As a siren approaches an observer, the pitch of its sound seems to get progressively higher. As it passes the observer and moves farther away from him, its pitch seemingly descends. Why does this occur, when it is apparent that the true pitch of the siren remains constant? This effect, commonly known as the Doppler effect, is attributable to the change in frequency of the siren’s sound waves relative to the observer, and relies on the theory that sound is emitted in waves. As the siren approaches, each sound wave takes less and less time to reach the observer’s ears and as it recedes, each wave takes a longer amount of time to travel back to the observer.

Which of the following statements is supported by the passage?

The Doppler effect is universally accepted by physicists.
The pitch from a siren moving away from an observer seems lower than it actually is.
The Doppler effect was proposed before the advent of electric sirens.



The passage provides information on each of the following except

A) what happens to the relative pitch of a sound wave as its source approaches an observer
B) what happens to the relative pitch of a sound wave as its source recedes from an observer
C) the physical property of sound necessary to produce the Doppler effect
D) whether the pitch of a sound at the observer’s location is the same as its pitch while it is approaching the observer
E) the distance from the observer at which the sound is most intense