Which factor affects the loudness of a sound?

The loudness of a sound is primarily determined by the amplitude of the sound wave. Amplitude refers to the maximum extent of a vibration or oscillation, which is essentially a measure of the energy contained in the sound wave. When the amplitude is greater, the sound wave carries more energy, resulting in a louder sound. This is why, for instance, turning up the volume on a speaker increases the loudness; the amplitude of the waves produced by the speaker is increased.

In contrast, volume level relates to how we perceive loudness, but it is not a physical property of the sound wave itself. Frequency, which refers to the pitch of the sound, does not affect its loudness directly; although our hearing sensitivity varies with frequency, it is not a determinant of loudness. The length of the sound wave, which may refer to the wavelength, is also not a factor affecting loudness. Thus, the amplitude is the correct choice since it directly correlates with the loudness we experience when hearing a sound.