C-weighting

C and Ctr : spectrum adaption terms under sound insulation

**Calculated Loudness Level Definition** (IEC 801-29-06) loudness level calculated by a specified procedure.

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

See also loudness and related terms

**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.

See also hand arm vibration syndrome

CB : critical band

CCLD : constant current line drive

**Characteristic Acoustic Impedance (z)**, the ratio of the effective sound pressure at a point in an acoustic medium to the effective particle velocity at that point. z = p ÷ v, the SI units are Pa·s/m.

**Characteristic Acoustic Impedance Definition** (ASA 6.35) at a point in a medium, the complex ratio of sound pressure to particle velocity in the direction of the wave propagation. SI Unit = Pa·s/m (pascal second per metre), also known as the specific acoustic impedance.

See other acoustic impedance and related terms

**Characteristic Impedance of a Medium Definition** (IEC 801-25-39) product of the equilibrium, the density and the speed of sound in a medium.

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*.

**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. 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

See also IEPE, integrated electronics piezoelectric

CIS : common intelligibility scale

Clarity

**Class 0** precision grade sound instrumentation, laboratory measurements, also known as *Type 0*

**Class 1** precision grade sound instrumentation, laboratory and field use, also known as *Type 1*.

**Class 2** general grade sound level meters for field use, also known as *Type 2*.

CNEL : community noise equivalent level

**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 a 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*.

Coincidence Effect

Common Intelligibility Scale

**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 22:00 and 07:00 hours and a 5 dB penalty added to the evening levels between 19:00 and 22:00 hours to reflect people's extra sensitivity to noise during the night and the evening, also known as Lden (day-evening-night equivalent noise level)

See also the IEC Definition of Level

**Complex Acoustic Impedance** in general a phase relation exists between the pressure and the particle velocity and the complex impedance is defined as Z = R +iX.
where R is the resistive part, and X is the reactive part of the impedance The resistive part represents the various loss mechanisms an acoustic wave experiences such as random thermal motion.

See related acoustic impedance and related terms

**Complex Sound Definition** (IEC 801-21-06) sound that is not a simple oscillation

**Compliance**, conforming to environmental laws, regulations, standards and other requirements.

Compression Wave

**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

**Conjugate Impedances Definition** (IEC 801-25-14) impedances whose real components (resistances) are equal and whose imaginary components (reactances) are equal but opposite in sign.

Note : conjugate impedances are expressed by conjugate complex quantities.

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

*Constant Bandwidth Filter* with fixed frequency
bandwidth, expressed in Hertz (Hz), regardless of centre frequency.

**Constant Percentage Bandwidth Filter** the 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 usually defined in octaves, 1/3 octaves or as a fixed percentage of the centre frequency of the filter.

**Continuous Spectrum Definition** (IEC 801-21-17) sound spectrum whose components are continuously distributed over a given frequency range.

**Control of Noise at Work Regulations 2005** came into force to ensure that workers' hearing is protected from excessive noise at their place of work, by the daily noise exposure, action and limit values

See also noise dose

**Control of Vibration at Work Regulations** came into force in July 2005 to protect workers from risks to their health from vibration, based on
hand arm and
whole body vibration *exposure action and limiting values*.

**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.

**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

**Coupled Modes Definition** (IEC 801-24-17) modes of oscillation which are not independent but influenced by the transfer of energy from one mode to the other.

See also uncoupled mode

CPB : constant percentage bandwidth filter

**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.

Criteria : the plural of criterion

Criterion Level

Criterion Time

**Critical Band** the human ear combines sounds of similar frequency into frequency bands, called 'critical bands'. Dr 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.

See also loudness.

**Critical Damping Definition** (IEC 801-24-20) minimum damping that will allow a displaced system to return to its initial state without oscillation.

See also other acoustic damping topics

Critical Distance under Diffused Field Distance

Critical Frequency

**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 presence of an unwanted sound via a break-in, also known as cross-talk

Ctr : spectrum adaption terms - sound insulation, single number values

**cubic-metre per second (m ^{3}/s)** under metre-cubed per second.

**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

Cylindrical Waves