Course:PHYS341/Archive/2016wTerm2/HarmoniumPitchBending
Harmonium Pitch Bending
Harmoniums are a part of the Free-Reed instrument family, along with harmonicas, pump-organs, and accordions. The reed is a small metal strip that is fastened at one end and free at the other. The free end sits above a hole that is just big enough for the reed to be able to pass through when vibrating up and down.
Harmonium
In a harmonium there is a different reed for every key on the keyboard, and separate divided columns for each reed [1]. Unlike the organ however, the reeds within the instrument are excited by air produced by pressure bellows rather than suction bellows. Bellows are commonly operated by pedals on the floor in Western harmoniums, and by a hand pump in different Eastern variations [2]. This means that there needs to be a large amount of air produced and pushed past all the reeds in order to produce sound from multiple keys at once. Air passing the reeds excites them into vibrating which creates the sound of the harmonium. In most western free-reed instruments the reed sits on top of a plate with a hole in it that allows air to escape and pressure to decrease when the reed is forced downwards [1].
The actual sound that is produced is a result of two conjoining forces. The vibration of the reed itself creates sound through its resonating body on the soundboard that is the heart of the harmonium. The second is the periodic stutter of air that is released through the reed as it vibrates, contributing to the timbre of the instrument [3].
The harmonium’s initial design was to create long lasting single notes that were achieved by the mechanically generated pressure forcing its way past the reeds. Commonly this was produced in the form of foot pedals, and then adapted as the instrument was adopted in India. There it was adapted to be played while sitting on the ground, with the addition of a hand pump instead of pedals. Either way, it was not designed to create wavering melodies, it was meant for sustained frequencies, or long notes [3].
Achieving Pitch Bending
When playing a harmonica, experienced musicians can control pitch of the available notes by varying the amount of air they force through the instrument as well as controlling their vocal tract (the resonator of the harmonica). This is not as easily achieved with a harmonium, or any other free-reed instrument, because the air flow is mechanically operated [4]. Even though it is more difficult, it isn’t impossible, and the same techniques apply to pitch bending a harmonium.
Varying Air Pressure
Varying the amount of air pressure being forced through the valves of a harmonium can change the frequency in which the reed vibrates, which will change the pitch of the instrument. This is achieved by either pushing the pedals or pumping the hand pump harder and more frequently. When pressure is increased in a free-reed instrument, the frequency (somewhat counterintuitively) decreases. This is because the pressure being applied to the reed becoming stronger results in the reed staying in its downward position longer, reducing frequency [5].
Partial Pallet Opening
The pallet valve controls the airflow to the reeds and is attached to the key on the keyboard, and when the key is pressed, the valve opens, allowing air to flow past the reed. This has a very similar effect as the varying air pressure because it’s doing the same thing in a different way. By depressing the key at different levels the amount of air that is passing the reed is changing, which will impact the frequency at which the reed vibrates. In fact, the less depressed the key is, the greater the frequency is capable of changing. Also comparable to the varying pressure technique is that the frequency decreases when the keys are less depressed. This is due to the fact that less air is entering the reed cell meaning the reed itself is not being excited as much, causing it to vibrate less [5].
Decreasing Reed Chamber Volume
The Reed Chamber is area of space in which the reed sits, and where air pressure accumulates once it is being forced past the reed. By decreasing the size of the reed chamber, more air is able to accumulate quicker when it passes the reed, meaning there is more pressure on the opposite side of the reed pushing it back. This causes the pressure to even out on either side of the reed, resulting in a decrease in frequency of the reed. In fact this has the greatest effect of the three techniques, however in a classical harmonium there is no built in adjustment feature that allows the user to change this parameter, so it can only be achieved by modifying the instrument [5].
The Problem with Pitch Bending
There is one main issue in pitch bending a harmonium because of its free reed system. The free reed system requires a base level of energy to excite each individual reed into vibration, and if there is not enough energy being put into the system, no sound will be created. Inversely, if there is too much energy being forced past the reed, then the material’s own restoring force is not enough to combat the wind pushing against it, causing the reed to vibrate less. This means that although it is possible to bend the pitch of a free reed instrument with a mechanically operated air vent, it can only be done in very slight amounts. The only way to allow for further pitch bending is to increase the resonating cavity, which is common for organs with large stacks of resonating pipes [4].
In summary, pitch bending on a Harmonium is scientifically possible, but when it comes to the practicality, it can only be achieved in very minimal amounts through partial pallet opening, varying air pressure, and decreasing the reed chamber volume. This shows that the instrument's original design and purpose (sustained single notes) is very difficult to overcome.
References
- ↑ 1.0 1.1 . Hilaire, Arthur O. St, Theodore A. Wilson, and Gordon S. Beavers. "Aerodynamic Excitation of the Harmonium Reed." Journal of Fluid Mechanics 49.04 (1971): 803-16. University of British Columbia Library. Web. 5 Mar. 2017
- ↑ Earl, S. G. Repairing the Reed Organ and Harmonium. Braintree: Organ Literature Foundation, 1960. HathitTrust. Web. 5 Mar. 2017.
- ↑ 3.0 3.1 Gellerman, Robert F. The American Reed Organ. 2nd ed. N.p.: Vestal, 1997. Print.
- ↑ 4.0 4.1 Cottingham, James P. "Acoustics of Free-Reed Instruments." Physics Today 64.3 (2011): 44-48. Physics Today. American Institute of Physics, 2011. Web. 5 Mar. 2017.
- ↑ 5.0 5.1 5.2 Cottingham, James P. Reed Vibration and Pitch Bending in Western Free Reed Instruments. Stanford: Stanford University, 15 Feb. 2013. PPT.