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A plenum chamber is essentially a type of ventilation or air transfer system. These air-filled chambers can be found on vehicles and machinery, or in the floors and ceilings of a room. The term plenum typically refers to a space with positive air pressure, where air pressure inside the space is greater than it is outside of the chamber. A plenum chamber can thus be pictured as the opposite of a vacuum chamber, which is characterized by negative air pressure, or interior pressure that is less than the surrounding air pressure. The positive pressure within a plenum generates natural forces that push air out, then replace it with fresh air from outside the chamber.
In homes and businesses, a plenum chamber is an alternative to traditional duct-based ventilation systems. Instead of relying on two separate networks of ducts to supply and return air, plenum systems utilize only a set of supply ducts. As these ducts pump fresh air into the room, stale air naturally exits through exhaust grills in the ceiling. This air enters the plenum space above the ceiling, where it is forced into a single exhaust vent and directed outdoors. This system helps to cut the cost of ventilation equipment and related ducts, and also results in reduced maintenance.
When used in engines and other machinery, the plenum chamber acts as an exhaust chamber to help cool the engine or motor. A positive-pressure plenum space is situated between the air intake assembly and the valves on the engine. Fresh air remains in the plenum until the valves open, allowing air into the engine. This allows each valve to open and receive fresh air as it's needed, resulting in enhanced performance and a long-lasting engine.
While the hovercraft may seem like a complex, space-age device, it actually relies on a simple plenum chamber to help it hover above the ground. The craft sits elevated above the ground, with a solid apron or casing around the perimeter acting as a support base. This apron also forms a plenum chamber between the bottom of the hovercraft and the ground. When a series of fans on top of the craft blow air into the plenum, it increases the pressure in this plenum, causing the craft to rise off the ground.
Plenum chambers have also been used in a variety of other applications. They are found in many large organs, where they help to create that characteristic sound associated with the organ. A plenum chamber below the floor or above the ceiling in a room can act as a sound barrier, making this type of design well-suited to recording studios. These chambers are also found at the base of kilns and certain types of furnaces, where they act as cooling chambers.
I have dealt with the Plenum Chamber with pneumatic wave machines in the swimming pool. The ones that actually makes the waves are extremely noisy.