Question

question 14 of 20 : select the best answer for the question. 14. in which of the following conditions would a person not hear an echo? a. a person is standing in the range of the sound waves coming from the original sound source as well as the reflected waves. b. a person is standing 30 meters in front of the reflecting surface. c. a sound wave reaches the ear, and the reflected wave reaches the ear less than 0.1 seconds later. d. the reflecting source is approximately 21.5 meters from the person. mark for review (will be highlighted on the review page)

Explanation:

To hear an echo, the time between the original sound and the reflected sound (echo) must be at least 0.1 seconds (so the brain can distinguish them). Let's analyze each option:

• Option A: Being in the range of both original and reflected waves is necessary for hearing an echo, so this is a condition for hearing an echo, not not hearing it.
• Option B: The distance of 30 meters from the reflecting surface: The total distance the sound travels (to the surface and back) is \( 2\times30 = 60 \) meters. The speed of sound in air is approximately 343 m/s. The time taken \( t=\frac{d}{v}=\frac{60}{343}\approx0.175 \) seconds, which is more than 0.1 seconds, so an echo would be heard.
• Option C: If the reflected wave reaches the ear less than 0.1 seconds after the original sound, the brain cannot distinguish them as separate sounds, so no echo is heard. This fits the condition of not hearing an echo.
• Option D: The reflecting source is 21.5 meters from the person. The total distance traveled by sound is \( 2\times21.5 = 43 \) meters. The time taken \( t = \frac{43}{343}\approx0.125 \) seconds, which is more than 0.1 seconds, so an echo would be heard.

Answer:

C. A sound wave reaches the ear, and the reflected wave reaches the ear less than 0.1 seconds later.