Which statement about the speed of sound in water is true?

Which statement about the speed of sound in water is true?

• A. It travels about twice as fast as in air
• B. It travels about four times as fast as in air
• C. It travels about six times as fast as in air
• D. It travels at the same speed as in air

Answer: (B)

Sound travels faster in a medium that is stiffer and not too densely packed. The speed roughly follows v ≈ sqrt(B/ρ), where B is the bulk modulus (stiffness) and ρ is the density. In water, the medium is much stiffer than air (large B) but its density isn’t prohibitive, so the speed ends up around 1500 m/s. In air, both stiffness and density are much lower, giving about 343 m/s at room temperature. That puts water’s sound speed roughly four times faster than in air. So the statement that it travels about four times as fast as in air is the best choice. Note that it’s not about being twice or six times faster, and it isn’t the same as in air.

Sound travels faster in a medium that is stiffer and not too densely packed. The speed roughly follows v ≈ sqrt(B/ρ), where B is the bulk modulus (stiffness) and ρ is the density. In water, the medium is much stiffer than air (large B) but its density isn’t prohibitive, so the speed ends up around 1500 m/s. In air, both stiffness and density are much lower, giving about 343 m/s at room temperature. That puts water’s sound speed roughly four times faster than in air. So the statement that it travels about four times as fast as in air is the best choice. Note that it’s not about being twice or six times faster, and it isn’t the same as in air.