I understand that a person outside of a moving car or siren hears a higher frequency when the car or siren is approaching and a lower frequency when it is going away. But what kind of frequency would a person inside of the moving car or siren hear?

My two thoughts were that they would hear a happy medium frequency because they are not moving in any direction relative to the sound. Another is that they would hear a constant high frequency because they are always so close to what is producing the sound.