Calculate the time domain. The time domain of a wave is how much of the time axis a wave occupies when plotted as voltage against time. In fact, for most constant signals, this domain is from 0 to infinity. Only when a wave is dampening or suddenly dies out will the time domain be different; in this case the time domain is from 0 to the time when the signal is no longer present. If the signal is in fact always present, then presenting the time domain as from 0 to infinity is rather pointless. Instead, look for the period of that wave and present the time domain as a period. The period of the wave is from 0 to the time at which the wave begins to repeat its pattern. For example, a sine wave begins to repeat itself at time 2pi. Thus, you could present the time domain as the domain from 0 to 2pi.
Calculate the frequency domain. Plot the wave as amplitude against frequency. Notice this is a totally different graph in terms of both axes when compared to the one used for the time domain. In this graph, look for all frequency values at which the amplitude is not zero. You may find a continuous set of frequencies or only a handful of frequencies. Regardless, the frequency domain is the set of frequencies for which the amplitude is not zero.
Write the frequency time domain. This domain is a two-dimensional domain. Write it in mathematical notation by combining the time domain and frequency domain. For example, if you have a dampening signal that dies out after five minutes and only presents itself in two frequencies: 4 and 8 Hz, then write the set {t: t e [0, 5]; f: f e {4, 8}}, where “e” represents epsilon, the mathematical notation for belonging in a set.