Calibration services are available for microphones, pistonphones and acoustic calibrators, sound level meters and filters, blast and noise monitors, accelerometers, vibration meters and monitors, geophones and velocity transducers, shock and impulse systems, portable calibration exciters and shakers, electronic filters and conditioning amplifiers, other forms of instrumentation and specialised measurements, ultrasonics and underwater acoustics. Click here:
Before consigning an instrument for calibration always consult us to discuss your requirements. For further information contact calibration.coordinator@measurement.gov.au.
Capacitor microphones including the 1-inch LS1P and ½-inch LS2P, as defined in IEC 61094-1 (2000–07), are calibrated by coupler reciprocity at the preferred frequency of 250 Hz for a single point reference, or at the secondary frequencies of 500 Hz and 1000 Hz. The uncertainty of a reciprocity calibration of a test microphone with the primary standard set is generally less than 0.05 dB depending on microphone type.
Microphones that cannot be calibrated by reciprocity with the primary standard set can be calibrated by comparison with an auxiliary standard microphone in an acoustic coupler. The frequency range is from 16 Hz to 2 kHz for 1-inch and from 16 Hz to 25 kHz for ½-inch microphones and the measurement uncertainty is generally less than 0.1 dB depending on microphone type.
The electrostatic actuator provides a means of measuring microphone frequency response. The technique uses an electrostatic force between the diaphragm and a grid placed over it. The electrostatic force simulates the action of a sound field closely approximating to a pressure response. The technique is normally restricted to 1-inch or ½-inch microphones where it is possible to gain access to the diaphragm. At NMI special adaptors are used to also allow calibration of other types including the IEC LS1P or LS1F and LS2P or LS2F types and the so called ¼-inch microphones. Electrostatic actuator calibrations are normally performed at third octave frequencies between 20 Hz and 20 kHz but may be extended to 100 kHz for ¼-inch microphones. The uncertainty would generally be less than 0.2 dB at frequencies up to 20 kHz but will depend on type.
Low frequency microphones can be calibrated at frequencies between 1 Hz and 250 Hz by comparison with a low frequency reference microphone in a special coupler. This low frequency reference microphone is calibrated using the electrostatic actuator technique. An assurance test is available to check the function of low frequency microphones at a set number of frequencies and amplitudes. Checks of amplitude linearity may be carried out up to 140 dB SPL, usually at 31.5 Hz.
For free field measurements, NMI maintains the free field primary reference standard consisting of a LS2P capacitor microphone. This microphone was initially calibrated by free field reciprocity technique as well as by pressure reciprocity, and its stability is periodically established by calibration using pressure reciprocity. Free field calibration of clients' microphones is carried out in an anechoic chamber between 31.5 Hz and 20 kHz by substitution with a working reference microphone which is traceable to the free field primary reference standard.
Pistonphones and acoustic calibrators are normally calibrated using laboratory standard microphones calibrated by the reciprocity technique. An insert voltage technique is used whereby the electrical output of the microphone, when used in the calibrator, is compared to an accurately measured ac voltage applied in such a way as to simulate the microphone output. The known open circuit sensitivity of the microphone allows a value for the output of the calibrator under test to be established. For best results it is recommended that the client advise NMI of the type of microphone that will be used with the device.
For pistonphones and acoustic calibrators that use other types of microphones, a substitution technique using a variable sound source is used. Using laboratory standard microphones calibrated by reciprocity, and an insert voltage technique, the variable source is set up to the nominal value of the device under test. The comparison is made using the microphone type specified for the device. For accurate results, it is recommended that the client submit an instrument with which the pistonphone or acoustic calibrator is intended to be used. This will be used as the detector/microphone in the substitution process.
Use of a pistonphone or calibrator on an instrument for which it was not designed or for which there is no calibration may lead to error.
The calibrators are tested with reference to AS IEC 60942 (2004).
Sound level meters are calibrated according to the tests and tolerances described in AS 1259.1-1990 Sound Level Meters, Part 1: Non-integrating and AS 1259.2-1990 Sound Level Meters, Part 2: Integrating-averaging. In addition, tests devised by NMI are used for integrating/averaging sound level meters which may include a statistical analyser function. The list of functions and characteristics tested are based on OIML R 58 and OIML R 88 and may include:
Most of these tests are performed using electrical signals input via a substitute 'dummy' microphone. AS 1259.1-1990 specifies that an alternative input to the microphone be available on the sound level meter for this purpose.
Band pass octave and one-third octave filters intended for the analysis of sound and vibration are calibrated according to the specifications in AS/NZ 4476-1997.
