Many types of input transducers can be found in a sound reinforcement system, with microphones being the most commonly used input device. Microphones can be classified according to their method of transduction, polar pattern or their functional application. Most microphones used in sound reinforcement are either dynamic or condenser microphones. One type of directional microphone, called "cardioid" mics, are widely used in live sound, because they reduce pickup from the side and rear, helping to avoid unwanted feedback from the stage monitor system.