Over the past few weeks, we’ve taken a look at the pipe organ, including the different sounds the pipes reproduce. But what actually makes the sound we all hear? Vibrating air. In a small room on the east side of the choir loft is a big air blower. This blower forces a large volume of low pressure air into a large box beneath the floor of the choir loft called a “plenum.” All of the pipes of the organ sit atop this giant air reservoir and are connected to it in some way. When the organist depresses a key or pedal, the corresponding valve opens, and air from the plenum rushes up the desired pipe, producing the sound using the same vibrating air column physics as an ordinary whistle. Since an organ’s sound is produced by pure acoustical energy, its volume cannot be controlled by a switch or dial. For this reason, some of the pipes are enclosed in the wooden box in the center of the choir loft known as the Swell Box. The Swell Box has shutters on the front of it that the organist can open and close to varying degrees with a foot pedal to adjust the volume as he or she desires. In the hands of a talented organist, this seemingly lifeless collection of wood, steel, fabric, and air comes alive to create the beautiful music we hear every week at Mass.