A damped wave is a wave whose amplitude of oscillation decreases with time, eventually going to zero. This term also refers to an early method of radio transmission produced by spark gap transmitters, which consisted of a series of damped electromagnetic waves. Information was carried on this signal by telegraphy, turning the transmitter on and off to send messages in Morse code. Damped waves were the first practical means of radio communication, used during the wireless telegraphy era which ended around 1920. In radio engineering it is now generally referred to as "Class B" emission. However, such transmissions have a wide bandwidth and generate electrical "noise" which interferes with other radio transmissions.
Because of their potential to cause interference and their resulting wasteful use of radio spectrum resources, there is an international prohibition against the use of class B damped wave radio emissions, established by the International Telecommunications Union in 1938.