User:John R. Brews/Reality
In physics and engineering the envelope of a rapidly varying signal is a smooth curve outlining its extremes in amplitude. The figure illustrates a sine wave varying between an upper and a lower envelope.
- C. Richard Johnson, Jr, William A. Sethares, Andrew G. Klein (2011). “Figure C.1: The envelope of a function outlines its extremes in a smooth manner”, Software Receiver Design: Build Your Own Digital Communication System in Five Easy Steps. Cambridge University Press, p. 417. ISBN 0521189446.