Question

exam lesson name: light and sound
exam number: 700699rr
exam guidelines
exam instructions
question 17 of 20 :
select the best answer for the question.

1. in which of the following locations can a person hear the echo of a sound?

a. at least 16.6 meters in front of the reflecting surface
b. less than 33.2 meters behind the reflecting surface
c. less than 16.6 meters in front of the reflecting surface
d. at least 33.2 meters behind the reflecting surface
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Explanation:

To hear an echo, the time between the original sound and its reflection must be at least 0.1 seconds (so the brain can distinguish them). The speed of sound in air is about 343 m/s. The distance the sound travels to the reflecting surface and back is \( d = v \times t \), where \( v = 343 \, \text{m/s} \) and \( t = 0.1 \, \text{s} \). So \( d = 343 \times 0.1 = 34.3 \, \text{m} \). This is the round - trip distance, so the one - way distance (from the person to the reflecting surface) is \( \frac{34.3}{2}\approx17 \, \text{m}\) (approx 16.6 m for simplicity). So the person must be at least 16.6 meters in front of the reflecting surface (so the sound has enough distance to reflect back and be heard as an echo). Option A fits this, while B, C, D have incorrect distance or position logic.

Answer:

A. At least 16.6 meters in front of the reflecting surface