Let's interpret the diagram! Remember that 100ct correspond to one halftone. See also the note names at the right margin. Each single reed can be a closing or an opening reed, which is indicated bei red and blue.
The eigen frequency of the draw reed D lies above the eigen frequency of the blow note C. This is true for holes #1 to #6 and is reversed for holes #7 to #10. This is a decisive observation. For holes #7 to #10 there should be an extra diagram (you may work out one for your own)!
- A closing reed will sound below its eigen frequency. This happens playing normal notes or bends. Mind that it is possible to blow bend or draw bend each reed on the whole harmonica if (!) the respective other reed plate is taped so that one is playing with single reeds.
- An opening reed will sound above its eigen frequency, which allows for overblows and overdraws. It is possible to bend such overbends to higher pitches.
Single reeds are perfectly described by the above rules of thumb. The frequency bands below resp. above eigen frequency allow for continuously bending down the normal note resp. bending up the lowest possible overbend.
Although it was emphasized above that always both reeds are collobarating, the diagram shows only major contributions for the sake of clarity. We will nevertheless always discuss the role of both reeds.
With one exception (no blow bend!) the diagram for coupled reeds looks like the "sum" of the two diagrams for single reeds. Similar diagrams exist for channels #7 to #10. The physically motivated idea of closing and opening reeds allows for a unified and simple discussion of normal blows, normal draws, blow bends, draw bends, overblows, and overdraws!
- Normal notes come mainly from the closing reeds, which sound somewhat below its respective eigen frequencies. For a normal D, the second reed (C) is opening and thus can oscillate with the higher frequency. For a normal C, the second reed (D) should be opening, so it should not join an oscillation with lower frequency. Actually measured oscillations are not very strong, but they exists. We must accept that this observation shows the limitations of our simple physical assumptions and models.
- Draw Bends can be explained with ease: The upper reed D is closing, so it can sound with lower frequencies. At the same time the lower reed C is opening, so it can join oscillations with higher frequencies. Within the region of the two overlapping frequency bands one can bend normal D continuously down. One further remark: It is obviously only a rule of thumb that the pitch of the lowest draw bend is one half step above the pitch of the lower reed - this depends on the instrument and on the player.
Comparing with the diagrams for the two single reeds, one might be tempted to describe a draw bend on coupled reeds as draw bending the draw note and overdrawing the blow note. This is especially true for the lowest draw bend which comes mainly from the blow reed.
It is not as easy to bend blow notes on the upper channels #7 to #10 continuously down. Playing the normal note as a starting point with small volume between teeth and tongue may help.
Overblows and Blow bends
: The lower reed C is closing
, so one should expect low
blow bends (as for a single reed). Instead the upper reed D "wins" in playing an overblow (or an upward bended overblow) as an opening
reed with higher
frequency. There is no explanation for this preference of overblows within our physical model. Nor can the small (but existing) oscillations of the lower (closing) reed above its eigen frequency be explained.
Starting with a normal note C and playing a smooth transition up to Eb is impossible, because closing reed C has to sound below normal C, while opening reed D has to sound above normal D. Therefore overblows seem to "pop up".
A look at the diagramm shows that normal C lies a scant below plucked C, plucked D lies two semitones above plucked C, and closing reed D sounds a scant above eigen frequency of D. Summing up results in a scant minor third, so the player has to bend this overblow note a little bit to play an Eb.