Organ pipes always play at the same volume.  The volume is controlled by swell shades placed between the organ pipes and the audience.  Swell shades look like giant wooden shutters.  They can be opened and closed by the "gas pedal" or swell shoe on the organ console.  The picture below shows the swell shoes of the Orpheum's Style 240 console.  The Style 216 has the same arrangement.  There is one swell shoe for each pipe chamber.  In the 216 and 240, which have two pipe chambers, there are two swell shoes.  These are the two leftmost pedals.

The rightmost pedal is the crescendo.  The crescendo pedal adds stops as it is pressed down, which is another way to increase the volume of an organ.  The crescendo pedal is slightly raised so the organist can distinguish it from the swell shoes by feel.  The swell and crescendo pedals are controlled with the right foot.  The right foot may also be needed to play upper pedal notes so it has to be able to find its way over a lot of territory.  Most often the organist will control both chambers together by centering the right foot  on the two swell pedals.  On larger organs with more than two chambers, there will be additional swell pedals.  On such organs, there is usually a general swell shoe just to the left of the crescendo to control all the swell shades together.

If you have a MIDI controller that generates MIDI Controller Change Messages, Controller 11 does the Main chamber, Controller 7 does the Solo chamber, and Controller 1 does the crescendo pedal.  You can also simulate the effect of using both swell shoes together by checking the "Use single swell pedal" at the bottom of the Settings panel.

  The crescendo pedal will be covered in a later part.  Effective use of the swell pedal adds considerably to a theatre organ performance.

When the swell shades are fully closed they reduce the sound but the organ is still quite audible.  Therefore, Miditzer does not send a zero volume message to the MIDI synthesizer when the MIDI expression is at zero.  You can adjust the minimum volume level for the zero expression input in the Settings panel

The swell shades reduce high frequencies more effectively than low frequencies.  As the swell shades open the organ will get brighter as well as louder.  The Miditzer sends signals to sound font devices to simulate this effect by altering the high pass filter setting of the synthesizer.  This works well with the Bruce Miles' cinema organ sound font.  It may interfere with the use of other sound fonts that use the high pass filter for other purposes.  The high pass filter can be disabled for such sound fonts. See the settings panel.