Facade Noise Levels
**Facade Noise Levels** measured at 1 metre from a building will include the noise reflected from the building and are typically 1 to 2 dB higher than a free field measurement.

Named after Michael Faraday

Far Sound Field

Fast Fourier Transform

**Fast Fourier Transform (FT)** 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).

FT Lines related to 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).

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

Field Normalised Impact Sound Pressure Level

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

See also narrowband noise • octave bands

Flanking Transmission

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.

See also loudness and related terms

**Flow Resistance Definition** (IEC 801-31-33) quotient of the difference of air pressure between the two faces of a sheet of porous material by the volume velocity of airflow through the sheet

See also specific flow resistance

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

**
Flux Density** the amount of flux passing through a unit area.

F = m·a = mass, x
acceleration

The SI unit of **Force** is the Newton 1 N = 1 kg·m/s² = 10^{5} dynes -

See also force reference level

**Forced Oscillation Definition** (IEC 801-24-01) oscillation caused by external excitation.

See also other oscillation terms

**Formant Definition** (IEC 801-29-44) of a Complex Sound, range of frequencies in which there is a local maximum in the sound spectrum.

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

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

Free Field Room

**Free Oscillation Definition** (IEC 801-24-02) oscillation that continues after removal of the external excitation.

See also other oscillation terms

Free Progressive Sound Wave

Free Sound Field

Frequency f, wavelength λ, speed of sound and time T are related by the formula λ = v/f and f = 1/T

See also angular frequency • centre frequency • natural frequency • nominal frequency • preferred frequenciesTypically the analysis in made using octave, one-third octave or narrow band (FFT) Analysis.

See also octave • 1/3 octave

Frequency Curve

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-weightings

**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.

**Fundamental Mode of Oscillation Definition** (IEC 801-24-16) mode of oscillation of a system having the lowest natural frequency.

See also other oscillation terms

**Fundamental Tone Definition** (IEC 801-30-01) sinusoidal component of a periodic sound wave having the same frequency as the periodic wave.