What is true of the frequency of the sound of an approaching siren compared to a stationary observer?

When considering the frequency of a sound as it approaches a stationary observer, the Doppler effect comes into play. As the source of the sound moves closer to the observer, the sound waves are compressed, resulting in an increase in frequency. This phenomenon occurs because the sound waves are produced at a constant rate, but as the source moves toward the observer, the waves take less time to reach the observer, effectively shortening the wavelength and increasing the frequency perceived.

Thus, the frequency of the sound of an approaching siren is perceived as higher by the stationary observer than if the siren were at rest relative to them. The other scenarios presented suggest varying combinations of higher or lower frequencies that do not accurately reflect the Doppler effect with a moving source toward a stationary observer.