Acoustic Glossary


 

F : Sound and Vibration Definitions, Terms, 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 Field
Far Sound Field


Fast Fourier Transform under FFT below.


Fast Time Weighting


FFT : 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).

See also • constant bandwidthconstant percentage bandwidthscontinuous spectrumline spectrumnarrowband noisenarrowband spectraoctave bands. pink noisewhite noisewideband noise


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 Quantity

for example a sound pressure is proportional to the sound power when the sound pressure is squared.

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 • narrowband noiseoctave bands


Flanking 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.


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 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 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/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 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 microphonespressure microphones


Free Field Room Definition

Free Oscillation Definition

IEC 801-24-02, oscillation that continues after removal of the external excitation.

See also other oscillation terms


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 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 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 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

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.

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