Ultrasonic flow meters are more complex than meters that are purely mechanical, as they require significant signal processing and computation capabilities. Ultrasonic meters measure the speed of gas movement by measuring the speed at which sound travels in the gaseous medium within the pipe. American Gas Association Report No. 9 covers the proper usage and installation of these meters, and it specifies a standardised speed-of-sound calculation which predicts the speed of sound in a gas with a known pressure, temperature, and composition.