Buy Harmonium online, sale of harmonium. A Harmonium is also called Reed Organ. It is a keyboard instrument that produces sound when wind sent by bellows through an air reservoir causes metal reeds screwed over slots in metal frames to vibrate through the frames with close tolerance. The pitch is determined by the size of the reed. Separate sets of reeds provide different tones. Volume is controlled by an air valve or directly from the bellows that allows the wind supply to bypass the reservoir. The instrument’s compass is normally four to five octaves.