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.

Charge Amplifier : an amplifier with low input impedance whose output voltage is proportional to the output charge from a Piezoelectric transducer. Has the advantage that the voltage output is not affected by length of connecting cable to the meter / analyser, and may also be normal screened cable not expensive accelerometer cable.

Charge Sensitivity : the charge sensitivity of an
Accelerometer is independent of the cable length. The voltage sensitivity on the other hand 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
IEPE - Integrated Electronics PiezoElectric

CIC : Charge Injection Calibration : is a technique patented by Bruel & Kjaer for on-line 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.

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.

Coherent Sources : A sound arriving directly from a source and sound arriving at the same point from the same source but modified by the Reverberant Field are said to be coherent. Changes in level, phase or time do not change the fact that the sound is coherent.

Community Noise Equivalent Level : CNEL : the 24 hour average noise level of all hourly
Leq measurements with a 10 dB penalty added to the night-time levels between 2200 and 0700 hours and a 5 dB penalty added to the evening levels between 1900 and 2200 hours to reflect people's extra sensitivity to noise during the night and the evening.

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

Conservation of Clean Air and Water in Europe.

Condenser Microphones : are widely used in noise measurements because they offer the best linearity, frequency range and high stability. Because of their importance we have prepared more details
Measurement Microphones

Constant Current Line Drive : CCLD : with built-in electronics to transmit the output signal over 'long' cables. also known as IEPE

Constant Bandwidth Filter : a filter with fixed frequency
Bandwidth, expressed in Hertz (Hz), regardless of centre frequency.

Constant Percentage Bandwidth Filter : a filter whose bandwidth is a fixed percentage of centre frequency. The width of the individual filters is defined relative to their position in the range of interest. The higher the centre frequency of the filter, the wider the bandwidth. The bandwidth is defined in octaves or as a fixed percentage of the centre frequency of the filter.

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.

Crest Factor : is the
Peak amplitude of a waveform divided by the RMS value. For example a sine wave (pure tone) has a crest factor of 1.414 or 3 dB and a square wave has a crest factor is 1.

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 : is the maximum Leq Sound Level allowed for an 8-hour period and corresponds to the 100% noise dose value. Used for calculating
Dose and
ProjDose.

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.

Critical Damping : minimum Damping that will allow a displaced system to return to its initial state without oscillation.

Critical Distance : the distance at which the sound pressure level of the direct sound and the reverberant sound are equal when dealing with a directional source.

Cross-correlation : cross-correlation is 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 cross-spectrum is 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.

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.