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

Explanation:

Sound waves can reflect off surfaces. In a closed room, the first sound wave might not reach Nina directly (maybe due to low amplitude or other factors). But the second - time, the sound wave reflects off the walls (a phenomenon called reverberation in a room). This reflection can change the path of the sound wave and make it reach Nina.

• Sound travels at a constant speed in a given medium (air in this case), so the second sound wave does not travel faster. So the option about speed is wrong.
• The density of air molecules in compressions (related to amplitude) would be a property of how Noah speaks (loudness). Since it's the same sentence, the initial amplitude (related to air - molecule density in compressions) should be similar. So the option about air - molecule density in compressions is wrong.
• Sound is a longitudinal wave, and the even spacing of air molecules (a property related to a uniform medium, not a change between the two sound waves) does not produce a high - frequency wave. Frequency is related to the source (Noah's vocal cords vibrating), and for the same sentence, the frequency should be similar. So the option about air - molecule spacing is wrong.

Answer:

The second sound wave reflects off the walls of the room, so Nina is able to hear the sound.