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Calculated PeakTerm used to describe the spectral overall RMS level multiplied by 2. Sometimes referred to as “derived peak” or “pseudo peak”.
CalibrationThe process of measuring to determine the accuracy of your measurement chain. This result can then be used to offset measured values and take account of this inaccuracy.
CalibratorA device that produces a known sound pressure on a microphone in a sound level measurement system, or a known vibration (acceleration, velocity, or displacement) on an accelerometer in a vibration measurement system. It is used to adjust the system to standard specifications.
Campbell DiagramA graph which shows frequency on axis and machine RPM on the horizontal axis. The spectral amplitude is indicated by the diameter of a circle (or the side length of a square) at each point in the diagram.
CapacitorAn electrical component that passes alternating currents but blocks direct currents. Also called a condenser, it is capable of storing electrical energy.
CCLDConstant current line drive, as used by DeltaTron™ accelerometers and microphone preamplifiers.
Centre FrequencyThe arithmetic centre of a constant bandwidth filter, or the geometric centre (midpoint on a logarithmic scale) of a constant percentage bandwidth filter. When specifying an FFT zoom frequency span, the centre frequency is the middle point of the frequency span that you want to analyse. For example, if measuring a 2 kHz zoom frequency span and the centre frequency is 6 kHz, the measured range is from 5 kHz to 7 kHz.
CepstrumThe cepstrum is the forward Fourier transform of the logarithm of a spectrum. It is thus the spectrum of a spectrum, and has certain properties that make it useful in many types of signal analysis. One of its more powerful attributes is the fact that any periodicities, or repeated patterns, in a spectrum will be sensed as one or two specific components in the cepstrum. If a spectrum contains several sets of sidebands or harmonic series, they can be confusing because of overlap. But in the cepstrum, they will be separated in a way similar to the way the spectrum separates repetitive time patterns in the waveform. Gearboxes and rolling element bearing vibrations lend themselves especially well to cepstrum analysis. The cepstrum is closely related to the auto correlation function.
Characteristic EquationThe mathematical equation whose solution defines the dynamic characteristics of the structure in terms of its natural frequencies, damping, and mode shapes. The mathematical formulation of the characteristic equation is called the Eigenvalue problem. The characteristic equation is obtained from the equations of motion for the structure.
Charge AmplifierAn amplifier with low input impedance whose output voltage is proportional to the output charge from a piezoelectric transducer. Has the advantage that voltage output is not affected by length of connecting cable from the transducer.
CICCharge Injection Calibration (CIC) is a technique patented by Brüel & Kjær for on-line verification of the integrity of the entire measurement chain, for example, microphone, preamplifier and cabling. Even microphones remote from the input stage/conditioning amplifier can be verified. The basic philosophy behind CIC is that if we have a known condition (for example, a properly calibrated microphone) and establish a reference measurement, then as long as the reference value does not change, nothing has changed, for example, the microphone calibration will still be valid. Additionally CIC verifies the cable and preamplifier.
Circle FitA single degree of freedom curve-fitting routine that tries to fit a mode to a circle (Nyquist plot of a single degree of freedom system). The modal coefficient is determined by the diameter of the circle and the phase by its location relative to the imaginary axis. For a real mode, it should be either completely above or completely below the imaginary axis.
Circular Correlation Effect
When using FFT to calculate auto- or cross-correlation from auto- or cross-spectra, rectangular weighting is used so that the signal is not distorted. This introduces leakage, which affects both the spectra and the correlation functions. Due to the discrete Fourier transform, the analyzer treats the signal as periodic. When correlation functions are calculated from such signals, there is a contribution from the end of one period multiplied by the beginning of the next. This is called the circular correlation effect:
To avoid this effect, zero pad is used. This also introduces an error into the calculations, but as the error is the same for all signals it can be compensated for using the bow-tie correction:
Further information can be found in Brüel & Kjær Technical Review, No. 2, 1984, p.29.
Classical Damping (in structural dynamics)It is assumed that the damping matrix may be expressed as a linear combination of the mass matrix and the stiffness matrix.
ClippingAn electrical signal is clipped if the signal level exceeds the capabilities of the amplifier. It is a distortion of the signal.
Coast-downThe slowing down of a rotating or reciprocating machine where no braking is applied.
CochleaA spirally coiled organ located within the inner ear that contains the receptor organs essential to hearing. Its hair cells respond to pressure fluctuations caused by mechanical vibrations of the cochlea fluid and generate nerve impulses that are interpreted by the brain as sound. It is the frequency-analysing portion of the auditory system.
Coefficient of DeterminationAn indicator of how well the equation resulting from regression analysis explains the relationship among the variables. It is obtained by comparing estimated and actual preference. It ranges in value from 0 to 1, where “1”, represents perfect correlation in the sample (that is, there is no difference between the estimated y-value and the actual y-value) and “0” represents no correlation (that is, the regression equation is not helpful in predicting a y-value). Used in Brüel & Kjær’s Psychoacoustic Test Bench program, BZ 5301
CoherenceCoherence 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). Coherence is thus a two-channel measurement, and does not apply to single-channel measurements of vibration signatures. In a frequency response measurement of a mechanical structure, if the structure is linear, the coherence will be one (100% coherent), but if there is some non-linearity in the structure, if there is noise in a measurement channel, or if the impulse response is truncated by the analysis (if the frequency resolution is inadequate) the coherence will be less than one. A dual-channel FFT analyzer is able to measure the coherence between the two channels, and it is a useful tool in determining good from noisy or meaningless data.
Coherent PowerIf the signal-to-noise ratio is high, it is equal to the product of the coherence and the measured output power spectrum. Useful for determining the contributions of sources in a measured signal that is made up of a number of sources.
Cold AlignmentMachine condition in which alignment procedures are normally performed. Changes in off-line to on-line running conditions should be allowed for during this procedure so that the machine can "grow" into alignment during operation. Also known as static alignment or primary alignment.
ColorationThe distortion of a signal detectable by the ear.
Comb FilterA distortion produced by combining an electrical or acoustical signal with a delayed replica of itself. The result is constructive and destructive interference that results in peaks and nulls being introduced into the frequency response. When plotted to a linear frequency scale, the response resembles a comb, hence the name.
Combination MetricA new objective metric created as a weighted sum of a number of single metrics. Can be created using Brüel & Kjær's Psychoacoustic Test Bench tool BZ5301.
Common-mode RejectionA measure of a measurement systems capability of rejecting common-mode signals.
Common-mode SignalsSignals of identical magnitude and phase that appear simultaneously at two inputs of an amplifier.
Common-mode VoltageThe voltage arising from common-mode signals.
Complex Acoustic AdmittanceThe ratio of the sound particle velocity through the surface to the surface sound pressure.
Complex Acoustic ImpedanceThe ratio of the surface sound pressure to the sound particle velocity through the surface.
Complex IntensityComplex intensity is the combined real intensity and imaginary intensity.
Complex ModesThe points on a structure have varying phase relationships between them at a natural frequency. This is unlike a real mode where the phase between points is either 0° or 180°.
Complex Reflection CoefficientThe complex ratio of the pressure of the reflected wave to the pressure of the incident wave.
Complex SpectrumIn a Complex or Fourier spectrum, the lines in the spectrum resulting from FFT analysis are equidistant, so the time signal is analysed in constant bandwidths. The analyzer analyses the time signal in blocks and each block is recorded in memory and a Fast Fourier Transform (FFT) is performed on each block.
Complex TimeThis is the analytical time signal where the real part is the time signal itself. The imaginary part is the Hilbert transform of the real part. The magnitude of the complex time signal is the envelope of the real signal.
ComplianceFrequency response function of displacement/force – the ease with which a system may be displaced or compressed for a given force. The reciprocal of Stiffness. Also known as Dynamic Compliance. See also Immittance.
CompressionReducing the dynamic range of a signal by electrical circuits that reduce the level of loud passages.
Conductive Hearing LossHearing loss caused by blockage of the outer ear or by derangement of the middle ear.
Constant Bandwidth AnalysisAnalysis using Constant Bandwidth Filters. This gives a uniform frequency resolution on a linear frequency scale.
Constant Bandwidth FilterA filter with fixed frequency bandwidth, expressed in hertz (Hz), regardless of centre frequency.
Constant Percentage Bandwidthsee CPB.
Constant Percentage FilterA 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 higher the bandwidth. The bandwidth is defined in octaves or as a fixed percentage of the centre frequency of the filter.
Continuous SoundSound having a steady nature that is not impulsive.
Contour GraphA graph that plots all point values and links together all points that belong within a band or level. Each of these levels is then shown in a unique colour. It divides all points into groups of closed curves. Every curve links together all points in the group that possess the same data value.
ContributionPart of the total sound that comes to a particular receiver in a vehicle’s interior from one or several sources
Control SystemA system in which deliberate guidance or manipulation is used to achieve a prescribed value of a variable.
CorrelogramA graph showing the correlation of one signal with another.
Cosine TaperAttenuation by a numerical factor that varies with time as a cosine function.
Coulomb DampingNon-linear damping that is a result of rubbing, looseness, etc.
CPBConstant Percentage Bandwidth (analysis); the width of the filters is defined relative to their position in the frequency range of interest. The higher the centre frequency of the filter, the higher the bandwidth. The bandwidth is defined in octaves, or as a fixed percentage of the centre frequency of the filter. A typical CPB analysis uses 1/3-octave filters, which correspond to 23% of the centre frequency.
Crest FactorThe term used to represent the ratio of the peak (crest) value to the rms value of a waveform. For example, a sine wave has a crest factor of 1.4 (or 3 dB), since the peak value equals 1.414 times the rms value. Music has a wide crest factor range of 4 – 10 (or 12 – 20 dB). This means that music peaks occur 12 – 20 dB higher than the rms value, which is why headroom is so important in audio design.
Critical BandIn human hearing, only those frequency components within a narrow band, called the critical band, will mask a given tone. Critical bandwidth varies with frequency but is usually between 1/6- and 1/3-octave. The ears act like a set of parallel filters, each with its own bandwidth. 24 critical bands make up the frequency range from 20 Hz to 16 kHz. Critical bands are approximated by 1/3-octave bands above 500 Hz and by 100 Hz wide bands at lower frequencies.
Critical DampingThe minimum viscous damping that will allow a displaced system to return to its original position without oscillation.
Cross CorrelationCross 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-spectrumThe cross-spectrum is the forward Fourier transform of the cross-correlation function. Generally, the cross-spectrum is a complex function.
Cross-talkThe signal of one channel, track, or circuit interfering with another.
Crossover FrequencyIn a loudspeaker with multiple radiators, the crossover frequency is the –3 dB point of the network dividing the signal energy.
Cumulative DistributionA 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.
CursorA thin hairline that can be displayed and positioned on 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.
Cursor HandlesPoints on a movable cursor that can be selected by the mouse and then dragged to a chosen position.
Cursor ReadingsInformation that can be read out from displayed functions when interrogated using cursors.
Curve FitThe process of fitting a curve to a set of polynomials.
Cut-off FrequencyThe frequencies that mark the ends of a band, or the points at which the characteristics of a filter change from pass to no-pass.
CycleThe complete sequence of values of a periodic quantity that occurs during one period.
Cycles Per SecondA measure of frequency numerically equivalent to hertz (Hz).
Cylindrical WaveA wave in which the surfaces of constant phase are coaxial cylinders. A line of closely spaced sound sources radiating into an open space produces a free sound field of cylindrical waves.Brüel & Kjær Sound & Vibration Dictionary End User Agreement