When discussing waves, what does the term 'amplitude' refer to?

Amplitude refers to the maximum displacement from the equilibrium position of a wave. In other words, it measures how far the wave oscillates away from its rest state, both upwards and downwards. This is crucial in understanding the energy of a wave; greater amplitude typically corresponds to a wave carrying more energy.

For example, when looking at sound waves, the amplitude is related to the loudness of the sound: the larger the amplitude, the louder the sound perceived by our ears. In the context of ultrasound testing, a higher amplitude can be indicative of a stronger signal, which can help in identifying materials or detecting flaws.

The other options represent different concepts in wave mechanics. The distance between wave crests relates to wavelength, the speed of the wave refers to how fast the wave travels through a medium, and frequency indicates how many cycles of the wave pass a specific point in a second. Each of these characteristics plays a different role in wave behavior and physics.