Gracey & Associates
Noise and Vibration
Glossary - F
GLOSSARY INDEX:
A B C
D E F
G H I
J K L
M N O
P Q R
S T U
V W X
Y Z
Far Field : 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 : reverberant field .
Fast : see Fast Time Weighting.
Fast Fourier Transform : : FFT
Fast Time Weighting : a standard time weighting applied by the sound level meter - 125 ms averaging time - also known as Fast Response and Fast Time Constant. See also exponential averaging.
FFT : Fast Fourier Transform : a digital signal processing technique that converts a time record into a narrow band constant bandwidth filtered spectrum. Measurements are defined by specifying the frequency span and a number of lines (or filters).
FFT Lines : related to the number of sample points in a 'block' of data to be analysed. For example if the frequency range is DC to 1000 Hz and the number of lines = 400 then each line represents = 2.5 Hz (1000/400).
Filter : a 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 un attenuated, and it attenuates components in other frequency bands. Modifies the frequency spectrum of a signal usually while it is in electrical form.
Flanking Sound Transmission : in sound transmission measurement, flanking is the transmission of sound from the source room to the receiving room by paths other than through the partition under test. For example, impact sound may be transmitted from one room to another through a timber floor, but also through the supporting wall. Other common mechanisms for flanking transmission include internal leaves of external cavity walls which adjoin separating walls; suspended ceilings and partition walls; and also pipework and ducting.
Fletcher-Munson Curves : in the 1930s Fletcher and Munson, after extensive testing produced their equal loudness contours to relate a decibel reading, at a given frequency to loudness. They called this unit a phon.
Flutter : a repetitive echo set up by parallel reflecting surfaces.
Flux : the rate of flow of a fluid or energy or particles across an area.
Flux Density : the amount of magnetic, electric, or other flux passing through a unit area.
Force : F : in physics, a force is whatever can cause an object with mass to accelerate. Force has both magnitude and direction, making it a vector quantity is defined as the rate of change of momentum.
F = m·a = mass, x acceleration
The SI unit of force is the newton 1 N = 1 kg·m/s2 = 105 dynes
Fourier Spectrum the 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).
Fourier Transform : a 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 FFT
Free Field : a free field or space occurs if there are no reflecting surfaces around the source. In the real world there is no such thing as a true free field - although an anechoic chamber comes pretty close. Any reflective surface will modify the arriving sound.
Free Field Microphone : at frequencies above 1 kHz the wavelength of sound is small enough for a half-inch microphone to 'disturb' or affect the sound field you are trying to measure. Free field microphones are tailored to compensate for this effect.
Frequency : the number of pressure variations per second. Frequency is measured in Hz (Hertz). The normal frequency range for a healthy young person ranges from approximately 20 Hz to 20,000 Hz (20 kHz).
The frequency f, wavelength λ and wave velocity v are related by the formulae λ = v/f
Frequency Analysis : analysing an overall broadband noise to identify the different contributions in different parts of the audio spectrum.
Typically the analysis in made using 1/1-octave, 1/3-octave of narrow band (FFT) analysis.
Frequency Curve : see Frequency Weighting.
Frequency Domain : Vibration 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 Network : see Frequency Weighting.
Frequency Response Function : the 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 Weighting : standard frequency weightings applied by sound level meters to correlate the objective measurement to human response. For details see the relevant type - A-weighting , B-weighting , C-weighting or Z-weighting .
Fundamental Frequency : the lowest frequency of a vibrating system. The spectrum of a periodic signal will consist of a fundamental component and possibly a series of harmonics of this frequency. The fundamental is also called the first harmonic.
GLOSSARY INDEX:
A B C
D E F
G H I
J K L
M N O
P Q R
S T U
V W X
Y Z
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