulance siren gets louder as it approaches you and quieter as it drives away due to the inverse square law of sound intensity. This means that as the distance between the source (the siren) and the listener (you) decreases, the sound intensity increases. Therefore, when the ambulance is approaching, the sound waves are directed towards you and the distance between you and the siren decreases, resulting in a louder sound. As the ambulance moves away, the distance increases, causing the sound intensity to decrease and the siren to sound quieter. This is not affected by the shape of the sound wave, as the inverse square law applies to all directions.

## Homework Statement

Q. Expalin why an ambulance siren gets louder as the ambulance approaches you and quieter as it drives away from you.

None

## The Attempt at a Solution

Hi,
So, I know it's not the Doppler effect, because this only affects frequency.
If the siren is only pointing in one direction, then the answer would be because the sound waves are directed at you when it's approaching and pointing away when it's moving away, yes?

However, if the siren is producing sound in all directions, I'm stuck. Is it something to do with the wavelengths being closer together on the approach, so there is more sound? But is this not the Doppler effect ie frequency?

Doc Al said:
How does sound intensity depend on the distance from the source? Read this: http://hyperphysics.phy-astr.gsu.edu/Hbase/acoustic/invsqs.html#c1"

OK, but I don't see why that means it should be louder when it's approaching if the sound wave is being emitted in the shape of a sphere. Why isn't it just as loud when it's moving away?

Last edited by a moderator: