If a source of solid is moving, a stationary listener will hear a sound of different frequency. This is called?

1. Analyze the Question

The question describes a physical phenomenon where the frequency of a sound wave perceived by a listener changes because the source of the sound is in motion relative to the listener. You are asked to identify the name of this specific effect.

2. Explain the Concept

This phenomenon is known as the Doppler Effect.

• How it works: When a source of sound moves towards a stationary listener, the sound waves are "compressed," leading to a shorter wavelength and a higher frequency (a higher pitch). Conversely, when the source moves away, the waves are "stretched," leading to a longer wavelength and a lower frequency (a lower pitch).
• Real-life example: Think of an ambulance siren. As it approaches you, the pitch sounds higher; as it passes you and moves away, the pitch drops significantly.

3. Evaluate the Options

• A. Doppler effect: This is the correct term for the change in frequency due to relative motion between a source and an observer.
• B. Resonance: This occurs when an object is forced to vibrate at its natural frequency by an external source, leading to an increase in amplitude.
• C. Ultrasound: This refers to sound waves with frequencies higher than the upper audible limit of human hearing (above 20,000 Hz).
• D. Rarefaction: This is a region in a longitudinal wave where the particles are furthest apart (the opposite of compression).

4. Conclusion

Based on the definition provided in the question, the phenomenon described is the Doppler effect.

Exam Tip: Whenever you see a question involving a "moving source" or "moving observer" and a change in "frequency" or "pitch," immediately think of the Doppler Effect. It is a very common topic in JAMB Physics!