Letter F

FFT: Fast Fourier Transform. A fast algorithm for performing a Discrete Fourier Transform (an orthogonal transform)

FLETCHER-MUNSON CURVES (EQUAL LOUDNESS CONTOURS): The ear is not equally sensitive to all frequencies, particularly in the low and high FREQUENCY ranges. Fletcher and Munson have charted the response to frequencies over the entire audio range in 1933, with later revisions by other authors, as a set of curves showing the sound pressure levels of pure tones that are perceived as being equally loud. The curves are plotted for each 10 dB rise in level with the reference tone being at 1 kHz. Also called loudness level contours and the Fletcher-Munson curves.
The curves are lowest in the range from 1 to 5 kHz, with a dip at 4 kHz, indicating that the ear is most sensitive to frequencies in this range. The intensity level of higher or lower tones must be raised substantially in order to create the same impression of loudness. The phone scale was devised to express this subjective impression of loudness, since the decibel scale alone refers to actual sound pressure or sound intensity levels.

FREQUENCY: The rate of repetition of the cycle of a periodic quantity, such as a sound wave. Thus, frequency is the inverse of the period. More generally, frequency can be thought of as the rate of change of phase.

FREQUENCY SPECTRUM: The FREQUENCY content of a sound or audio signal, often displayed as a graphic representation of amplitude (or intensity level) against frequency. Three-dimensional displays of a spectrum add the time variation on the third axis (see below). The spectrum of a sound is a primary determinant of its perceived timbre.

FUNDAMENTAL TONES (NOTES): If a sound is a complex of many tones of various frequency, amplitude and phase, repeating together in a basic cycle of definite frequency, the fundamental is the lowest frequency of this complex and corresponds to the unique pitch heard in such a complex tone.