Blues-harp

There are numerous techniques available for harmonica, including bending, overbending, and tongue blocking.

In addition to the 19 (draw 2 and blow 3 are the same pitch even though there are 10 holes) notes readily available on the diatonic harmonica, players can play other notes by adjusting their embouchure and forcing the reed to resonate at a different pitch. Although it is notoriously difficult and can be frustrating for beginners, one does this by relaxing and coordinating muscles in the throat, mouth, and lips. This technique is called "bending", a term borrowed from guitarists, who literally "bend" a string in order to create changes in pitch. Using bending, a player can reach all the notes on the chromatic scale. "Bending" also creates the glissando characteristic of much blues harp and country harmonica playing. Bending on a guitar bends the pitch upward. However, typically "bending" on a harmonica means the pitch falls downward. Bends are essential for most blues and rock harmonica due to the soulful sounds the instrument can bring out. The famous "wail" of the blues harp typically required bending.

The physics of bending are quite complex, but amount to this: a player can bend the pitch of the higher-tuned reed down toward the pitch of the lower-tuned reed in any given hole. In other words, on holes 1 through 6, the draw notes can be bent and on holes 7 through 10 the blow notes can be bent. Hole 3 allows for the most dramatic bending: in C, it is possible to bend 3 draw from a B down to a G♯, or anywhere in between.

In the 1970s, Howard Levy developed the "overbending" technique (also known as "overblowing" and "overdrawing".) Overbending, combined with bending, allowed players like Chris Michalek, Carlos del Junco, Otavio Castro and George Brooks to play the entire chromatic scale. When bending, the player forces the lower of the two reeds in a chamber to vibrate faster, while the higher pitched reed vibrates slower. When overbending, the player isolates the higher of the two reeds and by so doing can play higher pitched notes. By using both bending and overbending techniques a player can play the entire chromatic scale using a diatonic harmonica. This has allowed diatonic harmonica players to expand into areas traditionally viewed as inhospitable to the instrument such as jazz.

The overbend is a difficult technique to master. To facilitate overbending, many players use specially modified or customised harmonicas. Any harmonica can be set up for better overbending. The primary needs are tight tolerances between the reed and reed-plate and a general level of air-tightness between the reed-plate and comb. The former often necessitates lowering the "gap", the space between the tip of the reed and the reed-plate. Another often used technique called embossing is to make the space between the sides of the slots in the reed-plate and the reed itself as small as possible by drawing in the metal on the sides of the reed-plate slots towards the reed. While these modifications make the harmonica overbend more easily, overbending is often possible on stock diatonic harmonica, especially on an airtight design.

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Wikipedia