A small sound source emitting a continuous tone of constant frequency travels at a constant velocity along a straight track. A stationary detector is positioned at a fixed perpendicular distance \(d\) from the track, recording the observed frequency \(f\) as the source approaches, passes, and moves away. The graph below displays the observed frequency \(f\) as a function of time \(t\). Which of the following statements correctly interprets a feature of the graph?