## F : Sound and Vibration Terms, Definitions, Units, Measurements ...

### Facade Noise Levels

measured at 1 metre from a building will include the noise reflected from the building. To compare 'facade' measurements with free field values a correction of 2.5 or 3dB is normally applied

**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

Far Sound Field

Fast Fourier Transform see FFT below.

Fast Time Weighting

**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).

Field Normalized Impact Sound Pressure Level

### Field Quantity

, for decades a *field quantity* referred to any physical quantity measured at a specific point in space and time. Sound pressure for example met this definition, however not all physical quantities did. So a 'new' term, *root-power quantity* was introduced in ISO 80000-1 Annex C and defined as the square root of a power quantity, replacing and deprecating the term *field quantity*.

Note : **sound pressure squared** is proportional to sound power and is therefore a root-power quantity.

See also • sound fields

**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 • narrowband noise • octave bands

*Flanking Transmission* under

sound insulation.

Flat Weighting

### 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.

rapid but nearly even succession of echoes originating from the same sound source.

See also • echo

### Flux

the rate of flow of a fluid or energy or particles across an area.

###
Flux Density

the amount of 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 and 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

**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

Free Field Room

Free Progressive Wave

Free Sound Field

**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 Network

**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.

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.

sinusoidal component of a periodic sound wave having the same frequency as the periodic wave.

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