Integrated 3D Sound Intensity Sensor with Four-Wire Particle Velocity   Sensors
  A new symmetrical four-wire sensor configuration has resulted in a fully integrated sound intensity sensor with significant lower noise floor and smaller size than its predecessors. An integrated sound pressure sensor was further miniaturized by using a folded "back chamber" at both sides of the chip. 
Doekle Yntema, Joost Van Honschoten, Remco Wiegerink