Schematic representation of a sound wave. Top image: zones of compression (high pressure) and of rarefaction (low pressure) alternate; the distance between two pressure peaks is one wavelength (λ). Lower image: the course of pressure (on the y-axis) over distance (on the x-axis) can be represented as a sine wave. A similar wave would represent pressure over time at a fixed location; the time interval of two pressure peaks would be 1/f with f for frequency. Modified with permission from Weyman AE. Principles and Practice of Echocardiography, 2nd edn., 1994. Philadelphia, PA: Lea & Febiger.