Suchen Sie in unserem Wörterbuch nach Schlagworten aus dem Bereich Schall und Schwingungen.
Fall-off RateThe rate at which a signal’s amplitude diminishes.
Far FieldDescribes a region in free space at a much greater distance from a sound source than the linear dimensions of the source itself where the sound pressure level obeys the inverse-square law (the sound pressure level decreases 6 dB with each doubling of distance from the source). Also, in this region the sound particle velocity is in phase with the sound pressure. Closer to the source, where these two conditions do not hold, constitutes the near field region. In the far field, the sound waves can be considered planar. See also: Diffraction.
Fast Fourier Transform (FFT)The FFT is an algorithm, or digital calculation routine, that efficiently calculates the Discrete Fourier Transform from the sampled time waveform. In other words it converts, or “transforms”, a signal from the time domain into the frequency domain.
Feedback, AcousticUnwanted interaction between the output and input of an acoustical system, for example, between the loudspeaker and the microphone of a system.
Feedback Control SystemA control system that achieves prescribed relationships between selected system variables by comparing functions of these variables and using the differences to effect control.
FFTFast Fourier Transform. Gives the mathematical connection between time and frequency and vice versa and, given a time signal, allows the calculation of its spectrum. An iterative program that computes the Fourier Transform in a shorter time.
FFT AnalyzerAn FFT analyzer is a device that uses the FFT algorithm to calculate a spectrum from a time domain signal, and is the most common type of spectrum analyzer available today. The FFT analyzer is a very useful device, and is available in a great variety of models of varying complexity. It is the heart of any machinery predictive maintenance program. An FFT analyzer uses constant (or absolute) bandwidth resolution. This is often preferred in vibration analysis because it gives a better frequency resolution. See also Fast Fourier Transform.
FidelityAs applied to sound quality, the faithfulness to the original.
Field Impact Insulation Class (FIIC)A single-number rating for impact sound insulation, calculated from measured values of normalised impact sound pressure levels.
Field STC (FSTC)Sound Transmission Class based on field measurements of sound transmission loss of a partition.
FilterA device for separating components of a signal on the basis of their frequency. It allows components in one or more frequency bands to pass relatively unattenuated, and it attenuates components in other frequency bands. Modifies the frequency spectrum of a signal usually while it is in electrical form.
Filter, BandpassA filter that passes all frequencies between a low-frequency cut-off point and a high-frequency cut-off point.
Filter, High-passA filter that passes all frequencies above a cut-off frequency.
Filter, Low-passA filter that passes all frequencies below a cut-off frequency.
Finite Element Analysis or ModellingA computer-aided design technique for mathematically modelling a structure. Finite element modelling is used for structural analysis, heat transfer analysis, and modal analysis.
Fixed FormatFixed format numbering is where numbers be entered as their absolute values. The whole number is entered followed by a decimal point, if required, and the required number of significant figures.
FlangingThe term applied to the use of comb filters to obtain special sound effects.
Flanking PathAn indirect sound transmission path, such as the structure-borne path between two adjacent rooms.
Flat Top WindowThe flat top window is a special time window with low ripple that is used in some FFT analyzers in addition to the more common Hanning Window and Rectangular Time Window. The flat top window does not allow as fine a frequency resolution as the Hanning window, but it will accurately measure the amplitude level of a signal at any frequency, even if the frequency is between the lines of the FFT analysis. The maximum picket-fence-error is 0.008 dB. It is used in transducer calibration systems to increase amplitude accuracy.
Fluctuation StrengthA measure of the low-frequency (below 20 Hz) amplitude and frequency modulation of sound. It is perceived as changes in the frequency and volume of the sound with time. Fluctuating signals of this type sound louder, and more annoying, than a steady signal of the same RMS magnitude. The effect is most noticeable when the modulation frequency is around 4 Hz.
FlutterA repetitive echo set up by parallel reflecting surfaces.
Flyover NoiseMeasurement of aircraft noise as aircraft fly over.
Force WindowA special windowing function for minimising noise in impact testing. Since the duration of the actual impact is usually very short relative to the overall digitised time sample, the frequency response function of the force signal can have a low signal-to-noise ratio. The force window does not alter the actual force pulse but minimises the noise in the rest of the data block giving a much improved signal to noise ratio.
Forced Deflection ShapeWhen a structure is forced to vibrate by applying one or more excitation forces, the structure will exhibit what is known as a Forced Deflection Shape.
Forced Response AnalysisCalculating the system response to an arbitrary forcing function using modal analysis data as the system model.
Forced VibrationThe oscillation of a system under the action of a forcing function.
FoundationA surface to which a machine baseplate is mounted.
Fourier AnalysisApplication of the Fourier transform to a signal to determine its spectrum. Fourier analysis is another term for spectrum analysis, although it generally refers to analysis using an FFT analyzer.
Fourier, Jean BaptisteThe famous, many-talented French engineer, mathematician, and one time president of Egypt, who devised the Fourier series and Fourier Transform for the conversion of time functions into frequency functions and vice versa.
