F (fast) time weighting: an averaging time used in sound level meters, and defined in BS EN ISO 61672-1.

Façade noise level: a noise level measured close to (less than 3 m away) from the façade of a building, which contains a contribution arising from reflection of sound at the façade. The difference between the façade level and the free field level (in the absence of the façade) is called the façade correction factor.

Far field: of a sound source; that part of the sound field of the source where the sound pressure and acoustic particle velocity are substantially in phase, and the sound intensity is inversely proportional to the square of the distance from the source.

Fast Fourier transform (FFT): an algorithm or calculation procedure for the rapid evaluation of Fourier transforms; an FFT analyser is a device which uses FFTs to convert digitized waveform signals into frequency spectra, and vice versa.

Field measurements: measurements carried out on-site, away from controlled laboratory conditions; the results of field tests of sound insulation may include the effects of flanking paths as well as direct sound transmission, which would not be the case for laboratory tests.

Filter: a device, which transmits signals within a certain band of frequencies but attenuates all other frequencies; filters may be electrical, mechanical or acoustical.

Flanking transmission: the transmission of airborne sound between two adjacent rooms by paths other than via the separating partition between the rooms, e.g. via floors, ceilings and flanking walls.

Flutter echo: a series of repeating echoes caused by parallel reflecting surfaces.

Forced vibration: steady-state vibration of a system caused by a continuous external force.

Form factor: the ratio of the RMS value of a signal to the mean value between two successive zero-crossings.

Fourier analysis/series/spectrum: Fourier’s theorem shows that any periodic function may be broken down (or analysed) into a series of discrete harmonically related frequency components which may be represented as a line spectrum.

Fourier transform: a mathematical process which transforms a non-periodic function of time into a continuous function of frequency, and vice versa (in the case of the inverse function).

Fractional dose: a fractional component of a total noise exposure, defined in connection with assessment of workplace noise exposure in connection with the Noise at Work Regulations.

Free field conditions: a situation in which the radiation from a sound source is completely unaffected by the presence of any reflecting boundaries; see also under anechoic.

Frequency: of a sinusoidally varying quantity such as sound pressure or vibration displacement; the repetition rate of the cycle, i.e. the reciprocal of the period of the cycle, the number of cycles per second; measured in hertz (Hz).

Frequency analysis: the separation and measurement of a signal into frequency bands.

Frequency response: of measurement system or component of such system, e.g. a sound level meter or microphone; the variation in performance, e.g. sensitivity, with change of frequency.

Frequency spectrum: a graph resulting from a frequency analysis and showing the different levels of the signal in the various frequency bands.

Frequency weighting: an electronic filter built into a sound level meter according with BS EN ISO 61672-1; see also under A- and C-weighting.

Fundamental frequency: the lowest natural frequency of a vibrating system; the repetition rate of a harmonic waveform.