Swell pedal

An expression pedal is an important control found on many musical instruments including organs, electronic keyboards and pedal steel guitar. The musician uses the pedal to control different aspects of the sound, commonly volume. Separate expression pedals can often be added to a guitar amplifier or effects unit and used to control many different aspects of the tone.

Because the source of power with a pipe organ and electronic organs is not generated by the organist, the volume of these instruments has no relationship with how hard its keys or pedals are struck; i.e., the organ produces the same volume whether the key or pedal is depressed gently or firmly. Moreover, the tone will remain constant in pitch, volume, and timbre until the key or pedal is lifted, at which point the sound stops. The expression pedal gives the organist control over the external source of power, and thus the volume, of the instrument, while leaving his or her hands free.

This system of dynamic control is completely distinct from the act of adding stops (in the case of pipe organs) or pulling more drawbars (in the case of digital organs and synthesizers). Furthermore, the expression pedal can influence the volume (and, to a lesser degree, the timbre) of a note while it is being played; unlike other instruments, in which the note typically decays after it is first sounded, the organist can increase the strength of a chord or note as it sounds by increasing pressure on the expression pedal.

An organ expression pedal is typically a large pedal, resembling an oversized automobile accelerator, either partially or fully recessed within the organ console and located either directly above or to the right of the organ's pedalboard. As the pedal is pressed forward with the toes, the volume of the sound is increased; as it is depressed with the heel, the volume is decreased. A stand alone expression pedal used with electronic keyboards, amplifiers, and effects is usually a smaller pedal made of metal or plastic that can be placed on the floor and then connected to the device with an instrument cable.

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Wikipedia