Measurement microphone calibration
Encyclopedia
In order to take a scientific measurement with a microphone
, its precise sensitivity must be known (in volt
s per Pascal
). Since this may change over the lifetime of the device, it is necessary to regularly calibrate
measurement microphones. This service is offered by some microphone manufacturers and by independent certified testing labs. All microphone calibration is ultimately traceable to primary standard
s at a National Measurement Institute such as the National Physical Laboratory in the UK, PTB
in Germany and NIST in the USA, where the reciprocity calibration (see below) is the internationally recognised means of realising the primary standard. Laboratory standard microphones calibrated using this method are used in-turn to calibrate other microphones using comparison calibration techniques (‘secondary calibration’), referencing the output of the ‘test’ microphone against that of the reference laboratory standard microphone.
A microphone’s sensitivity varies with frequency
(as well as with other factors such as environmental conditions) and is therefore normally recorded as several sensitivity values, each for a specific frequency band (see frequency spectrum
). A microphone’s sensitivity can also depend on the nature of the sound field it is exposed to. For this reason microphones are often calibrated in more than one sound field, for example a pressure field and a free field. Depending on their application, measurement microphones must be tested periodically (every year or several months, typically), and after any potentially damaging event, such as being dropped or exposed to sounds levels beyond the device’s operational range.
for calibration of measurement microphones. The technique exploits the reciprocal nature of certain transduction mechanisms; measurement microphones are usually capacitor microphones (also called condenser microphones) which exhibit this behaviour. Reciprocity calibration is carried out using an acoustical coupler (to give the microphone’s pressure response), but can also be implemented in a free field
(to give the free-field response). In order to carry out ‘pressure field’ reciprocity calibration, three uncalibrated microphones A, B and C are used. Microphones A and B are placed facing each other in an acoustical coupler designed to create a cylindrical cavity between their diaphragms
, allowing the space to be easily modelled. One of the microphones is then driven electrically to act as the source of sound, and the other responds to the pressure generated in the coupler, producing an output voltage. Provided that the microphones are reciprocal in behaviour (the open circuit sensitivity in V/Pa as a receiver is the same as the free sensitivity in m³/s/A as a transmitter), it can be shown that the combined sensitivity product
of the coupled microphones is given by the ratio of electrical transfer impedance to acoustical transfer impedance. The first is measured in the calibration procedure, and the second deduced from transmission line analysis. Having determined the sensitivity product for one pair of microphones, the process is repeated with the other two possible pair-wise combinations (AC and BC). The set of three sensitivity product measurements then allows the individual microphone sensitivities to be deduced by solving three simultaneous equations. The technique provides a measurement of the sensitivity of a microphone without the need for comparison with another previously calibrated microphone, and is instead traceable to reference electrical quantities such as volts and ohms
, as well as length
, mass
and time
. Although a given calibrated microphone will often have been calibrated by other (secondary) methods, all can be traced (through a process of dissemination
) back to a microphone calibrated using the reciprocity method at a National Measurement Institute. Reciprocity calibration is a specialist process, and because it forms the basis of the primary standard for sound pressure, many national measurement institutes have invested significant research efforts to refine the method and develop calibration facilities. A system is also commercially available from Brüel & Kjær
.
For airborne acoustics, the reciprocity technique is currently the most precise method available for microphone calibration (i.e. has the smallest uncertainty of measurement). Free field
reciprocity calibration (to give the free-field response, as opposed to the pressure response of the microphone) follows the same principles and roughly the same method as pressure reciprocity calibration, but in practice is much more difficult to implement. As such it is more usual to perform reciprocity calibration in an acoustical coupler, and then apply a correction if the microphone is to be used in free-field conditions; such corrections are standardised for laboratory standard microphones (IEC/TS 61094-7) and are generally available from the manufacturers of most of the common microphone types.
A common method of comparison calibration requires the two microphones to be placed facing each other, closely spaced so that the pressure on both of the microphones’ diaphragms can be assumed to be equal at the frequencies of interest (the audible range). Both microphones are subjected to a broadband noise signal from a loudspeaker
(typically white noise
) so as to include every frequency of interest, and the electrical output signals from both microphones are subject to frequency analysis, resulting in the frequency response
of the microphones to the specific sound they were exposed to. By this method the ratio of sensitivities of the two microphones can be determined. This can then be multiplied by the known sensitivity of the calibrated reference microphone in order to obtain the absolute sensitivity of the test microphone. Comparison calibration may be carried out in a free-field environment (typically in an anechoic chamber
), a diffuse field (a reverb chamber) or a coupler, depending on the application the microphone it is intended for.
