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Playing Chromatically

The XB-40


High Notes on the XB-40 by Rick Epping, the XB-40 designer

The range of notes easily obtainable on an XB-40 is determined at its upper limit by the resonant frequency of the highest chambers relative to the resonant frequency of the reeds occupying those chambers. The smaller the air space within a reed chamber, the higher its resonant frequency will be and the higher pitched will be the reed that can be effectively played there. The chambers in the highest holes of the XB-40 have been designed as small as possible given the clearance requirements for the reeds and valves as well as the requirements of the injection molding process by which the comb is manufactured.

Difficulty begins to arise with the XB-40 keys of C and higher, when the resonant frequency of the reeds in Hole 10 exceeds that of their chamber and the lower frequency of the chamber begins to pull the oscillating frequency of the reeds downward in a phenomenon known as mode locking or frequency pulling. The greater the disparity between the higher frequency of a reed and the lower frequency of its chamber, the more difficulty there will be in obtaining good performance. It is for this reason that the key range for the XB-40, like that of the chromatics, is set lower than that of the standard, single-reed diatonics (C/C# is the highest chromatic key while high G is the highest diatonic key). The highest reed in a Hohner Richter 10-hole model, the g4 reed from Hole 10 of a high G harp, would play flat and with some difficulty if set over a Model 270 or 280 chromatic's highest chamber, which is larger and has a lower resonant frequency than that of a Hole 10 Richter chamber. The difficulty would be greater still were that same reed set over the highest chamber of an XB-40, which is actually a double chamber - the inner valve chamber combined with either the upper blow reed chamber or the lower draw reed chamber, depending on the direction of airflow. While the Hole 10 reeds in the lower XB-40 keys play easily, those in the highest few keys play with some difficulty as their resonant frequency is higher than the resonant frequency of the Hole 10 double chamber.

Technique can to some extent minimize this problem. For high notes on harmonicas with small reed chambers, the player's tongue can be positioned either forward in the mouth to create a resonant frequency in the reed chamber/mouth cavity matching that of the reed being played or down and back in the mouth to avail of the higher partials of the full vocal tract. On the higher keys of the XB-40 however, the player might not be able to position the tongue forward enough to reach the resonant frequency of the reed being played, resulting in an unintentional bend or a choked note. If the tongue is instead positioned back in the mouth to create a fundamental resonant frequency one octave lower than that of the reed, the second partial of this larger mouth cavity will then match that of the reed and allow it to play. From this position the tongue can be shifted upward and forward for a bend. Using this technique, the reeds in Hole 10 on an XB-40 in the key of C are playable though relatively stiff compared to the reeds in the lower holes due to their exceeding the resonant frequency of their reed chamber.

In conclusion, while the reeds in Hole 10 on the lower keys of the XB-40 are quite playable they are less easily so for the key of C where their frequency is somewhat higher than that of the reed cell. By the highest key of Eb, the reeds in Hole 10 may be effectively beyond the range of practical playability. While adjustment of technique can extend the range of playable high notes, it should be recognized that this range is limited in the highest keys of XB-40, where the highest reeds are pitched well above the resonant frequency of their reed chamber.