In order to create a musical sound in a wind instrument the player blows air into some sort of excitation mechanism. For the clarinet and saxophone this consists of a single piece of reed mounted onto the mouthpiece with a small gap between. Increasing the air pressure in the mouth cavity initiates an oscillatory regime with the reed opening and closing the gap, periodically letting air flow into the instrument to support pressure standing waves in the air column. The oboe and bassoon feature a double reed, with the gap between two pieces of cane regulating the flow.
When brass musical instruments are played, the player's lips are stretched across the mouthpiece. The lips behave much like the reeds of woodwind instruments, therefore brass instruments are said to have a lip reed mechanism. The operation of this lip reed can be understood by considering the forces acting on the lips and the pressure field in the instrument.
There are two simplified models which help to explain lip reed action, called the inward and outward striking reeds . Obviously a brass player's lips will blow open if the mouth cavity pressure is sufficiently large. A reed which follows this behaviour is called the outward striking reed. On the other hand a high pressure in the mouth will lead to air moving in the gap between the lips, lowering the pressure there. The lips are thus sucked together, experiencing a force known as the Bernoulli force. This is what we refer to as an inward striking reed. The actual nature of the lip reed is the study of current research, as reviewed in Campbell . The results seem to suggest that the lip reed can behave more like inward or outward striking reeds depending on the resonance frequencies of the lips and of the air column.