A more stable and repeatable calibration can be achieved by using an electrostatic actuator mounted on the microphone itself. This takes the form of a plate mounted very close to the microphone diaphragm, and normally replaces the standard protection grid. Some microphones incorporate the actuator in the weather protection system (e.g. rain cover). A variant is to electrically isolate the top plate of the standard microphone grille, so this doubles as the actuator (e.g. the MK255 capsule in the Svantek SV200).
A signal (typically at 1kHz) is supplied by a generator, via a special amplifier, to create an electrostatic modulation of the microphone diaphragm. This is effectively an acoustic signal, so it checks not only the integrity of the microphone, but also the sensitivity. This is similar to the way microphones are calibrated in the laboratory.
This method therefore requires the generator and amplifier to drive the actuator, and is completely separate to the signal and powering chain of the microphone. Outdoor microphones such as the MTG WME960H have all the necessary electronics integrated, and the actuation can be triggered by a simple contact closure on a serial port for example. This method is used in many long-term monitoring systems, such as the Sinus Swing.
The SV200 from Svantek is a complete outdoor noise analyser, and has the necessary system built-in. The electrostatic calibration can be triggered via a web page, either manually or automatically at predetermined intervals.
Although this method can be used as a ‘calibration’, the sensitivity of the system is not normally adjusted, but the levels logged to ensure accuracy of the results. Again, because the method requires external electronics and connections, it is not possible to do this via a single co-axial connection such as IEPE.