7.3.1—When a source of sound waves moves relative to a stationary observer, the

When a sound source moves relative to an observer, wavefront spacing changes, so the observed frequency differs from the emitted frequency.

Approaching motion produces compressed wavefronts and higher observed frequency; receding motion produces greater spacing and lower frequency.

A passing siren sounds higher before it reaches you and lower after it passes, even though the siren emits a steady tone.

The Doppler shift is not caused by the source changing its intrinsic frequency; it is a relative-motion observation effect.