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 unattenuated, and it attenuates components in other frequency bands. Modifies the frequency spectrum of a signal usually while it is in electrical form. A Helmholtz Resonator is an example of an physical acoustic filter.
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.
Fourier Spectrum : the line spectrum resulting from an FFT analysis is equally spaced, so the time signal is analysed in constant bandwidths. The analyser 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 -
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 and are the most common type in use. The presence of the microphone should not to effect the measurement. Free field microphones are also known as omnidirectional microphones.
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.
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.
Fundamental Tone : sinusoidal component of a periodic sound wave having the same frequency as the periodic wave.