When two people sing loudly at slightly different pitches, the frequencies can mix, causing a difference tone, or third pitch. You may also experience this phenomenon with fire whistles blaring through a town, or even the beeps or tones emitted by a clock radio. In fact, many difference tones are intentionally created to enhance the sound.

A harmonic is a mode of vibration that is a whole-number multiple of the fundamental mode. The first harmonic is the fundamental frequency. The second harmonic is twice its frequency, etc. Many instruments, especially bells, oscillate in modes that are not whole-number multiples of the fundamental frequency. These higher modes are called overtones. Overtones include harmonics, but harmonics do not include overtones. Another confusing point is that the first overtone is not the fundamental. The second harmonic is the first overtone.