Question

Why does the frequency of a siren get higher as an ambulance using that siren gets closer

Answer 1

Explanation:

Doppler effect: There is an apparent change in the frequency when there is relative motion between the source and the listener. This phenomenon is Doppler effect.

When the source and the listener are moving towards each other then the frequency of the wave increases.

In the given problem, the frequency of a siren gets higher as an ambulance using that siren gets closer. The frequency of the siren increases due to Doppler effect. The pitch of the sound also becomes louder as the pitch depends on the frequency.

Answer 2

The frequency of a wave becomes higher due to the object moving at a fast pace coming towards you with shorter wavelengths  (depending on the speed) aka the Doppler Effect. 
Hope this helps