Filter
An acoustic filter passes or blocks components of sound of different frequencies. The passing components may be amplified or attenuated. Most used filters are:
Octave band filter:
Only the desired frequency range inside the octave is transmitted, outside this range all frequencies are blocked.
dB(A)-filter: the different frequency components are attenuated (or amplified) according to the A-filter values (see dB(A)).
High-pass filter: All the low frequencies below the cut off frequencies are blocked.
Low-pass filter: All the high frequencies above the cut off frequencies are blocked.
Band pass filter: A combination of a high-pass and low-pass filter with different cut-off frequencies so that only a defined range of frequencies passes through the filter. Octave band filters and 1/3 octave band filters are typical band pass filters.

Flanking transmission
Flanking transmission occurs when sound is transmitted from one space to another indirectly, through adjoining parts of the structure. For example, impact sound may be transmitted from one room to another through a timber floor, but also through the supporting wall.

Flutter echo
In room acoustics, a series of specific reflective returns caused by large surfaces being parallel to each other.

Free field
A sound field whose boundaries exert a negligible influence on the sound waves. In a free-field environment, sound spreads spherically from a source and decreases in level at a rate of 6 dB per doubling of distance from a point source, and at a rate of 3 dB per doubling of distance from a line source. Free field conditions can be used as a reference situation for defining room acoustics parameters, e.g. Strength G

Frequency
Frequency is measured in Hertz [Hz] and is defined as the number of times an event is repeated per time unit. For example with sound it is the number of repeated waveforms per second.
Human hearing 20-20.000 Hz