Question

Explain the doppler effect for a listener moving away from the sound.

Answer 1

I believe this: The listener is moving away from the constant source, he is putting distance between the waves. The wavelength is decreased, and the pitch is lower.

Answer 2

@shamim

Answer 3

You are correct when you say "The listener is moving away from the constant source, he is putting distance between the waves". I would just change "distance between waves" to distance between antinodes, peaks (maximum air density), troughs (minimum air density), or something to that effect.

As for your second sentence - if you're INCREASING the distance between each peak/trough/etc, then you're INCREASING the wavelength as well. This in turn results in a sound with DECREASED frequency (i.e. pitch).