Question

The reason a fire truck’s siren sounds higher in pitch as it approaches you is because the truck pushes the sound waves together so that the sound wavelengths in front of the truck get shorter. How do the sound waves produced by the fire truck change as the truck drives away from you?

A.Wavelengths stretch out, which increases the pitch.
B.Velocity decreases, which increases the frequency.
C.Frequency decreases, which decreases the pitch.
D.Frequency increases, which increases the pitch.

Answer 1

As the fire truck moves away, the wavelengths of its siren sound increase, resulting in a lower frequency and thus a lower pitch, demonstrating the Doppler effect.

Step-by-step explanation:

The change in frequency and pitch of sound due to the motion of the source or the observer is known as the Doppler effect. As the fire truck moves away from you, the sound waves it produces begin to stretch out. This means the wavelengths are increased, which leads to a decrease in frequency. Therefore, the pitch of the sound, which is how we perceive frequency, is also reduced. The effect is that the siren that sounded high-pitched as the truck was approaching will transition to a lower pitch as the truck moves away. Hence, the correct answer is C: Frequency decreases, which decreases the pitch.