If the composite signal is periodic, the decomposition gives a series of signals with discrete frequencies.
If the composite signal is nonperiodic, the decomposition gives a combination of sine waves with continuous frequencies.
shows a nonperiodic composite signal. It can be the signal created by a microphone or a telephone set when a word or two is pronounced. In this case, the composite signal cannot be periodic, because that implies that we are repeating the same word or words with exactly the same tone.