F3 - Refers to a - 3 dB cutoff frequency, measured in Hz. Essentially, the frequency above or below a particular starting point where a loss of 3 dB in output occurs.
.

Fader - The control that adjusts the relative volume levels between the front and rear speakers in a four speaker system (or the front and rear pre-amplifier outputs).

Farad - The basic unit of capacitance. A capacitor has a value of one farad when it can store one coulomb ( the amount of electric charge carried by a current of 1 ampere) of charge with one volt across it.

Fb - The tuned frequency of a Vented (Ported) Enclosure , measured in Hz. Below this point the frequency response of a Tuned Port system rapidly falls off and it can become unstable (potentially damaging the speaker) if asked to reproduce lower frequencies at high volume levels.

Fc - System resonance of a sealed box systems, in Hz.

FCC - Acronym for the Federal Communication Commission, the U.S. government agency that oversees and regulates electronic communication.

Fidelity - A term used to describe the accuracy or general quality of audio processing, recording or reproduction.

Filter - An active or passive circuit or device designed to reject signals, vibrations or the radiation of certain frequencies while passing others through.

FL Backlight - Fluorescent illumination for the lighted portion of a liquid crystal display.

Flat Response - An output signal in which fundamental frequencies and harmonics are in the same proportion (uncolored) as those of the input signal being amplified.

Fletcher-Munson Curves - Fletcher and Munson were pioneering researchers who established many basic concepts of High Fidelity in the '30s. They accurately measured and published a set of plot curves that showed the human's ear's sensitivity to loudness verses frequency. The curves show the ear most sensitive to sounds in the 3 kHz to 4 kHz area and sounds above and below 3-4 kHz must be louder in order to sound balanced at lower volume levels. These are the curves that most "Loudness" circuits are based on.

Floating Ground - Grounding points that are not commonly connected. Found in higher powered amp outputs where the left and right channels do not share a ground path.

Floating Ground Adapter - A circuit that converts a floating ground amp output to common ground.

Flutter - Flutter is a rapid variation in the frequency of reproduced sound caused by slow speed variations in records, tape recorders, etc. Slower pitch fluctuations of one or two per seconds are classified as wow.

Flux - The flow of magnetic energy in a circuit. Flux density is the measure of the strength of the magnetic field (typically used in reference to speakers). Higher Flux density usually means higher power handling.

Former - The cylindrical portion of a speaker's voice coil section. A wire is wound around this cylinder to form the voice coil. Commonly made from Aluminum or Kapton. Kapton is known to usually be more resilient than Aluminum under high heat conditions (click here for Speaker parts diagram).

Frequency - Also referred to as Cycles per second. The number of wavelengths produced in a one second time period, measured in Hertz (Hz).

Frequency Modulation (FM) - A method of modulation in radio broadcasting. The frequency of the carrier voltage is varied with the frequency of the modulation voltage (Also see Amplitude Modulation).

Frequency Response - The range of frequency, lowest to highest, that can be reproduced by an audio component (within specific tolerances and limits).

Fs (a Theile Small parameter ) - Also referred to as Free Air Resonance. The resonant frequency of the driver's voice coil impedance with the driver suspended in free air (no enclosure). One of the three main properties used in Theile/Small Parameter for speaker frequency response calculations.

Fundamental Frequency - Also referred to as the First Harmonic. It is the lowest frequency in a complex waveform. The harmonics above this are multiples of it (x2, X3, X4 and so on). So, if a Fundamental Frequency is 300 Hz, the second harmonic would 600, the third would 900 Hz, etc.

Fuse - A device designed to provide protection for a given circuit or device. Rated in Amperes (Amps), they are designed to blow or physically open the circuit when the current being drawn through it exceeds its design rating. They come in two types, fast or slow blow.

Fusible Link - Similar to a fuse, but looks more like a wire. Fusible links are commonly used in ignition switches, power transformers and other high current circuits.