How does the Indian harmonium produce its lively music?

The Hindu

7 days ago

The Indian portable harmonium is a close cousin of the larger foot-pumped organs once found in European churches and homes. It's a wooden box roughly the size of a small suitcase. You sit on the floor behind it and pump a pair of bellows with one hand and play piano-like keys with the other. All these instruments work on the same simple idea: air is pushed past metal strips called reeds; when they vibrate, musical notes are created. The sound of the harmonium has been a staple of both Indian folk music and mainstream cinema.
How does a harmonium use air?
A harmonium doesn't have strings, membranes or electronic circuits. Instead, its 'fuel' is air in motion.
When the bellows attached to the instrument are pulled open, they suck some of the air in the room into their folded chambers, funnelled through inlet valves. When you push the bellows closed, the air is squeezed forward into an airtight wooden compartment that lies directly under the harmonium keys.
Because the compartment's walls don't move, the air pressure within rises above normal atmospheric pressure by a small amount, usually a few kilopascals. The compartment also holds a slender internal spring or a weighted flap that helps maintain this pressure, even if you don't pump the bellows at a regular rate.
In fact, as long as you pump the bellows every few seconds, the reeds will feel enough pressure to make sounds.
How does a harmonium make sound?
The keyboard on top of the harmonium resembles a piano in miniature. But unlike in a piano, each key is only a lever. If you press a key, its far end tilts upward inside the box, lifting a pallet lined with felt.
The pallet covers a hole that leads from the pressurised wooden compartment to a single metal reed. When the key is at rest, the pallet closes the hole and no air flows. When you press the key, the pallet opens, allowing high-pressure air to rush through the hole and towards the reed.
Most Indian harmoniums expose 1.5 to 3 reeds to each hole. A stop rod next to the keyboard allows you to choose which set of reeds, called a bank, is active at any time. If you slide out one stop rod, an extra airway opens so the same key can expose a second bank that is tuned to one octave higher or lower, creating a more organ-like tone.
How does each reed operate?
Every reed is essentially a tongue of brass or phosphor-bronze nailed on top of a rectangular slot in a metal frame. When air pressure rises on the front side, the tongue bends slightly into the slot, allowing a puff of air to slip past to the rear. The same puff now exerts pressure on the back side of the tongue, pushing it forward again.
This rapid seesawing motion sets up a vibration with a frequency of hundreds of hertz for higher notes and around 100 Hz for the lower ones.
The vibrating reed shreds the air stream into pulses, which bounce around inside the box before spreading into the room as sound waves. Because the reed is fixed at one end and free at the other, its pitch depends mainly on its length, thickness, and mass. The shorter or thinner the reed, the higher its natural frequency will be.
Unlike flutes or trumpets, the harmonium's reeds sit inside a wooden cavity rather than an open pipe. As a result, larger cavities produce lower notes and vice versa. The wooden compartment, the leather that makes the bellows, and even the player's lap all absorb or reflect certain frequencies, giving each instrument a unique timbre.
Because warmer air is less dense, reeds in such an atmosphere will also have a slightly higher pitch. Professional players thus often carry small screwdrivers to tweak the reeds' sounds before a performance.
Why does the sound seem 'alive'?
The harder you pump the bellows, the louder the sounds will be. This is simply because stronger pumping raises the pressure in the compartment and moves the reeds harder. Fluent players often use quick pulses on the bellows to produce sharp accents and ease off to create graceful decrescendos. Many designs also include a coupling lever that, when engaged, connects a key to the key one octave higher, so pressing a single note automatically depresses its octave partner without forcing the player's fingers to stretch. Second, because the reed's vibrations feed on the pumping, the instrument can be made to respond to the smallest motions. Players can brighten a note by pumping in a short burst or stall it to thin the note.
More fundamentally, the sounds of an Indian harmonium seem 'alive' because the instrument doesn't run on the clinical power of electrical energy but on a human body in motion. This may also explain why it became a staple of Indian classical, devotional, and folk music, including its willingness to accompany singers outdoors, through power cuts, and even play through the humid monsoons.