When a sound source is moving away from an observer, the observer will hear a lower pitch due to a decrease in the frequency of the sound wave. This is known as the Doppler's effect.
As the sound source moves closer to the observer, they hear a higher pitch in the reverse process.
A sound wave transmits vibrational energy from the source to the hearer.
A sound wave has frequency, amplitude, velocity and wavelength just like other wave types.
The frequency of a wave is the number of waves that passes through a point at a time.
This is related to the the wavelength of a wave.
Waves of a high frequency are known to have short wavelengths and vice versa.
To hear a low pitch, the source moves away and the wavelength between successive crests and troughs on a wave increases considerably.
