How is the sound produced in a modern pipe organ?

Wind

Deep in the basement is the electrically driven blower. Air from the blower is conveyed to reservoirs, or bellows, where it is held under constant pressure.

The various sections of the organ operate on different pressures: the softest pipes on the lowest pressure and the loud trumpet pipes on the highest pressure.

Stops & Keys

There are several systems by which the stops and keys can be connected to the pipes. The Auckland Town Hall organ uses an electro-pneumatic system: a combination of solid-state components, electromagnets and wind operated motors.

The organist sits at the console and controls the organ's sound by first pulling out a selection of stop knobs which are located in rows at either side of the keyboards. This causes electromagnetic devices to move sliders beneath the feet of pipes of the selected ranks to enable them to sound; holes bored in the sliders (wind-way) are lined up with the base of the pipes and the selected pipes are in standby mode, ready to receive the wind that enables them to sound.

Stop knob pulled out

Electric motors move the slider under the rank of pipes

A rank of pipes is ready to make sound

The organist then presses the keys on a keyboard. Further electromagnetic circuits are engaged, activating mechanisms (motors) in the wind chest which open valves (pallets) under the pipes. The stored wind then enters the pipes making them sound for as long as the organist holds down the keys.

The organist presses a key

Windchest magnets convert the electric signal from the console into mechanical movement

Note action inside the windchest

One or more pipes sound

Soft & Loud

The organ cannot be made to sound louder or softer by varying the pressure on the keys like a piano, or by putting down a loud or soft pedal. Instead, it is all to do with building harmonic pyramids in the various choruses. It works like this:

The basic pitch of the organ is the same as the piano and is represented in stops or ranks labelled 8 foot, which is the length of the longest pipe in the rank. In building a tonal pyramid, one adds a 4ft stop (an octave higher), a 2ft stop (two octaves higher), a 16ft stop, (an octave lower than 8ft), non unison stops such as a Twelfth (2 2/3ft), and finally Mixtures where several very small pipes sound at once to complete the pyramid and glue all the harmonics together. Therefore, by playing just one note, many pipes speak. In a two-handed chord of eight notes, 100 pipes can speak at once producing a sound unique to the organ. No electronic substitute, no matter how sophisticated, can produce this sound.

Apart from the building of such pyramids, a large palette of musical colours is available by mixing stops of various pitches and types. Loud stops on one keyboard can be played against the soft stops on another. Echo effects can be produced by moving from loud stop combinations on one keyboard to a softer combination on another. There are almost limitless possibilities.

Stop knobs are labelled with numbers for the basic pitch of the stop. Roman numerals (eg "IV") indicate a mixture - more than one rank together.

Different ranks have different length pipes, to sound the different pitches

The loudest pipes in the organ: the Solo Organ's Tubas and Orchestral Trumpet at the top level of the organ.

Soft and loud effects are also obtained by opening and closing the shutters on swell boxes. When the swell box is closed, less sound can escape and the organ is quieter. The Town Hall organ has three swell boxes, each controlled by a rocker pedal above the pedalboard. This is a direct mechanical connection: no electricity involved.

The Quality of the Sound

This is determined by the organ builder. Each pipe is hand made and the organ builder's skill determines how it will sound by itself and in combination with the other pipes. Like Stradivari in the world of stringed instruments, the exceptional organ builder is revered. And like Stradivari, the organ builders' instruments can last hundreds of years. Only the very best materials are used.

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