More portable than pipe organs, reed organs were widely used in smaller churches and in private homes in the 19th century, but their volume and tonal range are limited, and they generally had one or sometimes two manuals, with pedal-boards being rare. The finer instruments have a unique tone, and the cabinets of those intended for churches and affluent homes were often excellent pieces of furniture. Several million reed organs and melodeons were made in the U.S. between the 1850s and the 1920s. During this time Estey Organ and Mason & Hamlin were popular manufacturers.
The melodeon is another reed keyboard instrument, usually housed in a table-like casing, that predates the pump organ. In reference to the music of India, melodeon usually refers to a concertina accordion, while harmonium means the smaller hand-pumped variety.
Christian Gottlieb Kratzenstein (1723–1795), professor of physiology at Copenhagen, was credited with the first free-reed instrument made in the Western world, after winning the annual prize in 1780 from the Imperial Academy of St. Petersburg. The harmonium's design incorporates free reeds and derives from the earlier regal. A harmonium-like instrument was exhibited by Gabriel Joseph Grenié (1756–1837) in 1810. He called it an orgue expressif (expressive organ), because his instrument was capable of greater expression, as well as of producing a crescendo and diminuendo. Alexandre Debain improved Grenié's instrument and gave it the name harmonium when he patented his version in 1840. There was concurrent development of similar instruments. A mechanic who had worked in the factory of Alexandre in Paris emigrated to the United States and conceived the idea of a suction bellows, instead of the ordinary bellows that forced the air outward through the reeds. The firm of Mason & Hamlin, of Boston, in 1860 made their instruments with the suction bellows, and this method of construction soon superseded all others in America.
Harmoniums reached the height of their popularity in the West in the late 19th and early 20th centuries. They were especially popular in small churches and chapels where a pipe organ would be too large or too expensive. Harmoniums generally weigh less than similar sized pianos and are not as easily damaged in transport, thus they were also popular throughout the colonies of the European powers in this period not only because it was easier to ship the instrument out to where it was needed, but it was also easier to transport overland in areas where good-quality roads and railways may have been non-existent. An added attraction of the harmonium in tropical regions was that the instrument held its tune regardless of heat and humidity, unlike the piano. This "export" market was sufficiently lucrative for manufacturers to produce harmoniums with cases impregnated with chemicals to prevent woodworm and other damaging organisms found in the tropics.
At the peak of the instruments' Western popularity around 1900, a wide variety of styles of harmoniums were being produced. These ranged from simple models with plain cases and only four or five stops (if any at all), up to large instruments with ornate cases, up to a dozen stops and other mechanisms such as couplers. Expensive harmoniums were often built to resemble pipe organs, with ranks of fake pipes attached to the top of the instrument. Small numbers of harmoniums were built with two manuals (keyboards). Some were even built with pedal keyboards, which required the use of an assistant to run the bellows or, for some of the later models, an electrical pump. These larger instruments were mainly intended for home use, such as allowing organists to practise on an instrument on the scale of a pipe organ, but without the physical size or volume of such an instrument. For missionaries, chaplains in the armed forces, travelling evangelists, and the like, reed organs that folded up into a container the size of a very large suitcase or small trunk were made; these had a short keyboard and few stops, but they were more than adequate for keeping hymn singers more or less on pitch.
The invention of the electronic organ in the mid-1930s spelled the end of the harmonium's success in the West (although its popularity as a household instrument declined in the 1920s as musical tastes changed). The Hammond organ could imitate the tonal quality and range of a pipe organ whilst retaining the compact dimensions and cost-effectiveness of the harmonium as well as reducing maintenance needs and allowing a greater number of stops and other features. By this time, harmoniums had reached high levels of mechanical complexity, not only through the need to provide instruments with a greater tonal range, but also due to patent laws (especially in North America). It was common for manufacturers to patent the action mechanism used on their instruments, thus requiring any new manufacturer to develop their own version; as the number of manufacturers grew, this led to some instruments having hugely complex arrays of levers, cranks, rods and shafts, which made replacement with an electronic instrument even more attractive.
The last mass-producer of harmoniums in North America was the Estey company, which ceased manufacture in the mid-1950s; a couple of Italian companies continued into the 1970s. As the existing stock of instruments aged and spare parts became hard to find, more and more were either scrapped or sold. It was not uncommon for harmoniums to be "modernised" by having electric blowers fitted, often very unsympathetically. The majority of Western harmoniums today are in the hands of enthusiasts, though the instrument remains popular in South Asia.
