# Acoustic Glossary

## F : Sound and Vibration Terms ...

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.

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

Fast Time-weighting

FT : 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).

FT 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 it 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 noiseoctave bands

Flanking Sound

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.

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 a repetitive echo set up by parallel reflecting surfaces.

Flutter Echo Definition (IEC 801-31-23) 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/s² = 105 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 Level

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-fieldrandom incidence microphonespressure 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

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

See also angular frequencycentre frequencynatural 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 octave1/3 octave

Frequency Curve

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 Network

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.

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. 