Change in a siren's volume question

[Pyroadept]

Homework Statement

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

Homework Equations

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]

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

 

[Pyroadept]

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?

 

[Doc Al]

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?

You are asked to explain why it gets louder as it approaches. There's a very simple reason for that!

Don't confuse this with "why is the sound louder when approaching". That's not the same thing. (And you're right, there's no reason why it would be any louder coming or going.)