Home theater glossary

Definitions

Sensitivity

Sensitivity is how loud a speaker plays from a known drive: typically dB SPL at 1 metre with 1 watt, or with 2.83 V (1 W into 8 Ω). A 98 dB horn needs far less wattage than an 85 dB stand-mount for the same seat SPL. Marketing numbers are often anechoic or in-room; treat ±2 dB as noise. Inverse-square still applies: double the distance and you lose about 6 dB.

Impedance

Impedance is the AC load the amplifier sees. “8 Ω nominal” still dips; a 6 Ω bookshelf can look like 4 Ω in the bass. AVR wattage is usually specified two-channel into 8 Ω; 6 Ω numbers are higher current, not free headroom. If the speaker’s sensitivity is quoted per watt, compare the receiver rating at the speaker’s nominal load; if it is quoted at 2.83 V, compare the 8 Ω rating, because that figure already counts the extra power a lower impedance draws. Then check the minimum impedance in the manufacturer PDF.

Sound pressure level

Sound pressure level (SPL) is a log scale: 10 dB is ten times the power, 3 dB is a doubling of watts. THX/cinema calibration aims for 85 dB average at the main seat with peaks to 105 dB. Living-room movies often sit 5–10 dB quieter. Distance from the speaker, room gain and seating off-axis all move the number. Use an SPL meter app only as a sanity check, not as a calibration certificate.

Throw ratio

Throw ratio is distance from the lens to the screen divided by the image width (not the diagonal). A 1.4–2.8 zoom on a 2.5 m wide 16:9 image sits between 3.5 m and 7.0 m. Short-throw and UST lenses use ratios well below 0.5. Always use the manufacturer’s min/max, not a forum guess: offset, zoom memory and lens shift eat the theoretical window. Recalculate if you change aspect ratio.

THX

THX home-theater seating uses about a 40° horizontal field of view, which puts you at roughly 1.2× the screen width (or ~0.84× a 16:9 diagonal). The cinema soundtrack reference is 85 dB C-weighted per channel at the mix position, with 20 dB of peak headroom. Living rooms rarely hit that without complaints from the next room. Use THX as a starting geometry, then sit where the picture and the neighbours allow.

SMPTE

SMPTE and related ITU notes describe a ~30° field of view for conventional HD (about 1.6× screen width) and closer seating when the source is 4K so pixels stay below visual acuity. These are viewing-geometry conventions, not a promise that every seat in a seven-seat riser will match. Combine SMPTE distance with pixel-pitch if you are deciding 1080p vs 4K at a given diagonal.

Room modes

A rectangular room supports standing waves. The first axial mode along length L is f = c / (2L) with c ≈ 343 m/s (about 57 Hz in a 3 m room). Width and height add their own combs; tangential and oblique modes fill the gaps. Peaks and nulls of 10–20 dB are normal. Treatment, multiple subs and EQ after measurement beat guessing from a calculator. The tool maps where the problems likely sit, not how to EQ them.

First reflection

First-arrival reflections smear imaging and dialogue. The “mirror trick” puts a helper on the sidewall: sit in the money seat, have them slide a mirror along the wall, and mark where you see the tweeter. That point is where an absorber or diffuser belongs. Geometry from speaker, seat and room width estimates the same spot. Treat left and right, then the ceiling bounce if you can. Floor reflections are often a rug.

AWG

AWG is a US wire-size scale. 12 AWG is thicker than 16 AWG and drops less voltage on a long run. A common budget is round-trip resistance under 5% of the nominal impedance so you do not throw away damping factor or watts as heat in the cable. Two-way length counts: 10 m of run is 20 m of copper. In-wall cable must match local code; this calculator is resistance math, not a fire rating.

HDMI

HDMI bandwidth is raw video payload plus blanking, chroma and bits per channel. HDMI 2.0 (18 Gbit/s) covers 4K/60 8-bit 4:4:4 or 10-bit 4:2:0; 4K/120, VRR and 8K push you into HDMI 2.1 (48 Gbit/s) with Ultra High Speed cables. Chroma 4:2:0 halves colour payload. EDID, DSC and the display’s actual input board still win arguments that a calculator cannot. If the picture strobes, check cable length before you blame the AVR.

VESA

VESA mounting standards fix the four-hole pattern and the screw size (usually M4, M6 or M8). The pattern is quoted horizontal × vertical, centre to centre in millimetres. Screen size is only a rough guide: one 55-inch set can be 200×200 and another 300×300, and many patterns are rectangular, such as 300×200 or 400×200. A matching pattern is necessary but not sufficient: the TV must stay under the mount’s rated load, and the studs or anchors in the wall must take the extra pull of a full-motion arm at full extension. Measure the holes; do not trust a marketplace title. If the TV is heavier than the mount’s rating, stop.

STC

STC is a single-number rating of how a wall, door or window reduces speech-range airborne sound. A single layer of drywall on timber might sit in the low 30s; staggered studs, extra mass and a damped air gap climb toward the 50s. Low bass (subwoofer, HVAC) leaks around STC because the curve ignores the lowest octaves. This site’s estimator is mass-law plus a crude air-gap bonus, not a lab certificate. Flanking paths through ducts beat any spreadsheet.