When a bell is rung underwater compared to in air, what happens to the sound and why?

• A. It travels faster underwater because particles are closer together.
• B. It travels slower underwater because water is denser.
• C. It travels at the same speed as in air.
• D. It cannot travel underwater.

Answer: (A)

The speed of sound is determined by how the medium responds to compression: its stiffness (how hard it is to compress) and its density. The relevant relationship is that the wave speed increases with the medium’s ability to resist compression relative to its density. Water is much stiffer against compression than air (its bulk modulus is large), and although it is denser, that stiffness dominates the ratio that sets the sound speed. This means pressure disturbances transfer between molecules more quickly in water, so the sound wave travels faster underwater. The idea that particles are closer together helps intuition—denser packing supports quicker coupling of neighboring particles—but the key factor is water’s greater resistance to compression compared to air, which makes the speed higher than in air.