A siren, mounted on a tower, emits a sound whose frequency is 2140 Hz. A person is driving a car away from the tower at a speed of 27.0 m/s. As the figure illustrates, the sound reaches the person by two paths: the sound reflected from the building in front of the car, and the sound coming directly from the siren. The speed of sound is 343 m/s. Concepts:
(i) One way that the Doppler effect can arise is that the wavelength of the sound changes. For either the direct or reflected sound, does the wavelength change?
(ii) Why does the driver hear a frequency for the reflected sound that is different from 2140 Hz, and is it greater or smaller than 2140 Hz?
(iii) Why does the driver hear a frequency for the direct sound that is different from 2140 Hz, and is it greater or smaller than 2140 Hz? Calculations: What frequency does the person hear for the
(a) Reflected
(b) Direct sound?

One of the most important concepts we encountered in this chapter is the transverse wave. For instance, transverse waves travel along a guitar string when it is plucked or along a violin string when it is bowed. Problem 112 reviews how the travel speed depends on the properties of the string and on the tension in it. Problem 113 illustrates how the Doppler eff ect arises when an observer is moving away from or toward a stationary source of sound. In fact, we will see that it's possible for both situations to occur at the same time.

Direct Reflected