Home theatre systems
Calibrating a home theatre
What calibration sets — levels, distances, crossovers, subwoofer alignment and equalisation — the reference levels it works to, and what room correction can’t fix.
8 min read8 sourcesChapter 19 of 24
Schematic
A test signal played through each loudspeaker and captured at the seats. Levels, arrival times and the response at each position are what calibration is set from.
Animated illustration of the principle. Not measured data.
Calibration is the last step, not the first fix. Once the room is built and the loudspeakers are in their designed positions, calibration makes every channel arrive at the seats at the right level and the right time, integrates the subwoofers, and corrects what is left. It cannot move a loudspeaker or shorten reverberation; it finishes a room that was designed properly.
Levels
Every full-range channel is set to produce the same sound level at the main listening position. Dolby’s home installation guide uses pink noise at −20 dBFS and a target of 79 to 82 dB SPL, C-weighted, at the central listening position. The cinema equivalent is SMPTE RP 200’s 85 dBC per screen channel. Receivers with internal test noise use their own target — THX advises 75 dB on the meter with a certified receiver’s internal noise. What matters is that every channel reads the same.
The subwoofer level is set so redirected bass plays at the same level as the channels it came from, and so the LFE channel lands at its intended +10 dB. THX suggests checking the subwoofer level across several seats rather than only the main one.
Distances and delays
Sound travels about 343 m/s, so each metre is about 2.9 ms. Loudspeakers at different distances are delayed so that every channel’s sound arrives at the listening position together. The distances are measured and entered (or measured automatically), and the subwoofer delay is then adjusted so it sums correctly with the main loudspeakers through the crossover.
Automatic setup is not infallible: Dolby notes that auto-calibration systems can set the wrong distance for up-firing loudspeakers. Every value is checked, not just accepted.
Room correction
Room-correction systems measure at several positions and calculate filters. Audyssey recommends at least eight microphone positions; Dirac and Anthem ARC likewise measure across the listening area, because a single position produces corrections that are wrong everywhere else.
Its limits are worth stating plainly. Trinnov, which makes some of the most advanced processors, says that “no system can truly ‘correct’ a room with signal processing alone” and that it “cannot reduce reverberation time in the mid and high frequencies”. Correction works best on what all the seats share — mainly bass peaks — and on small tonal differences such as an acoustically transparent screen’s high-frequency loss.
The cinema “X-curve” (SMPTE ST 202) is a target for large rooms: its scope is rooms over 125 m³. It is not the target for a home theatre.
Target curves
Correction needs a target, and choosing one is not neutral. Toole argues that adjusting a room curve to match a target does not by itself ensure good sound — the loudspeakers’ own quality and the room acoustics come first. Listening research at Harman found that listeners preferred an in-room response with somewhat more bass and treble than a flat line, with large individual differences in how much. A calibrated room is set to a deliberate, documented target, then fine-tuned by listening.
What a handover should include
There is no single public standard for a calibration handover. In practice a complete one documents: the level of every channel, the distance and delay settings, the crossover for each loudspeaker, the subwoofer levels, delays and phase, the equalisation applied and its limits, the target used, and a saved copy of the processor’s settings — so the system can be restored after a firmware update or a factory reset.
For integrators and architectsThe numbers, the standards, and what to specify
- Level
- Dolby home: pink noise −20 dBFS RMS → 79–82 dB SPL(C) at the central listening position, every full-range channel equal. Cinema: 85 dBC per screen channel, C-weighting, slow (SMPTE RP 200).
- Delay
- ≈2.9 ms per metre at 343 m/s. Verify auto-set distances, especially for up-firing modules (Dolby).
- Subwoofer
- Level for redirected bass and LFE +10 dB; delay/phase aligned through the crossover; checked across seats (THX).
- Correction
- Multi-position measurement (Audyssey ≥8 positions). Cut-only below the transition region; correct AT-screen HF loss. No DSP fixes mid/high RT (Trinnov).
- Target
- X-curve only for rooms >125 m³ (SMPTE ST 202 scope). Small rooms: documented in-room target, fine-tuned by listening (Olive et al. 2013; Toole 2015).
What to write into the brief
- A calibration record: channel levels, distances/delays, crossovers, subwoofer level/delay/phase, EQ applied and its limits, the target, and the saved settings file.
Sources
- 01Dolby — Dolby Atmos Home Theater Installation Guidelines (2018)Level calibration 79–82 dB SPL(C) at −20 dBFS; auto-calibration caveats.
- 02SMPTE RP 200-2002 — Relative and absolute sound pressure levels for motion-picture multichannel sound systems
- 03THX — frequently asked questions (calibration levels and distances)
- 04SMPTE ST 202:2010 — B-chain electroacoustic response (X-curve)Scope: rooms over 125 m³.
- 05Toole, “The Measurement and Calibration of Sound Reproducing Systems”, JAES 63(7/8), 2015 (open access)
- 06Olive, Welti & McMullin, “Listener Preferences for In-Room Loudspeaker and Headphone Target Responses”, AES 135th Convention, 2013
- 07Trinnov — room correction explainedLimits of correction by signal processing.
- 08Audyssey — MultEQ-X user guideMulti-position measurement.
External links open in a new tab. Figures quoted in this chapter are taken from these sources; a Science of Sound measurement is only ever presented as one, with its file, date and instrument.
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