● Note 1 : such procedures are given in BS ISO 532-1:2017 methods for calculating loudness

See also • equal loudness contours • loudness level

**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.

Suitable hand-held calibrators are readily available for this purpose.

**Capacitor** an electrical component that passes alternating currents but blocks direct currents. Also called a condenser, it is capable of storing electrical energy.

**Carpal Tunnel Syndrome** pain, numbness and tingling sensation in the hands and fingers.

See also • hand arm vibration syndrome

**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/m^{3}

See also acoustic impedance and related topics

● Note : for a plane acoustic wave propagating in a non-dissipative medium, the specific acoustic impedance relative to this wave is equal to the characteristic impedance of the medium.

See also acoustic impedance and related topics

**Charge Sensitivity** the charge sensitivity of an
accelerometer is independent of the cable length. However the voltage sensitivity stated is for the accelerometer when used with its standard length of cable.

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** is the requirement 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

**Coherence** is a number between one and zero, and is a measure of the degree of linearity between two related signals, such as the excitation force (input) of a structure related to the vibration response to that force (output).

**Coherent Output Power Spectrum** a measure of what part of the (output) power spectrum is fully coherent with the input signal.

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'.

See also acoustic impedance and related topics

**Compliance** the inverse of Stiffness

Compression Wave

**CONCAWE** the propagation of noise from petroleum and petrochemical complexes to neighbouring communities 1981 - has been extensively used for many other industrial noise situations.

**CONCAWE** short for Conservation of Clean Air and Water in Europe.

**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

● Note : conjugate impedances are expressed by conjugate complex quantities.

See also acoustic impedance and related topics

See also •
fast fourier transform •
line spectrum •
narrowband noise •
narrowband spectra •
octave bands.
pink noise •
white noise •
wideband noise

Control of Noise at Work Regulations

See also •
noise exposure •
noise dose

Control of Vibration at Work Regulations

See also •
Aeq - acceleration equivalent value •
hand arm vibration syndrome •
whole body vibration

**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 picocoulomb = 1 x 10

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.

Criterion Level

Criterion Time

**Critical Band** the human ear combines sounds of similar frequency into frequency bands, called 'critical bands'. Zwicker divided the audio spectrum into 24 critical bands and named the units Bark.

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.

**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 (m ^{3}/kg)** specific volume.

**cubic metre per second (m ^{3}/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.

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