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How Bends Work

In a normal bend on a diatonic harmonica, both reeds can participate in making the sound.  Consider a draw bend (blow bends work the same only the reeds are the other way around).  At first, the draw reed is doing most of the speaking.  As the bend gets lower the blow reed starts taking over, and at the bottom of the bend the blow reed is producing almost all of the sound.

Bending lowers the pitch of the natural note of the highest reed in the hole.  However, since both reeds participate in producing the bent note, the natural note of the lower pitch reed in the hole actually raises while the higher pitch reed lowers in pitch.  For example, for a draw bend the pitch of the draw reed gets lower while the pitch of the blow reed gets higher.

The note in a hole can be bent down to about a semitone higher than the lower pitched reed in the cell.

This is the best I've been able to determine on the physics of how bends work on a diatonic harp.

  1. The blow/draw air flow contains a broad spectrum of air compression wave frequencies.
  2. Each reed has a range of vibration frequencies to which it will respond.
  3. Resonance adds energy to a frequency because of reinforcing wave forms.
  4. The range of frequencies to which a reed will respond overlaps for both reeds in a cell.
  5. By adjusting the resonant frequency of the "playing tract" (tongue, mouth, throat, and other airways) we alter the frequency that has the most energy.
  6. This frequency with the most energy will dominate the random broad spectrum of frequencies produced by the blow/draw air flow.
  7. The reed will respond to the driving compression wave frequency with the highest energy.
  8. Both reeds in a 2-reed cell will respond to the same driving frequency because that driving frequency falls in the range of frequencies to with each reed will respond, that is, where the response frequencies overlap for the two reeds.
  9. So the note that sounds depends on the resonant frequency of the airway tract, subject to the mechanical response characteristics of the reed pair.
Thus, bends are induced by changing resonance characteristics in the vocal tract, and the reed vibration rate is coupled to the playing tract.

If one uses only mouth adjustments to the resonance chamber, the range of resonant frequencies is smaller than if adjustments to other parts of the airway are included.  When the other airways in the vocal tract, e.g. the throat and below, are tuned to the same resonant frequency as the mouth, this will accentuate the frequency energy advantage, and the bending range and tone of the note will improve.  This is why it is best to play "from the diaphragm", using as much of the vocal tract as possible.

One of the few scientific papers on the physics of harmonicas is at:
http://www.bs.monash.edu.au/staff/johno/acust2.html

Also, see the section on Harp Physics for some interesting experimental results.