An observer perceives a sound to be lower in frequency when the sound source is moving away from the observer. This shift in the frequency of the perceived sound occurs because:

Question

An observer perceives a sound to be lower in frequency when the sound source is moving away from the observer.  This shift in the frequency of the perceived sound occurs because:

Quizplus · Accepted Answer

11ecba2d_1335_56bb_a3bf_57c70cb3befc_MD0008_00 The Doppler effect describes a waveform phenomenon in which the frequency of the waveforms perceived by an observer is greater or less than the real, native frequency of waveforms emitted from the source. The Doppler effect may be observed with both longitudinal (eg, sound) and transverse (eg, light) waveforms because both exhibit the same relationship among frequency (f), native velocity (v), and wavelength (λ): 11ecba2d_1335_56bc_a3bf_39cd1883a7f2_MD0008_00 Doppler shifts occur when the velocity of the waveform source (v_s) or the velocity of an observer (v_o) leads to a perceived change in the velocity of the emitted waveforms relative to the observer. A perceived change in v resulting from either cause produces a simultaneous apparent shift in the observed frequency (f_o): In other words, the motion of the observer or waveform source relative to each other leads to a shift in only the apparent, perceived velocity of the sound waveform as successive waveform crests and troughs are perceived closer together or farther apart. For example, when the source or observer is moving away from the other, the distance between successive waveforms increases and the perceived waveform velocity decreases. (Choice A) Doppler shifts describe only the perceived velocity of emitted waveforms. No amount of motion by either the source or the observer can modify the native velocity of the emitted waveform, because the speed of a waveform is primarily influenced by its type and by the medium through which it travels. (Choice B) The scattering of sound that occurs with the propagation of sound through open environments is associated with reduced waveform intensity, but this is not responsible for the frequency shift associated with the Doppler effect. (Choice D) The decay in soundwave intensity that occurs as the distance increases between the source and the observer is unrelated to the motion of the source and the observer; it cannot explain the frequency shifts associated with the Doppler effect. Educational objective: The Doppler effect describes apparent changes in the velocity of waveforms that lead to changes in the observed frequency of those waveforms, which are known as Doppler shifts; however, the actual velocity of waveforms does not change when Doppler shifts occur.

An Observer Perceives a Sound to Be Lower in Frequency