Harmonica
Overblowing is an important modern technique among players of some harmonica types, notably the standard Richter-tuned harmonica or blues harp. Combined with note bending, it yields the full chromatic scale across the instrument's range. Though pioneered on Richter-tuned harps, overblowing, or the related overdrawing, is possible on any harmonica having both a blow reed and a draw reed mounted in the same airway (i.e., behind the same mouthpiece hole), but no windsaver valve on the higher-pitched of the two reeds. While superficially resembling in its pitch-jumping effect the overblowing of other (beating-reed, aerophone, brass) wind instruments, harmonica overblowing is completely unrelated from the standpoint of the underlying physics. It does not induce the sounding reed to sound a higher overtone – free reed overtones do not even begin to approximate the harmonic series nor are they particularly musical – nor does it induce a higher vibrational mode in air in a pipe or other resonator – harmonicas generally have no such resonator. Rather, it silences the sounding reed while eliciting sound from the formerly silent one – the one that normally responds to air flowing in the opposite direction. A key fact for understanding both overblowing and bending on such an instrument: a free reed mounted over a reedplate slot will normally respond to air flows that pull it initially into the slot, i.e., as a closing reed, but, at only slightly higher air pressure from the opposite side, will also respond as an opening reed; the resulting pitch is generally just less than a semitone higher than the closing-reed pitch.
Overblown notes can be played as softly as any other note on the instrument. Proper embouchure alone will cause the closing reed to cease vibrating and induce the opening reed to start. Overblow notes are naturally flat but can be bent up to the correct pitch. An overblow consists of two steps: the closing reed must be choked (silenced), and the opening reed must be sounded. A clean overblow note requires that both of these steps be executed simultaneously. The steps can be learned separately: Remove both cover plates from an old Richter-tuned harmonica, use tape to block the draw reeds of holes 1-6 and blow reeds of 7-10, and reassemble the harp. Play any single note. Without blowing (or sucking) unduly hard, try to choke off that reed's sound. You may wish to try almost forming the letter K, or G way back in your throat, or on the high-hole draw reeds, pushing your tongue slightly forward. However you do it, when you can easily and completely silence the reed in every hole, then again disassemble the harp, remove the tape, tape the opposite reeds – 1-6 blow and 7-10 draw – and put cover plates back on. Now use your single-note technique while trying to play the reed that normally does not sound. Do not blow (or suck) hard. Try the same embouchure changes as above. When you can produce clear sound, both soft and loud, and can bend the pitches slightly upward on all 10 holes, then remove all tape, reassemble the harp, and try overblowing holes 1-6 and overdrawing holes 7-10. Overblowing technique also has been described as not much different than doing a blow bend, except on a draw-bend-only reed (holes 1-6), and doing a draw bend embouchure, except on a blow-bend-only reed (holes 7-10). The latter technique is also known as the "overdraw" due to the reversed airflow, and these techniques are sometimes collectively referred to as "overbends".
Certain modifications to factory-built harmonicas can increase the sensitivity of the instrument and make overblows far easier to achieve. Lowering the reed gap (over the reedplate) and slightly narrowing reed slots (a process called embossing) are probably the most common customization methods used to set up overblow-friendly harmonicas. Because it involves both reeds in the chamber, overblowing is not possible on fully valved harmonicas (including chromatic harmonicas or the Hohner XB-40, a harmonica with discrete reed chambers and extra sounding reeds).
Notable practitioners of overblowing are Howard Levy, a founding member of the Flecktones, Otavio Castro, Chris Michalek, Jason Ricci, and Carlos del Junco.
Read more about this topic: Overblowing