Which term refers to a sound wave's amplitude?

The correct answer is loudness, as it is directly related to the amplitude of a sound wave. Amplitude refers to the height of the sound wave, and in acoustics, it determines how loud a sound is perceived to be. A greater amplitude corresponds to a louder sound, while a smaller amplitude results in a quieter sound. This relationship is critical in understanding how we perceive different sounds in our environment.

Other terms listed do not pertain to amplitude in the same way. Timbre relates to the quality or tone color of a sound, allowing us to differentiate between different sound sources even when they produce the same pitch and loudness. Frequency refers to the pitch of a sound, determining how high or low it sounds, measured in Hertz (Hz). The middle ear is a part of the anatomy involved in hearing but does not describe any properties of sound waves themselves.