MTS Calibration

Measurement Microphones

The microphones normally used for sound measurements are fully described in the excellent standard BS EN 61094, which has several parts, each dealing with specific qualities of Measurement Microphones.

Part 1 (1994) describes the construction and specifies the dimensions of the various types and sizes of measurement microphone, in particular for Laboratory Standard Microphones.

Part 4 (1995) extends the specification to Working Standard Microphones, and defines the WS-2 Microphone - the half-inch (nominal) diameter microphone which is normally used on precision sound level meters.

The sound level meter standards specify whether a free-field response microphone is to be used (a microphone designed to measure the sound pressure which would exist in the absence of its own interference with the sound field - with a sensitivity independent of frequency) or whether a pressure response microphone is to be used (one designed to measure the pressure at its diaphragm with a sensitivity independent of frequency).

The two types of microphone are used for different types of measurements: free-field microphones are specified by the European standards for sound measurements: pressure response microphones are specified for closed coupler measurements, for example in measurements on hearing aids or audiometers. By coincidence, it happens that the pressure response microphone provides the better approximation to the random incidence microphones specified in USA sound level meter standards.

MTS Calibration is able to calibrate any version or make of WS-2 microphone. Calibration using the electrostatic actuator method (as described in BS EN 61094-6: 2004) can produce an accurate calibration of any of the three microphone variants, with suitable corrections (normally readily available from the microphone manufacturer). A free-field calibration produces a direct measurement of the microphones response in a free acoustic field, as required by the sound level meter standards.

In 2005, MTS Calibration was the first UK laboratory outside the National Physical Laboratory to receive UKAS accreditation for the free-field calibration of a measurement microphone. The calibration is achieved by reference to a known WS-2 microphone with traceable response in a reference environment which achieves free-field conditions across the specified frequency range (50Hz to 20kHz) to provide a measurement of the object microphone sensitivity with documented uncertainties.

Calibration of WS-2 microphones is a service offered by MTS Calibration independently of sound level meter calibration, if required by the customer. However, every UKAS-accredited calibration of a sound level meter includes the accredited free-field calibration of its microphone.