mechanically driven to move at a specified cyclic rate, pushing on a fixed volume of air to which the microphone under test is coupled. The air is assumed to be compressed adiabatically and the sound pressure level in the chamber can, potentially, be calculated from internal physical dimensions of the device and the adiabatic gas law, which requires that PVγ is a constant, where P is the pressure in the chamber, V is the volume of the chamber, and γ is the ratio of the specific heat of air at constant pressure to its specific heat at constant volume. Pistonphones are highly dependant on ambient pressure (always requiring a correction to ambient pressure conditions) and are generally only made to reproduce low frequencies (for practical reasons), typically 250 Hz. However, pistonphones can be very precise, with good stability over time.
However, commercially available pistonphones are not calculable devices and must themselves be calibrated using a calibrated microphone if the results are to be traceable; though generally very stable over time, there will be small differences in the sound pressure level generated between different pistonphones. Since their output is also dependent on the volume of the chamber (coupling volume), differences in shape and load volume between different models of microphone will have an influence on the resulting SPL, requiring the pistonphone to be calibrated accordingly.
Sound calibrators are used in an identical way to pistonphones, providing a known sound pressure field in a cavity to which a test microphone is coupled. Sound calibrators are different to pistonphones in that they work electronically and use a low-impedance (electrodynamic) source to yield a high degree of volume independent operation. Furthermore, modern devices often use a feedback mechanism to monitor and adjust the sound pressure level in the cavity so that it is constant regardless of the cavity / microphone size. Sound calibrators normally generate a 1 kHz sine tone; 1 kHz is chosen since the A-weighted SPL is equal to the linear level at 1 kHz. Sound calibrators should also be calibrated regularly at a nationally accredited calibration laboratory to ensure traceability. Sound calibrators tend to be less precise than pistonphones, but are (nominally) independent of internal cavity volume and ambient pressure.
Microphone
A microphone is an acoustic-to-electric transducer or sensor that converts sound into an electrical signal. In 1877, Emile Berliner invented the first microphone used as a telephone voice transmitter...
, its precise sensitivity must be known (in volt
Volt
The volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the...
s per Pascal
Pascal
Pascal or PASCAL may refer to:-People:* Pascal , a French given name* Pascal , a French and Italian surname* Adam Pascal , American actor and singer, best known for his role of Roger Davis in the Broadway musical Rent* Blaise Pascal , French mathematician and philosopher* Cleo Paskal, environmental...
). Since this may change over the lifetime of the device, it is necessary to regularly calibrate
Calibration
Calibration is a comparison between measurements – one of known magnitude or correctness made or set with one device and another measurement made in as similar a way as possible with a second device....
measurement microphones. This service is offered by some microphone manufacturers and by independent certified testing labs. All microphone calibration is ultimately traceable to primary standard
Primary standard
A primary standard in metrology is a standard that is accurate enough that it is not calibrated by or subordinate to other standards. Primary standards are defined via other quantities like length, mass and time. Primary standards are used to calibrate other standards referred to as working...
s at a National Measurement Institute such as the National Physical Laboratory in the UK, PTB
Physikalisch-Technische Bundesanstalt
The Physikalisch-Technische Bundesanstalt is based in Braunschweig and Berlin. It is the national institute for natural and engineering sciences and the highest technical authority for metrology and physical safety engineering in Germany....
in Germany and NIST in the USA, where the reciprocity calibration (see below) is the internationally recognised means of realising the primary standard. Laboratory standard microphones calibrated using this method are used in-turn to calibrate other microphones using comparison calibration techniques (‘secondary calibration’), referencing the output of the ‘test’ microphone against that of the reference laboratory standard microphone.
A microphone’s sensitivity varies with frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
(as well as with other factors such as environmental conditions) and is therefore normally recorded as several sensitivity values, each for a specific frequency band (see frequency spectrum
Frequency spectrum
The frequency spectrum of a time-domain signal is a representation of that signal in the frequency domain. The frequency spectrum can be generated via a Fourier transform of the signal, and the resulting values are usually presented as amplitude and phase, both plotted versus frequency.Any signal...
). A microphone’s sensitivity can also depend on the nature of the sound field it is exposed to. For this reason microphones are often calibrated in more than one sound field, for example a pressure field and a free field. Depending on their application, measurement microphones must be tested periodically (every year or several months, typically), and after any potentially damaging event, such as being dropped or exposed to sounds levels beyond the device’s operational range.
