Calibration, the best quality sound and vibration instruments are usually stable and reliable. However the 'accuracy' of the electronic components can 'drift so equipment should be calibrated from time to time by a competent laboratory. It is also good practice to carry out 'field calibrations' before and after measurements.
Capacitor, an electrical component that passes alternating currents but blocks direct currents. Also called a condenser, it is capable of storing electrical energy.
See also • hand arm vibration syndrome
CB ▷ critical band
CCLD ▷ constant current line drive
CE Marking, label to show an instrument conforms to the specification of a European Directive.
centimetre (cm) = 0.01 metre
Centimetre-gram-second (CGS) a system of measurement using the centimetre, the gram and the second as basic units of length, mass and time.
Centre Frequency the frequency in the middle of a band of frequencies, for example 1 kHz octave band.
For example at room temperature and normal atmospheric pressure, ρc = 415 N·s/m3
To take into account different cable lengths or to convert from charge sensitivity to voltage sensitivity, divide the charge sensitivity by the total capacitance of the accelerometer and the cable,
For example a 1 pC/ms-2 and 9.8 pC/g accelerometer with a capacitance of 900 pF and a 1.2 m cable (100 pF), would give 1 pC / (900+100) pF = 1 mV/ms-2 or 9.8 pC / (900+100) pF = 9.8 mV/g
By definition 1 g = 9.807 ms-2 and conversely 10 ms-2 = 1.01 g
Many accelerometers are supplied with a built-in preamplifier to transform the high impedance charge output to a low impedance voltage signal that can be transmitted over longer distances
See also • IEPE, integrated electronics piezoelectric
CIC (charge injection calibration) is a technique patented by Bruel & Kjaer for verification of the integrity of the entire measurement chain, for example, microphone, preamplifier and cabling. Manual CIC may be performed anytime a measurement is not running or automatically at the start and end of a logging measurement.
Class 0 precision grade sound level instrumentation, for laboratory measurements, also known as Type 0
Class 1 precision grade sound level meters for laboratory and field use, also known as Type 1.
Class 2 general grade sound level meters for field use, also known as Type 2.● Note : octave and one-third octave filters used for sound level measurements are also required to meet Classes 0, 1 or 2.
Clipping, an electrical signal is clipped if the signal level exceeds the capabilities of the amplifier. It is a distortion of the signal.
cm, centimetre = 0.01 metre
See also • incoherent sources
See also the IEC Definition of Level
For the case of propagation through a duct, wall vibrations and viscous forces at the air/wall interface (boundary layer) can also have a significant effect, especially at high frequencies for the latter. For resistive effects, energy is removed from the wave and converted into other forms. This energy is said to be 'lost from the system'.
Compliance, conforming to environmental laws, regulations, standards and other requirements.
CONCAWE (conservation of clean air and water in europe), established in 1963 by a small group of leading oil companies to carry out research on environmental issues relevant to the oil industry. .
Condenser Microphones widely used in noise measurements as they offer the best linearity, frequency range and high stability. Because of their importance we have a full page on measurement microphones
Constant Current Line Drive (CCLD) with built-in electronics to transmit the output signal over 'long' cables, also known as IEPE.
Correlation measures the degree to which two time domain signals are similar or identical. The higher the correlation, the stronger the relationship between both signals.
Correlation Coefficient when the correlation is 1 then perfect positive correlation exists and -1 equates to a perfect negative correlation. When the correlation is 0, there is no relationship between the two sets of data.
See also • autocorrelation • cross-correlation
Coulomb (C) a coulomb is a unit of electrical charge, defined as the quantity of electrical charge transferred by 1 ampere in 1 second. Symbol C.1 microcoulomb = 1 x 10-6 C = 1 μC
See also • uncoupled mode
Sound levels often have high Crest Factors, so hearing damage risk assessments require both the LAeq and the Peak values to be reported.
The notion of critical bands explains the auditory masking of a narrow band (sinusoidal) signal by a wideband noise source. Critical Band Frequencies.
See also • loudness.
Cross-correlation a measure of the similarity of two time domain signals. If the signals are identical, the cross correlation will be one, and if they are completely dissimilar, the cross correlation will be zero.See also • autocorrelation
Cross Power Spectrum measurement of two signals with an amplitude that is the product of the two signal amplitudes and a phase that is the difference of the two phases.
Cross Section the shape we get cutting through a object, normally at 90° For example the cross section of a cylinder is a circle and a pyramid is a rectangle. Other angles produces an infinite number of shapes and cross section areas. In acoustics the plane perpendicular to direction of propagation is assumed
Cross-spectrum the forward Fourier transform of the cross-correlation function. Generally, the cross-spectrum is a complex function.
Crosstalk the signal on one channel, track, or circuit interfering with another.
cubic metre per kilogram (m3/kg), specific volume.
cubic metre per second (m3/s), volumetric flow.
Cumulative Distribution a method of representing time-varying noise by indicating the percentage of time that the noise level is present above (or below) a series of amplitude levels.
Cursor a thin hairline that can be positioned on a spectrum or time signal graphs to obtain a readout. Various types of cursor are available. For example, single cursor - a line, delta cursor - selects a band, harmonic cursor - marks a set of harmonics.
Cut-off Frequency, the frequencies that mark the ends of a band, or the points at which the characteristics of a filter change from pass to no-pass.
Cycle the complete sequence of values of a periodic quantity that occurs during one period.
cycle per second (c/s) under frequency