Select and record the correct responses to complete the following statements. You have learned that the frequency of sound waves reaching a stationary observer will change depending on whether the source of the sound is approaching or receding. Suppose an ambulance is approaching and then passes you and drives away without changing speed. As an ambulance approaches, the sound waves (expand, compress), resulting in a (higher, lower)-frequency wave. As the ambulance passes, the pitch (increases to a higher frequency, drops to a lower frequency).