Fourier SpectrumThe line spectrum resulting from an FFT analysis is equally spaced, 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 (the old instantaneous spectrum). See also FFT Analyzer.
Fourier TransformA mathematical operation for decomposing a time function into its frequency components (amplitude and phase). The process is reversible, and the signal can be reconstructed from its Fourier components. See also Fast Fourier Transform.
Free FieldAn environment in which there are no reflective surfaces within the frequency region of interest and the sound is isotropic and homogenous.
Free-field Voltage SensitivityThe ratio of the output open-circuit voltage of a microphone to the free-field sound pressure in an undisturbed plane progressive wave. Frequency and angle of incidence must be specified.
Free RunWhen using free run triggering, the analyzer runs continuously capturing time records and calculating the corresponding spectra.
Free Sound FieldA sound field in which the effects of obstacles or boundaries on sound propagated in that field are negligible.
FrequencyFrequency is the reciprocal of time. If an event is periodic in time, that is, if it repeats itself at a fixed time interval, then its frequency is one divided by the time interval. If a vibrating element takes one tenth of a second to complete one cycle and return to its starting point, then its frequency is defined to be 10 cycles per second, or 10 hertz (Hz). Although the SI standard unit of frequency is the Hz, when analysing machinery vibration it is sometimes more convenient to express frequency in cycles per minute (cpm), which corresponds to rpm. Frequency in cpm is simply frequency in Hz times 60. Another common frequency representation used in machinery monitoring is multiples of turning speed, or “orders”. Frequency in orders is frequency in cpm divided by the turning speed of the machine. The second order is then the second harmonic of turning speed, etc. This is especially convenient if the machine is varying in speed, for the frequency representation on a spectrum will be the same regardless of speed. Two spectra from the same machine can therefore more easily be compared if they are both expressed in orders. Conversion of the frequency axis of a spectrum to orders is called “order normalisation”, and is done by vibration monitoring analyzers.
Frequency DomainVibration exists in time, and it is said to be in the “time domain”. The representation of a vibration signal in the time domain is a “wave form”, and this is what one would see if the signal were displayed on an oscilloscope. If the waveform is subjected to a spectrum analysis, the result is a plot of amplitude versus frequency, called a spectrum, and the spectrum is in the frequency domain. The waveform is “transformed” from the time domain to the frequency domain. Most detailed analysis of machinery vibration data is done in the frequency domain, but certain information is more easily interpreted in the time domain.
Frequency RangeThis is the maximum frequency span that it is possible to measure. See also Frequency Span.
Frequency ResponseThe changes in the sensitivity of a circuit or device with frequency. The frequency response function, also called the FRF, is a characteristic of a system that has a measured response resulting from a known applied input. In the case of a mechanical structure, the frequency response is the spectrum of the vibration of the structure divided by the spectrum of the input force to the system. To measure the frequency response of a mechanical system, one must measure the spectra of both the input force to the system and the vibration response, and this is most easily done with a dual-channel FFT analyzer. Frequency response measurements are used extensively in modal analysis of mechanical systems.
Frequency Response FunctionThe output to input relationship of a structure. Mathematically, it is the Fourier transform of the output divided by the Fourier transform of the input.
Frequency Response MatrixFor an N degree of freedom system, it is an N x N symmetrical matrix whose elements are the frequency response functions between the various points on the structure. Rows correspond to response points and columns to excitation points. For example, H23 is the frequency response with excitation at point 3 and response at point 2.
Frequency SpanThe frequency range of interest in a measurement. This can be the measured or displayed frequency span. The frequency span can be equal to or less than the Frequency Range that the hardware/software in use allows to be measured.
Frequency Sweep TestA test in which a specimen is excited at a constant force or acceleration level while the frequency is swept (varied from low to high, and then back to low again).
Frequency WeightingModification of the frequency spectrum of a signal by means of a filter having a conventional characteristic known as A, B, C or D. A-weighting is the most commonly used.
Front-endData acquisition hardware.
FTCFrequency Time Curve. A graph of time versus frequency with magnitude displayed as dB contour lines.
FundamentalThe basic pitch of a musical note.
Fundamental FrequencyThe lowest frequency of a vibrating system. The spectrum of a periodic signal will consist of a fundamental component at the reciprocal of the period and possibly a series of harmonics of this frequency. The frequency is directly related to the phase-locked, rotational speed being measured and its amplitude may be low enough that it is difficult to see in the spectrum (or it may be missing altogether). The spectrum of a periodic signal will consist of a fundamental component at the reciprocal of its period and a series of harmonics of this frequency. The fundamental is also called the “first harmonic”. Even if the fundamental is so low in level that it cannot be seen, the harmonics will still be spaced apart by the fundamental frequency.
Fundamental Train Frequency (FTF)The rotation frequency or rate of the cage supporting the rolling elements in an anti-friction bearing. The FTF is always less than one-half shaft turn speed.
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Fusion ZoneAll reflections arriving at the observer’s ear within 20 to 40 ms of the direct sound are integrated, or fused together, with a resulting apparent increase in level and possible pleasant change of character. This is the Haas Effect.