In a longitudinal wave, the amplitude refers to the maximum displacement of particles from their equilibrium positions as the wave passes through a medium. However, it's important to note that the concept of amplitude is more commonly associated with transverse waves.
In a longitudinal wave, such as a sound wave, particles of the medium oscillate back and forth parallel to the direction of wave propagation. The displacement of the particles corresponds to variations in pressure or density, rather than physical displacement as in transverse waves. The amplitude of a longitudinal wave is typically measured as the maximum pressure or density deviation from the equilibrium state.
For example, in a sound wave, the amplitude represents the maximum variation in air pressure caused by the compressions and rarefactions of the wave. The amplitude determines the loudness or intensity of the sound wave. However, unlike in transverse waves where the amplitude corresponds to the physical displacement of particles, in longitudinal waves it relates to the magnitude of the pressure or density fluctuations.