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In order to take a scientific measurement with a microphone, its precise sensitivity must be known (in volts 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 testing laboratories. Microphone calibration by certified laboratories should ultimately be traceable to primary standards a (National) Measurement Institute that is a signatory to International Laboratory Accreditation Cooperation. These could include the National Physical Laboratory in the UK, PTB in Germany, NIST in the USA and the National Measurement Institute, Australia, 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 sound levels beyond the device’s operational range.
Reciprocity calibration is currently the favoured primary standard for calibration of measurement microphones. The technique exploits the reciprocal nature of certain transduction mechanisms such as the electrostatic transducer principle used in condenser measurement microphones. In order to carry out a reciprocity calibration, three uncalibrated microphones , and are used. Microphones and are placed facing each other with a well known acoustical coupler between their diaphragms, allowing the acoustic transfer impedance to be easily modelled. One of the microphones is then driven by a current to act as the source of sound and the other responds to the pressure generated in the coupler, producing an output voltage resulting in the electrical transfer impedance . Provided that the microphones are reciprocal in behaviour, which means the open circuit sensitivity in V/Pa as a receiver is the same as the sensitivity in m³/s/A as a transmitter, it can be shown that the product of the transmission factors , , and the acoustical transfer impedance equals the electrical transfer impedance.
Having determined the product of the transmission factors for one pair of microphones, the process is repeated with the other two possible pair-wise combinations and . The set of three measurements then allows the individual microphone transmission factor to be deduced by solving three simultaneous equations.
The electrical transfer impedance is determined during the calibration procedure by measuring the current and voltage and the acoustic transfer impedance depends on the acoustical coupler.
Commonly used acoustical couplers are free field, diffuse field and compression chamber. For free field conditions between the two microphones the sound pressure in the far field can be calculated and it follows
where is the distance between the microphones. For diffuse field conditions follows
where is the equivalent absorption area and is the critical distance for reverberation. For compression chamber conditions follows
where is the air volume in the chamber.
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 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 piston 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 dependent 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 from 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.
The decibel is a relative unit of measurement equal to one tenth of a bel (B). It expresses the ratio of two values of a power or root-power quantity on a logarithmic scale. Two signals whose levels differ by one decibel have a power ratio of 101/10 or root-power ratio of 101⁄20.
In electrical engineering, impedance is the opposition to alternating current presented by the combined effect of resistance and reactance in a circuit.
A viscometer is an instrument used to measure the viscosity of a fluid. For liquids with viscosities which vary with flow conditions, an instrument called a rheometer is used. Thus, a rheometer can be considered as a special type of viscometer. Viscometers only measure under one flow condition.
A calorimeter is an object used for calorimetry, or the process of measuring the heat of chemical reactions or physical changes as well as heat capacity. Differential scanning calorimeters, isothermal micro calorimeters, titration calorimeters and accelerated rate calorimeters are among the most common types. A simple calorimeter just consists of a thermometer attached to a metal container full of water suspended above a combustion chamber. It is one of the measurement devices used in the study of thermodynamics, chemistry, and biochemistry.
A microphone, colloquially called a mic or mike, is a device – a transducer – that converts sound into an electrical signal. Microphones are used in many applications such as telephones, hearing aids, public address systems for concert halls and public events, motion picture production, live and recorded audio engineering, sound recording, two-way radios, megaphones, radio and television broadcasting. They are also used in computers for recording voice, speech recognition, VoIP, and for non-acoustic purposes such as ultrasonic sensors or knock sensors.
The speed of sound is the distance travelled per unit of time by a sound wave as it propagates through an elastic medium. At 20 °C (68 °F), the speed of sound in air is about 343 metres per second, or one kilometre in 2.9 s or one mile in 4.7 s. It depends strongly on temperature as well as the medium through which a sound wave is propagating. At 0 °C (32 °F), the speed of sound is about 331 m/s.
Flow measurement is the quantification of bulk fluid movement. Flow can be measured in a variety of ways. The common types of flowmeters with industrial applications are listed below:
Sound pressure or acoustic pressure is the local pressure deviation from the ambient atmospheric pressure, caused by a sound wave. In air, sound pressure can be measured using a microphone, and in water with a hydrophone. The SI unit of sound pressure is the pascal (Pa).
Sound intensity, also known as acoustic intensity, is defined as the power carried by sound waves per unit area in a direction perpendicular to that area. The SI unit of intensity, which includes sound intensity, is the watt per square meter (W/m2). One application is the noise measurement of sound intensity in the air at a listener's location as a sound energy quantity.
