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Home theater glossary

Every term gets a definition, why it matters, and one number worth remembering, with links to the tool or guide that uses it.

Video and displays

Acoustically transparent screen: A perforated or woven projection screen that lets sound pass through, so front speakers can sit behind the image as in a cinema.
ALR screen: An ambient light rejecting screen that reflects projector light toward the seats while absorbing or redirecting light arriving from other angles.
ANSI lumens: A standardized projector brightness rating averaged from nine points on a white test pattern, far more comparable than marketing lumens.
Dolby Vision: A licensed HDR format that adds dynamic, scene-by-scene metadata and up to 12-bit color on top of the PQ curve for more consistent tone mapping.
Foot-lambert (fL): A unit of screen luminance used for projection, equal to 3.426 nits, with 16 fL as the SMPTE reference for a dark cinema.
HDR10: The baseline open HDR format using 10-bit color, the PQ transfer curve, and static metadata that applies one set of brightness values to a whole film.
Input lag: The delay between a signal arriving at a display and the corresponding image appearing on screen, critical for gaming responsiveness.
Keystone correction: A digital fix that squares up a trapezoid-shaped image caused by a tilted projector, at the cost of resolution and sharpness.
Lens shift: An optical adjustment that moves the projected image up, down, or sideways without tilting the projector or distorting the picture.
Local dimming: A backlight technique that divides an LCD into zones that brighten or darken independently to improve contrast and black levels.
Mini-LED: An LCD backlight built from thousands of tiny LEDs, allowing far more local dimming zones and higher peak brightness than standard full-array sets.
Nit: A unit of luminance equal to one candela per square meter, used to rate the brightness of TVs and flat panel displays.
OLED: A self-emissive display in which each pixel produces its own light and can switch fully off, giving perfect black and per-pixel contrast.
Pixel pitch: The center-to-center distance between adjacent pixels on a display, which sets how close you can sit before individual pixels become visible.
QLED: A marketing term for an LCD TV that uses a quantum dot film to widen color gamut and improve brightness, not a self-emissive technology.
Refresh rate: How many times per second a display redraws its image, measured in hertz, which sets the ceiling for smooth motion and high frame rate input.
Screen gain: The ratio of light a projection screen reflects toward the viewer compared with a reference matte white surface, with 1.0 as unity.
Throw ratio: The distance from the projector lens to the screen divided by the image width, which tells you how far back a projector must sit.
Ultra short throw (UST): A projector design with a throw ratio below about 0.4 that sits inches from the wall and projects steeply upward onto the screen.
Viewing angle: The horizontal angle a screen fills in your field of view, measured from the seat to the screen's left and right edges.

Audio and speakers

Bi-amping: Driving a speaker's low and high frequency sections with two separate amplifier channels through its dual sets of binding posts.
Center channel: The speaker placed directly below or behind the screen that carries most dialogue and anchors sound to the picture.
Crossover frequency: The frequency below which a receiver redirects bass from a speaker to the subwoofer, with 80 Hz as the THX standard for most systems.
Damping factor: The ratio of speaker impedance to amplifier output impedance, describing how tightly the amplifier controls the motion of the driver.
Dolby Atmos: An object-based surround format that adds overhead height channels and places sounds anywhere in a three-dimensional space around the listener.
DTS:X: DTS's object-based immersive audio format, which renders sound objects to any speaker layout and does not require a fixed height speaker arrangement.
Height channels: Speakers mounted in or on the ceiling, or upward-firing modules, that reproduce the overhead layer in Dolby Atmos and DTS:X.
LFE channel: The Low Frequency Effects channel, the '.1' in 5.1, a dedicated bass-only track with 10 dB of extra headroom that feeds the subwoofer.
Object-based audio: A mixing approach in which sounds are stored as objects with position metadata and rendered to the listener's speaker layout at playback time.
Pre-outs: Line-level outputs on a receiver that carry each channel's processed signal to an external power amplifier, bypassing the built-in amps.
Speaker impedance: The electrical load a speaker presents to an amplifier, nominally 4, 6, or 8 ohms, which determines how much current the amplifier must supply.
Speaker sensitivity: How loud a speaker plays for a given input, stated in decibels at one meter with either 2.83 volts or one watt applied.
Timbre matching: Using speakers with the same tonal character across all channels so that sounds keep their voice as they pan around the room.

Room acoustics

Bass trap: A thick absorber, usually placed in corners, that damps low frequency room modes and shortens bass decay time.
Diffusion: Scattering sound reflections evenly in many directions with a shaped surface, so a room stays lively without discrete echoes or flutter.
First reflection point: The spot on a wall, floor, or ceiling where sound from a speaker bounces once and reaches the listener, arriving just after the direct sound.
NRC (Noise Reduction Coefficient): A single-number rating of how much sound a material absorbs, averaged over 250 Hz to 2 kHz, from 0 (fully reflective) to 1 (fully absorptive).
Room modes: Standing waves at frequencies set by a room's dimensions that create strong peaks and nulls in bass response at different seats.
RT60: The time it takes sound in a room to decay by 60 dB after the source stops, the standard measure of how live or dead a space is.
Schroeder frequency: The transition frequency below which a room behaves as a set of discrete resonances and above which reflections blend into a diffuse field.

Cables and connections

ALLM (Auto Low Latency Mode): An HDMI signal that tells a display to switch into its low-lag game mode automatically when a console or PC starts sending game content.
AWG (American Wire Gauge): The standard scale for wire diameter, where a smaller gauge number means a thicker conductor with lower resistance per foot.
CL2 / CL3 rating: Fire-safety ratings for low-voltage cable jackets that make speaker and HDMI runs legal to install inside walls and ceilings.
eARC: Enhanced Audio Return Channel, an HDMI feature that sends full-bandwidth, lossless audio from a TV back to a receiver or soundbar.
HDMI 2.1: The HDMI specification with 48 Gbps of bandwidth, enabling 4K at 120 Hz, 8K, VRR, ALLM, and eARC over a single cable.
OFC vs CCA: Oxygen-free copper is solid copper wire; copper-clad aluminum is aluminum with a thin copper skin that has about 60 percent more resistance.
VRR (Variable Refresh Rate): A display feature that lets the refresh rate track a game's frame rate in real time, eliminating tearing and stutter without added lag.

Setup and calibration

38 percent rule: A placement guideline that puts the main listening position about 38 percent of the room length from the front or rear wall to sit in the smoothest bass.
Reference level: The calibrated playback volume at which each main channel peaks at 105 dB SPL and the LFE channel at 115 dB, as film mixes were monitored.
Room correction: Automated measurement and equalization built into receivers that adjusts speaker levels, delays, and frequency response to compensate for the room.