F

Far Field
A region that is sufficiently far off from the source so that the direct sound pressure level due to the source decreases by 6 dB per doubling of distance.

Field Impact Insulation Class (FIIC)
An impact insulation class(IIC) rating that is based on measurements at an actual installation, as opposed to the measurements obtained in a laboratory.

Field Sound Transmission (FSTC)
A rating of the field-derived airborne sound transmission loss data for structure

Flanking paths
Any paths for sound transmission that bypass or circumvent the primary path through the structure under consideration.

Flutter
A rapid reflection or echo pattern between opposing sound-reflective walls with sufficient time between each reflection to cause a listener to be aware of separate discrete signals.

Focusing
Concentration of reflected acoustic energy within a limited location in a room as the result of reflections from concave surfaces.

Free Field
An enclosure or space in which there is no reflected sound. The sound field in an anechoic chamber, or in an open space, in which the ground surface is absorptive, is free field.

Frequency
The number of complete cycles per second of a vibration (or other periodic motion). The unit is hertz (Hz).

Frequency Analyser
An instrument capable of measuring the acoustical energy present in various frequency subdivisions of a complex sound.

Frequency Band
A subdivision of the frequency range of interest for measurement of analysis purposes.

Frequency Response
The changes in the sensitivity of a circuit or device, usually a loudspeaker, with frequency. Usually refers to the frequency range capability of a loudspeaker.

Frequency Spectra
The distribution of sound energy over a frequency range of interest.