Sound power or acoustic power is the rate at which sound energy is emitted, reflected, transmitted or received, per unit time. It is defined as "through a surface, the product of the sound pressure, and the component of the particle velocity, at a point on the surface in the direction normal to the surface, integrated over that surface." The SI unit of sound power is the watt (W). It relates to the power of the sound force on a surface enclosing a sound source, in air. For a sound source, unlike sound pressure, sound power is neither room-dependent nor distance-dependent. Sound pressure is a property of the field at a point in space, while sound power is a property of a sound source, equal to the total power emitted by that source in all directions. Sound power passing through an area is sometimes called sound flux or acoustic flux through that area.
Particle velocity is the velocity of a particle in a medium as it transmits a wave. The SI unit of particle velocity is the metre per second (m/s). In many cases this is a longitudinal wave of pressure as with sound, but it can also be a transverse wave as with the vibration of a taut string.
Acoustic impedance and specific acoustic impedance are measures of the opposition that a system presents to the acoustic flow resulting from an acoustic pressure applied to the system. The SI unit of acoustic impedance is the pascal-second per cubic metre, or in the MKS system the rayl per square metre, while that of specific acoustic impedance is the pascal-second per metre, or in the MKS system the rayl. There is a close analogy with electrical impedance, which measures the opposition that a system presents to the electric current resulting from a voltage applied to the system.
Particle displacement or displacement amplitude is a measurement of distance of the movement of a sound particle from its equilibrium position in a medium as it transmits a sound wave. The SI unit of particle displacement is the metre (m). In most cases this is a longitudinal wave of pressure, but it can also be a transverse wave, such as the vibration of a taut string. In the case of a sound wave travelling through air, the particle displacement is evident in the oscillations of air molecules with, and against, the direction in which the sound wave is travelling.
A quartz crystal microbalance (QCM) measures a mass variation per unit area by measuring the change in frequency of a quartz crystal resonator. The resonance is disturbed by the addition or removal of a small mass due to oxide growth/decay or film deposition at the surface of the acoustic resonator. The QCM can be used under vacuum, in gas phase and more recently in liquid environments. It is useful for monitoring the rate of deposition in thin film deposition systems under vacuum. In liquid, it is highly effective at determining the affinity of molecules to surfaces functionalized with recognition sites. Larger entities such as viruses or polymers are investigated as well. QCM has also been used to investigate interactions between biomolecules. Frequency measurements are easily made to high precision ; hence, it is easy to measure mass densities down to a level of below 1 μg/cm2. In addition to measuring the frequency, the dissipation factor is often measured to help analysis. The dissipation factor is the inverse quality factor of the resonance, Q−1 = w/fr ; it quantifies the damping in the system and is related to the sample's viscoelastic properties.
A sound level meter is used for acoustic measurements. It is commonly a hand-held instrument with a microphone. The best type of microphone for sound level meters is the condenser microphone, which combines precision with stability and reliability. The diaphragm of the microphone responds to changes in air pressure caused by sound waves. That is why the instrument is sometimes referred to as a sound pressure level meter (SPL). This movement of the diaphragm, i.e. the sound pressure deviation, is converted into an electrical signal. While describing sound in terms of sound pressure metrics, such as Pascals, is possible a logarithmic conversion is usually applied and the sound pressure level is stated instead, with 0 dB SPL equal to 20 micropascals.
A Rayl, rayl or Rayleigh is one of two units of specific acoustic impedance or, equivalently, characteristic acoustic impedance; one an MKS unit, and the other a CGS unit. The units are named after John William Strutt, 3rd Baron Rayleigh, and not to be confused with the rayleigh unit of photon flux, used to measure airglow, and named after his son, Robert John Strutt, 4th Baron Rayleigh. It has the same dimensions as momentum per volume.
Electroacoustic phenomena arise when ultrasound propagates through a fluid containing ions. The associated particle motion generates electric signals because ions have electric charge. This coupling between ultrasound and electric field is called electroacoustic phenomena. The fluid might be a simple Newtonian liquid, or complex heterogeneous dispersion, emulsion or even a porous body. There are several different electroacoustic effects depending on the nature of the fluid.
Acoustic radiation force (ARF) is a physical phenomenon resulting from the interaction of an acoustic wave with an obstacle placed along its path. Generally, the force exerted on the obstacle is evaluated by integrating the acoustic radiation pressure over its time-varying surface.
Flow conditioning ensures that the “real world” environment closely resembles the “laboratory” environment for proper performance of inferential flowmeters like orifice, turbine, coriolis, ultrasonic etc.
Transmission loss (TL) in duct acoustics, together with insertion loss (IL), describes the acoustic performances of a muffler-like system. It is frequently used in the industry areas such as muffler manufacturers and NVH department of automobile manufacturers. Generally the higher transmission loss of a system it has, the better it will perform in terms of noise cancellation.