Resistive attenuators and sine generators for use in acoustic measurements can also be tested over a range of attenuations, from 0 to 120 dB. It is usual to make these tests at three frequencies in the audible range, e.g. 31.5 Hz, 1 kHz and 12.5 kHz. The preferred impedance is 600 Ω unbalanced.
Low frequency microphones used to monitor blast overpressures can be calibrated by comparison with suitable reference microphones in a special coupler capable of completely enclosing both the test and reference devices. Such microphones can be examined at a suitable number of frequencies in the range 1 to 250 Hz and at selected amplitudes.
Instrumentation and special outdoor microphones used for environmental noise monitoring are calibrated between 31.5 Hz and 20 kHz by comparison with a free-field reference microphone. The NMI anechoic chamber is used for these free-field measurements.
NMI is mainly concerned with the calibration of reference grade accelerometers and accelerometers that are used for precise or critical measurements of vibration. Most popular makes and models of accelerometers are catered for. A calibration of an accelerometer is performed either by comparison with the response characteristics of a calibrated NMI standard, or by applying absolute measurement techniques through laser interferometry. Typically an accelerometer is calibrated with its associated charge amplifier or power supply, comparing its voltage output with the acceleration measured with our standard or laser interferometer, for a number of frequencies and acceleration amplitudes. The normal units of this type of calibration is millivolts per metre per second squared (mV/(m/s2)) or millivolts per standard gn (mV/gn, where gn = 9.80665 m/s2). Charge accelerometers can also be calibrated without a conditioning amplifier, in this case the sensitivity of the accelerometer is expressed in picocoulombs per metre per second squared or per gn (pC/(m/s2) or pC/gn).
A vibration meter is usually regarded as any vibration measurement system that provides a direct output of the vibration level monitored on an instrument display, chart recorder, printer, oscilloscope or any other means of presenting information to an operator. A vibration monitor is usually equipment that logs readings of vibration levels over long periods of time. Calibration of these instruments is generally carried out in a similar manner to calibrating an accelerometer as detailed above, with a reading made from the vibration meter or monitor for each calibration frequency and amplitude.
Horizontal and vertical omni-directional, and uni-directional velocity transducers and geophones can be calibrated up to a frequency of 250 Hz. The limitation of mass for vertically orientated transducers is 0.5 kg, and horizontal transducers is 5 kg. Velocity transducers are usually calibrated against NMI reference accelerometers. The normal calibration units of geophones and other velocity transducers is the millivolt per millimetre per second (mV/(mm/s)).
Calibration of accelerometers with charge amplifiers set to provide an output proportional to velocity, can be calibrated as an accelerometer. The unit of this type of calibration would be mV/(mm/s).
Calibration and verification of compliance with the requirements of SAE J211b, SAE J211/1 and ISO 6487 for shock measuring systems which include accelerometers and filtering units. Calibration of shock and impulse systems is performed using steady state excitation and compared with an NMI standard. Nominal units of shock systems is mV/(m/s2) or mV/gn, and relative response in decibels (dB).
Measurement of instrumented hammers can also be undertaken.
Portable calibration exciters and shakers can be calibrated by comparison with an NMI reference accelerometer. A typical calibration would involve ascertaining the excitation amplitude and drive frequency of the shaker for up to two or three external mass loadings (normally 20, 60 and 90 g). Results are usually quoted as drive excitation in acceleration (m/s2), velocity (mm/s) and displacement peak (µm).
Normally a calibration is performed on complete systems including transducers, amplifiers and any associated filters. However a charge amplifier or filter can be calibrated independently of a transducer. This is typically done where a number of settings or configurations for an amplifier (such as range and filter settings) are required to be used with a single transducer. In this case a normal calibration of the transducer with the amplifier is performed for one setting of the amplifier/filter. The amplifier is then calibrated independently of the transducer for one or more additional configurations. Charge amplifiers are calibrated by comparing a known charge, from a calibrated standard capacitor, to the amplifier's output. The units of a charge amplifier calibration are typically millivolt per picocoulombs (mV/pC). For independent calibration of electrical filters, the results are usually given as a ratio, or decibels (dB), of the input voltage compared to the output voltage.
Specialised or unusual acoustic or vibration instrumentation may be accepted for testing. These include dosimeters where access is available to the microphone input and detector output.
Contact calibration.coordinator@measurement.gov.au for general inquiries on the measurement aspects of other applications such as sonar and medical ultrasound.
BH Meldrum and S Thwaites (1994) Primary Microphone Calibration at CSIRO Australia, American Institute of Physics Handbook of Condenser Microphones, Chapter 12 BH Meldrum, KJ Hews-Taylor and NH Clark (1994) High-pressure Low-frequency Microphone Calibration at CSIRO Australia, American Institute of Physics Handbook of Condenser Microphones, Chapter 15