**Farad** the derived SI unit of electric capacitance; the capacitance of a capacitor between the plates of which a potential of 1 volt is created by a charge of 1
Coulomb

Named after Michael Faraday

Fast Fourier Transform *under* FFT below.

**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).

Related Terms •
line spectrum •
narrowband noise •
pink noise •
white noise •
wideband noise

See also •
constant bandwidth, constant percentage bandwidths •
octave bands.

**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).

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*.

**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, see also octave bands

Flanking Transmission *under* Sound Insulation.

**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.

See also • specific flow resistance

**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/s^{2} = 10^{5} dynes -

See also • force reference level

● 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. See also FFT (fast fourier transform)

**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.

**Because of their importance in acoustics we have a full page on** measurement microphones

See also • diffuse-field or random incidence microphones • Pressure Microphones

Free Field Room Definition

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

**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 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

Frequency Curve *under* 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 waveform, 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 Interval** ratio of two frequencies, see also logarithmic frequency interval

**Frequency** Nominal Frequency

**Frequency** Preferred Frequency

**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.

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

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