 Take a 440 Hz tone and a 441 Hz tone and add them together. What happens? They interfere with each other. Sometimes the interference is constructive, producing a louder signal and sometimes the interference is destructive, cancelling the signal out altogether. This getting louder and softer again is called a beat frequency and is equal to the difference between the frequencies of the two tones. Add more tones at adjacent frequencies with carefully controlled amplitudes and we can make the tone shorter. However, the beat frequency remains at 1 Hz. To silence the 440 Hz tone for longer, we have to reduce the difference between adjacent frequencies. Looking at what we just did in the frequency domain, we can see all the frequencies present in our time domain signal. We are building our signal out of discrete frequencies. This is why signals that are discrete in the frequency domain produce periodic signals in the time domain.