When a sound source and a listener are moving in the same direction with the same velocity, the phenomenon known as the Doppler effect comes into play. The Doppler effect describes the change in frequency (and hence, pitch) of a wave as observed by an observer moving relative to the source of the wave.
In this scenario, if both the source and the listener are moving in the same direction with the same velocity, the apparent frequency heard by the listener will be equal to the actual frequency of the sound. The ratio of the actual frequency to the apparent frequency will be 1:1.
This is because when the source and the listener move at the same velocity, there is no relative motion between them to cause a change in the frequency of the sound waves. Therefore, the listener perceives the sound at the same frequency as emitted by the source.
It's worth noting that the situation would be different if the source and the listener were moving at different velocities or in opposite directions. In such cases, the relative motion would cause a shift in the frequency heard by the listener, and the ratio of the actual frequency to the apparent frequency would differ from 1:1.