Reciprocity Calibration
Reciprocity calibration is currently the favoured primary standardPrimary standard
A primary standard in metrology is a standard that is accurate enough that it is not calibrated by or subordinate to other standards. Primary standards are defined via other quantities like length, mass and time. Primary standards are used to calibrate other standards referred to as working...
for calibration of measurement microphones. The technique exploits the reciprocal nature of certain transduction mechanisms; measurement microphones are usually capacitor microphones (also called condenser microphones) which exhibit this behaviour. Reciprocity calibration is carried out using an acoustical coupler (to give the microphone’s pressure response), but can also be implemented in a free field
Anechoic chamber
An anechoic chamber is a room designed to stop reflections of either sound or electromagnetic waves.They are also insulated from exterior sources of noise...
(to give the free-field response). In order to carry out ‘pressure field’ reciprocity calibration, three uncalibrated microphones A, B and C are used. Microphones A and B are placed facing each other in an acoustical coupler designed to create a cylindrical cavity between their diaphragms
Diaphragm (acoustics)
In the field of acoustics, a diaphragm is a transducer intended to faithfully inter-convert mechanical motion and sound. It is commonly constructed of a thin membrane or sheet of various materials. The varying air pressure of the sound waves imparts vibrations onto the diaphragm which can then be...
, allowing the space to be easily modelled. One of the microphones is then driven electrically to act as the source of sound, and the other responds to the pressure generated in the coupler, producing an output voltage. Provided that the microphones are reciprocal in behaviour (the open circuit sensitivity in V/Pa as a receiver is the same as the free sensitivity in m³/s/A as a transmitter), it can be shown that the combined sensitivity product
Sensitivity (electronics)
The sensitivity of an electronic device, such as a communications system receiver, or detection device, such as a PIN diode, is the minimum magnitude of input signal required to produce a specified output signal having a specified signal-to-noise ratio, or other specified criteria.Sensitivity is...
of the coupled microphones is given by the ratio of electrical transfer impedance to acoustical transfer impedance. The first is measured in the calibration procedure, and the second deduced from transmission line analysis. Having determined the sensitivity product for one pair of microphones, the process is repeated with the other two possible pair-wise combinations (AC and BC). The set of three sensitivity product measurements then allows the individual microphone sensitivities to be deduced by solving three simultaneous equations. The technique provides a measurement of the sensitivity of a microphone without the need for comparison with another previously calibrated microphone, and is instead traceable to reference electrical quantities such as volts and ohms
Ohms
OHMS may refer to:* The plural of ohm, a unit of resistance, named after Georg Ohm* Ohm's Law of electric currents, first proposed by Georg Ohm* O.H.M.S., On His/Her Majesty's Service...
, as well as length
Length
In geometric measurements, length most commonly refers to the longest dimension of an object.In certain contexts, the term "length" is reserved for a certain dimension of an object along which the length is measured. For example it is possible to cut a length of a wire which is shorter than wire...
, mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
and time
Time
Time is a part of the measuring system used to sequence events, to compare the durations of events and the intervals between them, and to quantify rates of change such as the motions of objects....
. Although a given calibrated microphone will often have been calibrated by other (secondary) methods, all can be traced (through a process of dissemination
Dissemination
To disseminate , in terms of the field of communication, means to broadcast a message to the public without direct feedback from the audience. Dissemination takes on the theory of the traditional view of communication, which involves a sender and receiver...
) back to a microphone calibrated using the reciprocity method at a National Measurement Institute. Reciprocity calibration is a specialist process, and because it forms the basis of the primary standard for sound pressure, many national measurement institutes have invested significant research efforts to refine the method and develop calibration facilities. A system is also commercially available from Brüel & Kjær
Brüel & Kjær
Brüel & Kjær is a subsidiary of UK-based Spectris plc. Brüel & Kjær is a manufacturer and supplier of sound and vibration measurement apparatus. The company headquarters is based in Nærum, Denmark...
.
For airborne acoustics, the reciprocity technique is currently the most precise method available for microphone calibration (i.e. has the smallest uncertainty of measurement). Free field
Anechoic chamber
An anechoic chamber is a room designed to stop reflections of either sound or electromagnetic waves.They are also insulated from exterior sources of noise...
reciprocity calibration (to give the free-field response, as opposed to the pressure response of the microphone) follows the same principles and roughly the same method as pressure reciprocity calibration, but in practice is much more difficult to implement. As such it is more usual to perform reciprocity calibration in an acoustical coupler, and then apply a correction if the microphone is to be used in free-field conditions; such corrections are standardised for laboratory standard microphones (IEC/TS 61094-7) and are generally available from the manufacturers of most of the common microphone types.
