What do we call the observed frequency of a wave when either the source or observer is in motion?

The observed frequency of a wave when either the source or observer is in motion is referred to as the apparent frequency. This concept arises from the Doppler effect, which describes how the frequency of a wave changes due to the relative motion between the source of the wave and the observer.

When the source of sound moves towards the observer, the waves are compressed, leading to an increase in the observed frequency. Conversely, when the source moves away from the observer, the waves are stretched, resulting in a decrease in frequency. This phenomenon is crucial in various fields, such as astrophysics, where it helps in understanding the movement of stars and galaxies, as well as in everyday experiences, such as hearing the changing pitch of a siren as an emergency vehicle approaches and then recedes.

Understanding apparent frequency is vital for interpreting how motion impacts wave observation, differentiating it from other types of frequency concepts, such as resonance frequency, which pertains to a system's natural frequency of oscillation, or musical frequency, which relates to sounds perceived as musical notes.