How a pressure release valve organ works
In the world of mechanical music instruments there are several ways to record music and play it again. The data medium can be in different forms. In music boxes you often find a metal barrel with pins. In old hand cranked organs, there is a wooden barrel with brass pins and bridges. In Dutch street organs you will find cardboard books with rectangular holes. As the cardboard moves, metal pins fall into these holes and cause valves to open, allowing air into the corresponding pipes. In German organs you find cardboard books or paper rolls with round holes. When air flows through these holes , it causes a valve to move and open the holw which leads to the pipe. There are two systems in use: the pressure system and the pressure release system. We will look more closely at the last one.
An organ needs air under pressure. A pair of opposite acting bellows is used to provide constant air pressure. In the above animation, the air under pressure is coloured blue. The bellow spring provides the air pressure by compressing the bellows, there is a pressure relief valve in the top of the bellows to prevent over inflation. The pressurised air is fed into a wind chamber. In this wind chamber there are air valves with membranes. The key to the operation is the air pressure above and below the membrane. If there is a no note to be played, the air pressure is equal though the bypass channel. When a note is required the hole in the paper roll releases the air pressure on the upper side and the membrane in the valve will move upwards though the pressure of the wind chamber. The air channel to the pipe is opened and there will be a tone produced. With the adjusting screw you can control the wind for correct valve working.