Disclosed is a silicon condenser microphone including a backplate having a plurality of perforations therethrough, a diaphragm opposed from the backplate for forming a capacitor. Conditions may be identified from low frequency variations in a frequency spectrum measured on the line. The circuit also includes a volatile active dispenser to control volatile active emission rates. The driven plate 15 is fixed to the turbine 4. An optical apparatus includes the eyepiece lens, the objective lens and an erecting prism.