Problem 18 - An Introduction to Physical Science

Question

Choose the word or phrase that best answers the question. An ambulance siren speeds away from you. What happens to the pitch of the siren? A) It becomes softer. B) It becomes louder. C) It decreases. D) It increases.

Step-by-Step Solution

Answer

C) It decreases.

Step 1 - Understand the phenomenon

The effect in question is known as the Doppler Effect. This occurs when there is relative motion between a source of sound and an observer, leading to a perceived change in frequency and therefore pitch.

Step 2 - Identify the direction of movement

The problem states that the ambulance siren is speeding away from you. This means the source of sound is moving away from the observer.

Step 3 - Determine the effect on pitch

When the sound source moves away from the observer, the sound waves are stretched, leading to a decrease in the frequency of the waves. A lower frequency corresponds to a lower pitch.

Step 4 - Choose the correct answer

Based on our understanding of the Doppler Effect and the direction of the movement, the pitch of the siren decreases. Therefore, the best answer is C) It decreases.

Key Concepts

sound frequency

Sound frequency refers to the number of sound wave cycles that occur in one second. It is measured in Hertz (Hz). When an ambulance siren produces sound, it generates sound waves that propagate through the air.
Higher frequency sound waves result in a higher-pitched sound, while lower frequency sound waves result in a lower-pitched sound.
The frequency of the sound determines how high or low we perceive a sound to be.
Objects moving towards us compress sound waves, increasing frequency and pitch. Objects moving away stretch sound waves, lowering frequency and pitch.

pitch

Pitch is our perception of the frequency of a sound. It's how we interpret the highness or lowness of a sound.
High-frequency sounds, like a whistle, have a high pitch, whereas low-frequency sounds, like thunder, have a low pitch.
The Doppler Effect plays a crucial role in pitch perception when there is relative motion between the sound source and the observer.
As the ambulance siren moves away from you, you perceive a lower pitch because the sound waves' frequency decreases.
This change in perception is due to the stretching of the sound waves, which reduces the frequency reaching your ears.

relative motion

Relative motion describes how two objects move in relation to each other. In the context of sound, it significantly affects how we perceive sound's frequency and pitch.
The Doppler Effect is a prime example of relative motion's impact on sound.
When the ambulance with its siren speeds away from you, relative motion between you and the ambulance influences your perception of the sound.
The key factor here is the direction of movement. As the sound source (ambulance) moves away, the sound waves stretch, decreasing the frequency and pitch you hear.
Conversely, if it were moving towards you, the opposite would happen; sound waves would compress, increasing the frequency and pitch.