Microphone checks and calibration for remote microphone systems
Condenser microphones are extremely stable and sensitive transducers when used in their normal operating conditions. When used for long-term noise monitoring, they can be exposed to more extreme environments (e.g. high temperatures, high windspeeds, moisture, etc) which may cause a change in sensitivity, and even damage. The nature of this type of monitoring often precludes a site visit to calibrate the microphone in the normal way, such as using a sound level calibrator or pistonphone mounted directly on the microphone capsule.
We therefore need a method of checking that all is well at the microphone end, so this article describes two methods that can be used.
The first is SysCheck from GRAS, which simply injects an electrical signal into the microphone circuit, via the preamplifier, to check the signal path integrity. This includes the microphone capsule itself, which means that any change in the resulting measured signal can be used to deduce if the microphone capacitance has changed, which might be an indicator of damage (e.g. damage or corrosion on the diaphragm, or physical damage). A similar method is also called charge injection calibration (CIC).
It’s important to note that this is not a ‘calibration’ as such, it is simply a means to check that nothing has changed out of tolerances, which can be preset by the vibration measurement system. It is not an acoustical signal.
In order to inject the signal, an additional connection is required on the preamplifier, and this is normally available via an industry standard 7-pin Lemo connector, used by many GRAS power supplies, such as 12AK, 12AA etc. These power supplies include the signal generator, and will activate the signal when the Syscheck button is pressed, or remotely by software trigger.
This method therefore precludes a simple co-axial connection to the
microphone, such as IEPE, which is used solely to provide power to the preamplifier, and return the measured signal. Some front-ends have Syscheck built-in, such as Apollo from Sinus, where the Samurai software can provide the necessary signal on the correct pin of the Lemo connector.
For this method, a suitable preamplifier must be used, such as the 26AJ from GRAS, and this feature is also built-in to the GRAS 41AC-2 Environmental Microphone.
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).