Doppler shift

Luke1121

I've just finished some past exam papers and when marking them using the mark scheme something confused me a little
One question said when a fire engine moves away from an observer, the pitch of the siren decreased, why and I chose the wavelength of the sound wave decreases
The mark scheme said the answer was the distance traveled by each wave front increases
I learned from my mistake and moved on. Today I've done another paper and a question says explain how the movement of the ambulance causes the frequency of the sound the man hears to change. Remembering my mistakes I talked about as the ambulance moves closer the distance traveled by each wavefront, decreases so there are more waves per second so f increases and the opposite for moving away, but the mark scheme said "higher frequency, wavelengths shorter as ambulance moves closer" Am I actually correct in what I'm saying or has the mark scheme got a case of double standards. Thanks in advance.

Fewmet

Those answers look like different ways of expressing the same thing.

Personally, I'd focus on the the motion of the sound source decreasing the distance between wave fronts in the direction of motion and decreasing it behind.