Whenever you play two pitches at the same time, additional pitches are produced, called combination tones, ghost tones or Tartini tones - the lower one is the difference or differential tone; the upper one is the summational tone. The frequency of the difference tone is the difference between the frequencies of the two original pitches; the frequency of the summational tone is the sum of the frequencies of the original tones.
So, if you play a note of 440Hz at the same time as one of 660Hz, you will get additional pitches of 220Hz and 1100Hz. Or, to put it in musical terms, if you play an A at the same time as the E above it, you will get a pitch equal to one octave below the original A and a C# above the E. Similar "ghost" notes are produced by other intervals. On a justly intonated harmonica they are harmonically related to the generating pitches, but on a tempered tuning the combination tones are "out of tune" and often produce very harsh effects (except of course when playing octaves, or less tempered intervals such as fourths or fifths). This happens with all instruments, but the effect is particularly noticeable with the harmonica.
On a harmonica, the difference tones are usually much stronger than the summational tones, although the summational tones contribute to the overall timbre or tonal quality. In fact, in the top octave of the harp, the difference tones are almost as loud as the original notes. The difference tones can be greatly emphasised by the player's technique, particularly if there is a mike and amplifier involved - the use of combination tones is one of the "secrets" to getting a big amplified tone. If you alter the frequencies of the original notes by bending, the pitch of the difference tone will also change. This can produce an interesting sound, but it can also be quite distracting. The classic example of difference tones in an amplified blues harp context is by Corky Siegel on the Jimmy Reed tune "Hush Hush", from the album "The Siegel-Schwall Band", where his release of a bend on the 3 draw and 4 draw notes causes the difference tone to dive downwards in pitch:
The easiest way to demonstrate them is to take a high pitched harp - an F harp is ideal. Play first 7 draw, then 8 draw. Now play both notes together. Depending on how well the harp is tuned, you should be able to hear a note which sounds an octave lower than 2 draw on the same harp - that is the difference tone.
Here is an audio demonstration. Here is 7 draw on an F harp; here is 8 draw; here is 7 and 8 draw played together. You should be able to hear the difference tone at this pitch, an octave below the pitch of 2 draw on the same harp. This example was played on a justly intonated harp. If it were played on a harp with a tempered tuning, the difference tone would be slightly flatter in pitch.
For more about difference tones, please visit this page.
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