Named after Michael Faraday

Far Sound Field

Fast Fourier Transform see FFT below.

**FFT : Fast Fourier Transform**, a digital signal processing technique that converts a time record into a narrow band constant bandwidth filtered fourier spectrum. Measurements are defined by specifying the frequency span and a number of lines (or filters).

See also •
constant bandwidth •
constant percentage bandwidths •
continuous spectrum •
line spectrum •
narrowband noise •
narrowband spectra •
octave bands.
pink noise •
white noise •
wideband noise

A 'new' term root-power quantity was introduced in ISO 8000 Annex C and defined as the square root of a power quantity. It replaces and deprecates the term *field quantity*.

See also • narrowband noise • octave bands

Flat Weighting

See also • specific flow resistance

**Flutter Echo** IEC 801-31-23, rapid but nearly even succession of echoes originating from the same sound source.

F = m·a = mass, x
acceleration

The SI unit of **Force** is the Newton 1 N = 1 kg·m/s^{2} = 10^{5} dynes -

See also • force reference level

See also other oscillation terms

● Note : the frequency at the maximum is the formant frequency.

**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 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.

See also • diffuse-field or random incidence microphones • pressure microphones

Free Field Room

See also other oscillation terms

Free Progressive Wave

Free Sound Field

**Frequency (f)** the number of times that a
periodic function or vibration occurs or repeats itself in a specified time, often 1 second - cycles per second. It is usually measured in Hertz (Hz).

Frequency f, wavelength λ and wave velocity v are related by the formulae λ = v/f

See also • angular frequency • centre frequency • natural frequency

Typically the analysis in made using octave, one-third octave or narrow band (FFT) Analysis.

**Frequency Band** continuous range of frequencies between two limiting frequencies

See also • Octave • 1/3 octave

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.

See also • logarithmic frequency interval

Frequency Network

**Frequency** - Nominal Frequency

**Frequency** - Preferred Frequency

Frequency Weightings

**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 Frequency Definition** IEC 801-24-11,

a) frequency of the sinusoidal component of a periodic quantity that has the same period as the periodic quantity

b) lowest natural frequency of an oscillatory system.

See also other oscillation terms

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