Comparison Calibration
A comparison calibration determines a microphone’s sensitivity by comparing its electrical response to a sound field (ie. its sensitivity) against that of a previously calibrated microphone (a reference microphone). Since the sensitivity of the reference microphone is already known, if the 2 microphones are exposed to the same stimulus (sound-field), then the responses can be directly compared. Comparison calibration can be carried out against a microphone which was itself calibrated by comparison method (or another secondary method); however it would more normally be referenced against a reciprocity calibrated microphone, due to the added uncertainty of measurement introduced by each ‘step’ in the calibration chain.A common method of comparison calibration requires the two microphones to be placed facing each other, closely spaced so that the pressure on both of the microphones’ diaphragms can be assumed to be equal at the frequencies of interest (the audible range). Both microphones are subjected to a broadband noise signal from a loudspeaker
Loudspeaker
A loudspeaker is an electroacoustic transducer that produces sound in response to an electrical audio signal input. Non-electrical loudspeakers were developed as accessories to telephone systems, but electronic amplification by vacuum tube made loudspeakers more generally useful...
(typically white noise
White noise
White noise is a random signal with a flat power spectral density. In other words, the signal contains equal power within a fixed bandwidth at any center frequency...
) so as to include every frequency of interest, and the electrical output signals from both microphones are subject to frequency analysis, resulting in the frequency response
Frequency response
Frequency response is the quantitative measure of the output spectrum of a system or device in response to a stimulus, and is used to characterize the dynamics of the system. It is a measure of magnitude and phase of the output as a function of frequency, in comparison to the input...
of the microphones to the specific sound they were exposed to. By this method the ratio of sensitivities of the two microphones can be determined. This can then be multiplied by the known sensitivity of the calibrated reference microphone in order to obtain the absolute sensitivity of the test microphone. Comparison calibration may be carried out in a free-field environment (typically in an anechoic chamber
Anechoic chamber
An anechoic chamber is a room designed to stop reflections of either sound or electromagnetic waves.They are also insulated from exterior sources of noise...
), a diffuse field (a reverb chamber) or a coupler, depending on the application the microphone it is intended for.
Calibration using Pistonphones and Sound Calibrators
A pistonphone is an acoustical calibrator (sound source) that uses a closed coupling volume to generate a precise sound pressure for the calibration of measurement microphones. The principle relies on a pistonPiston
A piston is a component of reciprocating engines, reciprocating pumps, gas compressors and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder and is made gas-tight by piston rings. In an engine, its purpose is to transfer force from...
mechanically driven to move at a specified cyclic rate, pushing on a fixed volume of air to which the microphone under test is coupled. The air is assumed to be compressed adiabatically and the sound pressure level in the chamber can, potentially, be calculated from internal physical dimensions of the device and the adiabatic gas law, which requires that PVγ is a constant, where P is the pressure in the chamber, V is the volume of the chamber, and γ is the ratio of the specific heat of air at constant pressure to its specific heat at constant volume. Pistonphones are highly dependant on ambient pressure (always requiring a correction to ambient pressure conditions) and are generally only made to reproduce low frequencies (for practical reasons), typically 250 Hz. However, pistonphones can be very precise, with good stability over time.
However, commercially available pistonphones are not calculable devices and must themselves be calibrated using a calibrated microphone if the results are to be traceable; though generally very stable over time, there will be small differences in the sound pressure level generated between different pistonphones. Since their output is also dependent on the volume of the chamber (coupling volume), differences in shape and load volume between different models of microphone will have an influence on the resulting SPL, requiring the pistonphone to be calibrated accordingly.
Sound calibrators are used in an identical way to pistonphones, providing a known sound pressure field in a cavity to which a test microphone is coupled. Sound calibrators are different to pistonphones in that they work electronically and use a low-impedance (electrodynamic) source to yield a high degree of volume independent operation. Furthermore, modern devices often use a feedback mechanism to monitor and adjust the sound pressure level in the cavity so that it is constant regardless of the cavity / microphone size. Sound calibrators normally generate a 1 kHz sine tone; 1 kHz is chosen since the A-weighted SPL is equal to the linear level at 1 kHz. Sound calibrators should also be calibrated regularly at a nationally accredited calibration laboratory to ensure traceability. Sound calibrators tend to be less precise than pistonphones, but are (nominally) independent of internal cavity volume and ambient pressure.