The telegraph line carries the current pulses that are recognized by the human receiver as Morse code. In this case, human operators acted to modulate and demodulate the information signal onto and off of the telegraph line. For more complex information, such as speech or audio, the information signal is time-varying and continuous, and more complex modulation techniques are used. Sinusoidal voltage or current signals optimized for a specific channel are used to 'carry' the information signal. These carrier waves have some physical property modulated such that the information signal is contained in the carrier wave.
Many forms of energy propagate as traveling waves.
Mechanical waves, such as sound waves in air or pressure waves in water, require a physical medium to propagate. The wave speed of mechanical waves is highly dependent upon the material characteristics of the medium. Whether the traveling wave is mechanical or electromagnetic in nature, one set of characteristics is sufficient to describe the traveling wave. Although much of the development of wave characteristics is independent of the shape of the wave, a sinusoidal waveform is used since it is easy to generate, easy to model, and more complex waveforms can be described by sums of sinusoidal waves. We look at the standing wave, then, in order to easily recognize some characteristics that will be common to more complex waves.