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.
Thus, bends are induced by changing resonance characteristics in the vocal
tract, and the reed vibration rate is coupled to the playing tract.
The blow/draw air flow contains a broad spectrum of air compression wave
Each reed has a range of vibration frequencies to which it will respond.
Resonance adds energy to a frequency because of reinforcing wave forms.
The range of frequencies to which a reed will respond overlaps for both
reeds in a cell.
By adjusting the resonant frequency of the "playing tract" (tongue, mouth,
throat, and other airways) we alter the frequency that has the most energy.
This frequency with the most energy will dominate the random broad spectrum
of frequencies produced by the blow/draw air flow.
The reed will respond to the driving compression wave frequency with the
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.
So the note that sounds depends on the resonant frequency of the airway
tract, subject to the mechanical response characteristics of the reed pair.
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
One of the few scientific papers on the physics of harmonicas is at:
Also, see the section on Harp Physics
for some interesting experimental results.