The sound of an organ is generated by a constant flow of air that is blown through the different pipe sizes. The sound is made either by the air being blown through a narrow gap against a lip, as is the case with the recorder, or by a vibrating reed inside the pipe, as is the case with the clarinet.
Until about a century ago, the compressed air required for the so-called organ wind used to be generated by bellows that were operated with the feet. On large instruments, up to 12 people were needed to perform this strenuous task. Only with the emergence of electricity did organ building make increased use of electric bellows, which, compared to the foot-operated ones, generated a very constant and quiet air flow. But it was always a challenge to design the electric wind generator so that it would not be heard either during quiet pieces of music.
When it comes to restoring old instruments, however, organ builders try wherever possible to keep the historical bellows in order to preserve the original sound of the organ. Electric motors are often used to move the bellows up and down, so the wind for an organ is generated automatically instead of by a human hand. This was precisely how the historical organ at the College of Catholic Church Music and Musical Education in Regensburg, which was built in 1752 by Antonio Pilotti from Bologna in typical Italian style, was restored – by installing an automatic bellows unit.