The acoustical effects described below are a result of the free-reed mechanism. Therefore, they are essentially identical for the Western and Indian harmoniums and the reed organ. In 1875, Hermann von Helmholtz published his seminal book, On the Sensations of Tone, in which he used the harmonium extensively to test different tuning systems:
"Among musical instruments, the harmonium, on account of its uniformly sustained tone, the piercing character of its quality of tone, and its tolerably distinct combinational tones, is particularly sensitive to inaccuracies of intonation. And as its vibrators also admit of a delicate and durable tuning, it appeared to me peculiarly suitable for experiments on a more perfect system of tones."
Using two manuals and two differently tuned stop sets, he was able to simultaneously compare Pythagorean to just and to equal-tempered tunings and observe the degrees of inharmonicity inherent to the different temperaments. He subdivided the octave to 28 tones, to be able to perform modulations of 12 minor and 17 major keys in just intonation without going into harsh dissonance that is present with the standard octave division in this tuning. This arrangement was difficult to play on. Additional modified or novel instruments were used for experimental and educational purposes. Notably, Bosanquet's Generalized keyboard, constructed in 1873 for use with a 53-tone scale. In practice, that harmonium was constructed with 84 keys, for convenience of fingering. Another famous reed organ that was evaluated was built by Poole.
Lord Rayleigh also used the harmonium to devise a method for indirectly measuring frequency accurately, using approximated known equal temperament intervals and their overtone beats. The harmonium had the advantage of providing clear overtones that enabled the reliable counting of beats by two listeners, one per note. However, Rayleigh acknowledged that maintaining constant pressure in the bellows is difficult and fluctuation of the pitch occurs rather frequently as a result.
In the generation of its tones, a reed organ is similar to an accordion or concertina, but not in its installation, as an accordion is held in both hands whereas a reed organ is usually positioned on the floor in a wooden casing (which might make it mistakable for a piano at the very first glimpse). Reed organs are operated either with pressure or with suction bellows. Pressure bellows permit a wider range to modify the volume, depending on if the pedaling of the bellows is faster or slower. In North America and the United Kingdom, a reed organ with pressure bellows is referred to as a harmonium, whereas in continental Europe, any reed organ is called a harmonium regardless of whether it has pressure or suction bellows. As reed organs with pressure bellows were more difficult to produce and therefore more expensive, North American and British reed organs and melodeons generally use suction bellows and operate on vacuum.
Reed organ frequencies depend on the blowing pressure; the fundamental frequency decreases with medium pressure compared to low pressure, but it increases again at high pressures by several hertz for the bass notes measured. American reed organ measurements showed a sinusoidal oscillation with sharp pressure transitions when the reed bends above and below its frame. The fundamental itself is nearly the mechanical resonance frequency of the reed. The overtones of the instrument are harmonics of the fundamental, rather than inharmonic, although a weak inharmonic overtone (6.27f) was reported too. The fundamental frequency comes from a traverse mode, whereas weaker higher traverse and torsional modes were measured too. Any torsional modes are excited because of a slight asymmetry in the reed's construction. During attack, it was shown that the reed produces most strongly the fundamental, along with a second transverse or torsional mode, which are transient.
Radiation patterns and coupling effects between the sound box and the reeds on the timbre appear not to have been studied to date.
The unusual reed-vibration physics have a direct effect on harmonium playing, as the control of its dynamics in playing is restricted and subtle. The free reed of the harmonium is riveted from a metal frame and is subjected to airflow, which is pumped from the bellows through the reservoir, pushing the reed and bringing it to self-exciting oscillation and to sound production in the direction of airflow. This particular aerodynamics is nonlinear in that the maximum displacement amplitude in which the reed can vibrate is limited by fluctuations in damping forces, so that the resultant sound pressure is rather constant. Additionally, there is a threshold pumping pressure, below which the reed vibration is minimal. Within those two thresholds, there is an exponential growth and decay in time of reed amplitudes .
The harmonium was somewhat embraced by European and American composers of classical music. It was also used often in the folk music of the Appalachians and South of the United States.
Harmoniums played a significant part in the new rise of Nordic folk music, especially in Finland. In the late 1970s, a harmonium could be found in most schools where the bands met, and it became natural for the bands to include a harmonium in their setup. A typical folk band then—particularly in Western Finland—consisted of violin(s), double-bass and harmonium. There was a practical limitation that prevented playing harmonium and accordion in the same band: harmoniums were tuned to 438 Hz, while accordions were tuned to 442 Hz.
