What happens to wave frequency when a sound source moves towards a listener?

When a sound source moves towards a listener, the frequency of the sound waves perceived by the listener increases. This phenomenon is known as the Doppler effect. As the source approaches, the sound waves are compressed, resulting in a shorter wavelength and therefore a higher frequency. This is why the pitch of the sound heard by the listener becomes higher as the source comes nearer. Conversely, if the source were moving away, the frequency would decrease due to the stretching of the sound waves. Thus, the principle of the Doppler effect is key to understanding why the frequency increases when the source moves towards the listener.