Bending and Overblowing
Everyone knows the unique sound of note bending, that evocative pitch modulation effect which is probably the main reason for the lasting popularity of the diatonic harmonica. But it used to be a mystery, a secret known only to the initiated. The Harp Handbook (Steve Baker 1990, Hal Leonard LLC) contained the first description in harmonica literature of how and why note bending works.
Here’s today’s version:
Harmonica reeds are mounted on one side of the reed plate and sit over the slot, slightly offset from it. The reeds on the inside of the upper reed plate sound when you exhale, the reeds on the outside of the lower reed plate sound when you inhale. They’re usually referred to as “blow” and “draw“ reeds and every channel contains one of each, of different pitches. In both cases, the airstream pushes the reed tip through the slot until it’s elasticity causes it to spring back. The frequency of its passage through the slot determines the pitch of the note. This is called a “closing reed“, because the gap between reed tip and slot closes when the reed starts to move. Here’s a diagram showing a section through the harmonica from the side:
What was not generally known at that time, however, was that reeds can also vibrate in a reverse airstream. Under the right conditions, it’s therefore possible to get any reed to sound by making it first move away from the slot, before it springs back to pass through it. This type of reed behaviour is referred to as an “opening reed“ and the pitch it produces is higher than its pitch as a closing reed.
If the higher-pitched of the two reeds in any channel is closing, it can be bent down by changing the shape of the vocal tract to correspond to a lower pitch. Then the lower-pitched reed will start to open, sounding together with the higher-pitched closing reed. This enables an accomplished player to bend the higher of the two notes downwards in a glissando or slide, until the sound is coming almost entirely from the opposing, opening reed. This sounds at a slightly higher pitch than when closing, so you can bend the higher note in any hole down to a point slightly above the pitch of the the lower note. It isn’t possible to bend any further, so every bend has a fixed lower limit. Both draw and blow bends function this way. Because in holes 1-6 the draw note is the higher-pitched of the two, those draw notes can be bent downwards. Because in holes 7-10 the blow note is the higher-pitched of each pair, in those holes the blow note can be bent downwards. When bending, the two reeds in each hole work together as a dual reed system, making it possible to hit bent notes directly, without first playing the unbent note. This is how you play upward slides.
Bending = higher-pitched reed is closing, lower-pitched reed is opening
If this relationship is reversed, it’s also possible to persuade the higher-pitched of the two reeds in any channel to function as an opening reed, producing a note above its pitch as a closing reed. When this happens, the opposing (lower-pitched) reed doesn’t sound, even though the airstream would normally cause it to vibrate as a closing reed. The note produced by the higher-pitched of the two reeds in any channel when sounding as an opening reed is called an overblow or overdraw. Both are also referred to collectively as “overbends”. You can play overbends by blowing in holes 1-6 and by drawing in holes 7-10. This technique enables the player to produce all missing notes of the chromatic scale which cannot be created by bending. Overbends are however less stable than regular notes or bends and it takes a lot of practice to hit and sustain them. Many overblow players use custom harps, which make the technique easier to execute.
Overbending = higher-pitched reed is opening, lower-pitched reed is silent
Because an opening reed can sound over a range of pitch above its lowest point, overbends can be bent upwards in a similar way to how bends work, but because the reed is sounding in isolation, there is no fixed upper limit and accomplished players can bend overbends up by a considerable amount. Any reed can function both as a closing reed and as an opening reed, and will produce notes of different pitches in each case. The combination of bending and overbending permits the skilled player to produce all notes of the 12 tone chromatic scale, as well as countless microtonal notes which lie in between the keys of the piano, including slides or glissandi. This is what makes the diatonic harmonica such a unique instrument.
To put it in a nutshell:
Closing reeds can be bent downwards from their highest possible pitch when sounding in isolation
Opening reeds can be bent upwards from their lowest possible pitch when sounding in isolation
Of course this doesn’t tell you how to do it. Both bending and overbending are executed by modulating the shape of the tongue and vocal tract to correspond to the wavelength of the altered note, thus creating the acoustic conditions where these notes can resonate. There are many instructional videos online which purport to tell you how to approach learning this. Here’s a link to my video on basic bending technique:
Illustrations are from The Harp Handbook, published by Hal Leonard LLC