Final answer:
The correct option is b) Decreases.
As a sound source approaches someone, the frequency of the sound wave increases, which is explained by the Doppler Effect. The observed pitch sounds higher as the source approaches due to the compression of sound waves. The amplitude of a sound wave decreases with distance from the source due to energy spreading over a larger area, separate from the frequency change.
Step-by-step explanation:
As a sound source approaches someone, the frequency of the sound wave increases. This phenomenon is known as the Doppler Effect, which describes the change in frequency or wavelength of a wave in relation to an observer who is moving relative to the wave source. The observer perceives a higher pitch as the source approaches due to the sound waves being compressed, effectively increasing their frequency.
Conversely, as the sound source moves away, the waves are stretched, leading to a decrease in frequency and a lower pitch. It is important to clarify that the actual speed of the sound wave does not change, nor does the emitted frequency from the source; it is the received frequency that changes for the observer.
For example, when an ambulance with a blaring siren moves towards you, the siren's pitch sounds higher than it actually is; when it moves away, the pitch lowers. This example illustrates how the observed frequency changes due to the movement of the sound source relative to the observer. One can note that the amplitude of a sound wave does decrease with distance from its source, but this is due to the energy of the wave being spread over a larger area, rather than the frequency change addressed by the question.