A source of sound travels towards an observer.

When a source of sound moves towards an observer, the successive wavefronts emitted by the source are compressed in the direction of motion.

The apparent wavelength $\lambda'$ in the medium towards the observer is given by $\lambda' = \dfrac{v - v_s}{f}$, where $v$ is the speed of sound in the medium, $v_s$ is the speed of the source, and $f$ is the actual frequency of the source.

Since $v_s > 0$, we have $\lambda' < \lambda$. Thus, the wavelength of the sound in the medium towards the observer decreases.

The actual frequency of the source and the speed of sound in the medium depend on the source characteristics and medium properties respectively, and remain unchanged.