This phenomenon was first demonstrated in the late 19th century by Pierre Curie, who later won a Nobel prize with his wife Maria for their work on radiation, and his brother Jacques. Substances including crystals, some ceramics and bone produce an electrical voltage when a mechanical stress such as an impact is applied. In the context of electronic percussion, a piezo sensor is attached to a playing surface. When struck, the sensor produces an electrical pulse, the strength of which reflects that of the hit; this allows the drum module, or "brain", to reproduce the dynamics of the player's performance.
Most of Alesis' electronic drum sets consist of separate pads, each incorporating one or more piezo sensors, which can be arranged on a rack to mimic the layout of an acoustic kit. Some, like the high-end DM10 Pro, use Mylar drum heads like those on acoustic drums for the playing surface, since realism is a major goal in electronic drum design. Others, like the DM6, use rubber pads. Some pads are "dual-zoned", with two piezo sensors allowing separate head and rim voices like that of an acoustic snare drum.
Alesis' Surge cymbal pads have as many as three piezo sensors mounted on a real bronze cymbal, making them look and feel very much like normal cymbals. Like Mylar-headed drum pads, these include dampening to restrain the natural sound of impact. Cymbal pads made of rubber are also available. Some cymbal triggers feature a "choke" sensor, which mimics the drummer's ability to shorten a cymbal sound by grasping it with his hand.
Alesis also produces the Control and Performance Pads which, like the classic Roland Ocatapad, mount several pads on a single surface. This design is extremely compact, and, therefore, ideal for use with an acoustic kit. However, this also makes it particularly vulnerable to one of the piezo sensor's major weaknesses, "cross-triggering"; when one pad is struck, vibrations can travel to others, resulting in an accidental trigger. This problem is addressed by carefully isolating each pad from the others.
An alternative to piezo transducers used in, for example, Alternate Mode's drumKat is the Force-Sensing Resistor, a thin film of conductive material whose electrical resistance decreases when it is subjected to a force such as a drummer's stroke. This produces a brief spike in the amount of electricity which is able to pass through the FSR, which is then interpreted by the drum brain in the same way as the pulse from a piezo resistor. The FSR offers resistance to accidental triggering, as transmitted vibrations do not affect the film's resistance. However, it is less precise than the piezo transducer.