Question

A person behind a moving siren will interpret the frequency of the siren as being

Answer 1

Answer: Smaller than it actually is.

Explanation:

The person is behind the moving siren, and in this case, we assume that the siren is moving away from the man. By the Doppler effect, we know that a wave that comes from a source that is moving away will suffer a "redshift", which means that the wavelength increases a little bit, now, as you may know, the relation between the wavelength and the frequency comes by the equation:

velocity = wavelength/frequency.

If we keep the velocity constant (the velocity of the sound is the same independent of the frequency) then, if we have an increase in the wavelength, we must have a decrease in the frequency to maintain the velocity constant. From this, we can see that a redshift is also associated with a decrease in the frequency of the wave.

Then, the person behind the moving siren will interpret the frequency as being smaller than it actually is.

Answer 2

A person that is standing behind a moving siren will interpret the frequency to be lower compared to the person that would be standing in front of it. This is because the sound is now moving away from the person behind the siren and at this point the sound is juts lingering but, still very low.