Why does the siren from an ambulance that is approaching you seems louder?

The phenomenon you describe is known as the Doppler effect. It occurs when a source of sound is moving relative to an observer, resulting in a change in the perceived frequency of the sound. In the case of an approaching ambulance, the sound waves emitted by the siren are compressed as the ambulance moves towards you. This compression leads to an increase in the frequency of the sound wave, causing it to sound higher in pitch or louder. Conversely, as the ambulance moves away from you, the sound waves will be stretched out, resulting in a decreased frequency and a lower pitch or softer sound. This effect is not limited to sound waves and can be observed with other wave phenomena such as light and water waves.