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
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 : 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 bandwidth •
constant percentage bandwidths •
continuous spectrum •
line spectrum •
narrowband noise •
narrowband spectra •
pink noise •
white noise •
, 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
, 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
, 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
under sound insulation
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
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
the rate of flow of a fluid or energy or particles across an area.
the amount of magnetic, electric, or other flux passing through a unit area.
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
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 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 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 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, wave
velocity v and time T are related by the formulae λ = v/f and f = 1/T
See also • angular frequency • centre frequency • natural frequency
analysing an overall broadband noise to identify the different contributions in different parts of the audio spectrum.
Typically the analysis in made using
one-third octave or narrow band
Frequency Band continuous range of frequencies between two limiting frequencies
See also • octave • 1/3 octave
under frequency weighting
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.
ratio of two frequencies
See also • logarithmic frequency interval
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.
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.