The harmonium repertoire includes many pieces written originally for the church organ, which may be played on a harmonium as well, because they have a small enough range and use fewer stops. For example, Bach's Fantasia in C major for organ BWV 570  is suitable for a four-octave harmonium.
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The harmonium is popular to the present day, and the harmonium remains an important instrument in many genres of Indian music. For example, it is a staple of vocal North Indian classical music concerts. It is commonly found in Indian homes. Though derived from the designs developed in France, the harmonium was developed further in India in unique ways, such as the addition of drone stops and a scale-changing mechanism.
In Kolkata, Dwarkanath Ghose of the Dwarkin company modified the imported harmony flute and developed the hand-held harmonium, which has subsequently become an integral part of the Indian music scenario. Dwijendranath Tagore is credited with having used the imported instrument in 1860 in his private theatre, but it was probably a pedal-pumped instrument that was cumbersome or possibly some variation of the reed organ. Initially it aroused curiosity, but gradually people started playing it, and Ghose took the initiative to modify it. It was in response to the Indian needs that the hand-held harmonium was introduced. All Indian musical instruments are played with the musician sitting on the floor or on a stage, behind the instrument or holding it in his hands. In that era, Indian homes did not use tables and chairs. Also, Western music being harmonically based, both a player's hands were needed to play the chords, thus assigning the bellows to the feet was the best solution; Indian music, being melodically based, only one hand was necessary to play the melody, and the other hand was free for the bellows.
The harmonium was widely accepted in Indian music, particularly Parsi and Marathi stage music, in the late 19th century. By the early 20th century, however, in the context of nationalist movements that sought to depict India as utterly separate from the West, the harmonium was portrayed as an unwanted foreigner. Technical concerns with the harmonium included its inability to produce meend (slides between notes) and the fact that, once tuned, it cannot be adjusted in the course of performance. The former prevents it from articulating the subtle inflections (such as andolan, gentle oscillation) so crucial to many ragas; the latter prevents it from articulating the subtle differences in intonational color between a given svara in two different ragas. For these reasons, it was banned from All India Radio from 1940 to 1971; a ban still stands on harmonium solos. On the other hand, many of the harmonium's qualities suited it very well for the newly reformed classical music of the early 20th century: it is easy for amateurs to learn; it supports group singing and large voice classes; it provides a template for standardized raga grammar; it is loud enough to provide a drone in a concert hall. For these reasons, it has become the instrument of choice for accompanying most North Indian classical vocal genres, with top vocalists (e.g., Bhimsen Joshi) routinely using harmonium accompaniment in their concerts. However, it is still despised by some connoisseurs of Indian music, who prefer the sarangi as an accompanying instrument for khyal singing.
A popular usage is by followers of the Hindu and Sikh faiths, who use it to accompany their devotional songs (bhajan or kirtan). There is at least one harmonium in any mandir (Hindu temple) or gurdwara (Sikh temple) around the world. The harmonium is commonly accompanied by the tabla as well as a dholak. To Sikhs, the harmonium is known as the vaja or baja. It is also referred to as a peti (literally, box) in some parts of North India and Maharashtra. The harmonium plays an integral part in Qawwali music. Almost all Qawwals use the harmonium as their sole musical accompaniment. It has received international exposure as the genre of Qawwali music has been popularized by renowned Pakistani musicians, including Nusrat Fateh Ali Khan. There is some discussion of Indian harmonium makers producing reproductions of Western-style reed organs for the export trade.
Vidyadhar Oke has developed a 22-microtone harmonium, which can play 22 microtones as required in Indian classical music. The fundamental tone (Shadja) and the fifth (Pancham) are fixed, but the other ten notes have two microtones each, one higher and one lower. The higher microtone is selected by pulling out a knob below the key. In this way, the 22-shruti harmonium can be tuned for any particular raga by simply pulling out knobs wherever a higher shruti is required.
Bhishmadev Vedi is said to have been the first to contemplate improving the harmonium by augmenting it with a swarmandal (harp-like string box) attached to the top of the instrument. His disciple, Manohar Chimote, later implemented this concept, also making the instrument more responsive to key pressure, and called the instrument a samvadini—a name now widely accepted. Bhishmadev Vedi is also said to have been among the first to contemplate and design compositions specifically for the harmonium, styled along the lines of "tantakari"—performance of music on stringed instruments. These compositions tend to have a lot of cut notes and high-speed passages, creating an effect similar to that of a string being plucked.
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