Question

Noah and Nina are 15 m apart in a closed room. Noah says the same sentence twice. The first time, Nina does not hear the sound, but she does hear the sentence the second time.

What best describes the difference between the second sound wave and the first?

The second sound wave reflects off the walls of the room, so Nina is able to hear the sound.
The second sound wave travels faster to Nina, so less energy is dissipated through the air.
The air molecules in the compressions of the second wave are more dense, so the sound is louder.
The air molecules in the second wave are spaced evenly, so a high-frequency wave is produced.

Answer 1

The first option.

Reflections from the room’s walls can provide a path for the sound to reach Nina (an echo or indirect path) even if the direct sound didn’t. The sound speed in air is essentially unchanged, and saying the same sentence twice doesn’t by itself make the compressions denser or the frequency higher.