A baby can easily surpass 100 decibels. Which of the following part of a sound wave is increased to create such a loud cry?

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Correct Answer: C. The wave’s amplitude.
Loudness is a measure of intensity or power per area. The area of the ear is fixed, but the power of the wave can be increased. Power is energy over time, or force times velocity.

Sound is a pressure wave. The force can be written in terms of pressure times area, and the velocity can be written in terms of speed over a bulk modulus. Together these give a power independent on frequency. The period, frequency, wavelength, and wavenumber all relate to one another through the phase velocity. If you are dependent on one of them, you are dependent on the rest. The correct choice is C.
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