Secondary Pressure and Free-Field Calibration of Measurement Microphones
NPL Acoustics Group's secondary calibration services are suitable for customers who require traceable microphone calibration, but not the extremely high precision of measurement attained from calibration by reciprocity technique. Calibration involves direct comparison of the customer's microphone with one of NPLs own reciprocity calibrated reference microphones in order to determine the frequency-dependent sensitivity. Both pressure and free-field calibration is available by such methods. Services are available for laboratory standard and working standard microphones, though secondary pressure calibration is restricted to IEC type LS2 and WS2 microphones only. These services are UKAS accredited.
Pressure comparison calibration is available over the frequency range 63 Hz to 20 kHz, and is carried out in an active coupler or NPL's reverberation chamber depending on the frequency; this method typically gives an uncertainty of 0.07 dB.
Free-field comparison calibration is available between 31.5 Hz and 12.5 kHz and is carried out by sequential comparison in NPL's free-field room (anechoic chamber) or plane-wave duct; this method typically gives an uncertainty of 0.2 dB.
Measurements are traceable through the calibration of the reference microphone, to reference electrical quantities such as volts and ohms, and to length, mass and time derived elsewhere at NPL. An NPL Certificate of Calibration will be issued giving the sensitivity in one-third octave bands, enabling traceability to UK national standards to be demonstrated. Re-calibration at yearly intervals is recommended to maintain this traceability.
A Complete Service
The specification and calibration of measurement microphones is covered in IEC Standard 61094. NPL plays an active role in the development of this series of standards and offers a complete range of calibration services to satisfy the needs of product testing, research and development and manufacturing. For a full list of calibration services offered for measurement microphones, as well as details of other sound-in-air related services, see the Acoustics In Air Measurement Services page, or make an enquiry using the details below.
Contact
Customer Services tel: +44 20 8943 8631
E-mail: acoustics_enquiries@npl.co.uk