What measures the difference between the maximum value and the average or undisturbed value of a wave?

The measure of the difference between the maximum value and the average or undisturbed value of a wave is known as amplitude. Amplitude represents the extent of a wave's displacement from its resting position, indicating how strong or intense the wave is. In the context of sound waves or electromagnetic waves, higher amplitude corresponds to louder sounds or brighter light, while lower amplitude indicates softer sounds or dimmer light.

This concept is fundamental in understanding wave behavior, as it affects how waves interact with media and how they are perceived by human senses. The amplitude is crucial in various applications, including medical ultrasound, where the amplitude of the detected signal can correspond to tissue characteristics.

Other choices—frequency, wavelength, and period—relate to different properties of waves. Frequency measures how many cycles of a wave occur in a given time frame, wavelength is the distance between successive crests or troughs of a wave, and period refers to the time it takes for one complete cycle of the wave to pass a point. These properties do not directly measure the variation in displacement from the undisturbed value, making amplitude the correct measure in this context.