C : Sound and Vibration Terms, Definitions, Units, Measurements ...
C and Ctr (spectrum adaption terms)
under sound insulation
, 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.
Hand-held calibrators 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.
CB • critical band
CCLD • constant current line drive
, label to show an instrument conforms to the specification of a European Directive.
= 0.01 metre
a system of measurement using the centimetre, the gram and the second as basic units of length, mass and time.
, the frequency in the middle of a band of frequencies, for example 1 kHz octave
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.
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 charge injection calibration may be performed anytime a measurement is not running or automatically at the start and end of a logging measurement.
CIS • common intelligibility scale
, precision grade sound level instrumentation, for laboratory measurements, also known as Type 0
, precision grade sound level meters for laboratory and field use, also known as Type 1
, 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 one of the above classes.
Clipping, an electrical signal is clipped if the signal level exceeds the capabilities of the amplifier. It is a distortion of the signal.
= 0.01 metre
CNEL • community noise equivalent level
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.
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.
See also • incoherent sources
Common Intelligibility Scale
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 also acoustic impedance and related topics
Complex Sound Source
under sound source
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.
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.
See also acoustic impedance and related topics
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
, expressed in Hertz (Hz), regardless of centre frequency.
Constant Percentage Bandwidth Filter
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.
Control of Vibration at Work Regulations
, came into force in July 2005 to protect workers from risks to their health from vibration, based on
whole body vibration exposure action and limiting values
measures the degree to which two time domain signals are similar or identical. The higher the correlation, the stronger the relationship between both signals.
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 •
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.
CPB • constant percentage bandwidth filter
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.
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.
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.
Ctr (spectrum adaption terms)
under sound insulation
cubic-metre per kilogram (m3/kg)
under metre-cubed per kilogram
cubic-metre per second (m3/s)
under metre-cubed per second
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)