Acoustic Glossary


 

C :  Sound and Vibration Definitions, Terms. Units, Measurements ...


C-weighting


C and Ctr : Spectrum Adaption Terms, under Sound Insulation


Calculated 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


CB : Critical Band


CCLD : Constant Current Line Drive


CE Marking a 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.


Characteristic Acoustic Impedance : Zo is the ratio of the effective sound pressure at a given point to the effective particle velocity at that point. In a free progressive wave, it is equal to the product of the density ρ of the medium times the speed of sound c in the media. Zo = ρc the SI units are N·s/m3.

For example at room temperature and normal atmospheric pressure, ρc = 415 N·s/m3

Characteristic Impedance of a Medium IEC Definition,

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.

 
See also : Acoustic Impedance

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

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 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.


CIS : Common Intelligibility Scale


Clarity


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.


Classes : 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


CNEL : Community Noise Equivalent Level below


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.

See also : Incoherent Sources


Coincidence Effect


Common Intelligibility Scale : CIS


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.

Also known as Lden : day-evening-night equivalent 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.

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'.


Complex Sound IEC Definition,

sound that is not a simple oscillation

 
Complex Sound Source composed of various sources, multiple frequencies and directivity patterns.

Compliance the inverse of Stiffness


Compression Wave
Compressional 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.

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 under Measurement Microphones


Conjugate Impedances IEC Definition,

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 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.

Related Terms : Narrow Band NoisePink Noise NoiseWideband Noise
See also : Octave BandsFFT- Fast Fourier Transform


Continuous Spectrum IEC Definition,

sound spectrum whose components are continuously distributed over a given frequency range.


Control of Noise at Work Regulations
See also : Noise ExposureNoise Dose


Control of Vibration at Work Regulations
See also : Aeq - acceleration equivalent valueHand Arm Vibration SyndromeWhole 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 : AutocorrelationCross-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
1 picocoulomb   = 1 x 10-12 C = 1 pC.

Coupled Modes IEC Definition,

modes of oscillation which are not independent but influenced by the transfer of energy from one mode to the other


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.


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.


Criterion Time is the time in hours used to calculate % Dose and Estimated % Dose. Normally 8 hours.


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.


Critical Damping IEC Definition,

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.


Critical Frequency


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.

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 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.


Ctr : Spectrum Adaption Terms, under Sound Insulation


cubic metre, m3 : volume.


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


Cylindrical Wave

 

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