Harmonium-Construction

Harmoniums consist of banks of brass reeds (metal tongues that vibrate when air flows over them), a pumping apparatus, stops for drones (some models feature a stop that causes a form of vibrato), and a keyboard. The harmonium's timbre, despite its similarity to the accordion's, is actually produced in a critically different way. Instead of the bellows causing a direct flow of air over the reeds, an external feeder bellows inflates an internal reservoir bellows inside the harmonium from which air escapes to vibrate the reeds. This design is similar to bagpipes, as it allows the harmonium to create a continuously sustained sound. (Some better-class harmoniums of the 19th and early 20th centuries incorporated an "expression stop" that bypassed the reservoir, allowing a skilled player to regulate the strength of the air flow directly from the pedal-operated bellows and so to achieve a certain amount of direct control over dynamics.) If a harmonium has two sets of reeds, it is possible that the second set of reeds (either tuned unison or an octave lower) can be activated by a stop, which means each key pressed plays two reeds. Professional harmoniums feature a third set of reeds, either tuned an octave higher or in unison to the middle reed. This overall makes the sound fuller. In addition, many harmoniums feature an octave coupler, a mechanical linkage that opens a valve for a note an octave above or below the note being played, and a scale changing mechanism, which allows one to play in various keys while fingering the keys of one scale.

Harmoniums are made with one, two, three or, occasionally, six sets of reeds. Classical instrumentalists usually use one-reed harmoniums, while a musician who plays for a qawaali(Islamic devotional singing) usually uses a three-reed harmonium.