HARMONIUM. — This is a keyboard instrument whose sound is produced by free reeds without resonators, placed horizontally beneath the keyboard. The air is supplied by a bellows operated by two pedals. It derives from the ancient “regal,” a small portable organ with reed stops without resonators or with very small resonators. After the various experiments carried out by C. Perrault, S. Erald, C. Buffet, and Cavaille Coll, Debain (1809–77) constructed the first models of the present-day h. Subsequent makers introduced improvements: Vittorio Mustel (1815–90) may be called the father of the modern instrument.
It can be constructed according to two different systems: pressure or suction, depending on the different manner in which the air acts upon the reeds. The complete series of reeds producing a sound of a given timbre throughout the entire range of the keyboard is called a stop. The flute, clarinet, bourdon, oboe, English horn, bassoon, dulciana, aeoline, celeste voice, etc., are among the principal stops, which, with regard to their pitch, may be of 16, 8, 4, or 2 feet. The mechanical device by which the performer operates the stop and other mechanisms is called a “register.” The principal mechanical registers are: expression, a register of great aesthetic importance that gives the h. a particular character; percussion, extremely useful for the clear execution of passages of agility and pianissimos; fixed forte, which opens or closes a kind of shutter; expressive forte (Mussel), by means of which the shutter is opened to a greater or lesser degree by a small bellows, depending on the varying pressure exerted on the pedals; prolongation, by means of which one or more sounds in the low register can be lengthened indefinitely without hindering the free use of the two hands; super and sub octave; tremolo; mute. Two knee levers serve to operate a grille divided into two parts, one for the low register and the other for the high register of the keyboard, which opens or closes the sound. The two parts of the grille can be operated independently (double expression). Another device makes it possible to open all the stops simultaneously.
The most common type of h. has a single keyboard, but there are also models with two keyboards and a pedalboard. It is most commonly used to support choirs in church and in the school; but the concert harmonium is tending to become a concert instrument, having been enriched by an important repertoire consisting above all of works by C. Frank, L. Boëllmann, A. Guilmant, C. M. Widor, Max Reger, and S. Karg-Ellert. Alessandro Santini