You will need to occasionally calibrate your microphone to be confident in its readings. You may already have an acoustic calibrator or pistonphone, or you will have purchased one with your meter. This will need to be professionally calibrated once per year with a certified calibration laboratory. If your acoustic calibrator has a valid certificate, you can calibrate the 97x meters in the Function > Calibration > Calibration by Measurement part of the meter’s menu. For the 958 series, usually Channel 4 is the sound channel. Enter the level of the tone being emitted from the calibrator (usually 94dB or 114dB – check the manual for the instrument) and place the calibrator firmly on the microphone. Take care not to carry this procedure out with high background noise.
If you need a formal instrument calibration carried out, which is recommended every 18-24months, please contact firstname.lastname@example.org with your requirements.
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 three methods that can be used.
The SV307 outdoor noise monitor is fitted with a MEMS microphone, for additional ruggedness and reliability. As such, none of the other two methods can be used. In the case of the SV307, a small microspeaker is installed in the weather protection, which generates an acoustical signal which can be used for a calibration check.
The SV307 also includes the possibility of sending alerts if the measured level is outside tolerances, or there is another issue with the patented microphone system.
In this way, you can be sure that measured results are valid.
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.
SysCheck or Charge Injection Calibration 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).
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 measuring system. It is not a traceable 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 MTG power supplies. 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 this calibration method built-in, such as Apollo from Sinus, where the Samurai software can provide the necessary signal on the correct pin of the Lemo connector.