Also known as the rain-barrel effect, echo is signal reflection. At any point in a circuit where an electromagnetic wave meets a discontinuity, a portion of the wave is reflected back in the direction of the transmitter. Such discontinuities can be caused by impedance mismatches, mismatches between line and balancing networks, and irregular spacing of loading coils. Echo is imperceptible in human-to-human conversations as long as the echo return is weak and the total roundtrip delay is not longer than 30