The THX 0 dB reference level is a calibrated volume setting used in dubbing, cinema, and home theaters. The 0 dB (decibel) setting represents an average 85 dB SPL (sound pressure level). Movie soundtracks are created and mixed in dubbing theaters. Movies are presented and played back in cinema and home theaters. The general goal of the reference level is to preserve the directors intent which is to ensure that the volume level that the movie was created (mixed) at is the same as the playback volume.
Pink noise and a sound pressure level (SPL) meter with C-weighting are used to calibrate the individual 5.1 or 7.1 speaker levels. This calibration balances all of the speakers levels so that they are equal and that no speaker is louder than any other. The calibration also sets the 0 dB volume setting to equal the THX reference level. Both internal and external pink noise signal sources can be used to calibrate the speaker levels. External calibration signals found on video discs are typically full range pink noise at a 85 dB SPL. A preamp/receiver's internal calibration signal is typically banded pink noise played at a 75 dB SPL which is -10 dB down from the reference level. The quieter -10 dB signal is chosen by manufacturers because it is less harsh for users. It is also important to note that banded pink noise and full range pink noise will produce dB level settings that are slightly different. Which one equates to the true reference level depends on who you ask and how flat the frequency response of your speakers are.
The 0 dB reference level represents an average 85 dB SPL and a maximum 105 dB SPL. The LFE (Low Frequency Effects) channel used by AC3 (Dolby Digital) and DTS is +10 dB higher. This means that the maximum output for bass peaks is 115 dB SPL. The purpose of the +10 dB gain for the LFE channel is to increase the dynamic range of bass events such as explosions, crashes, punches, and rumbles. This means when the volume is set to 0 dB that the subwoofer could be asked to produce an output of 115 dB SPL. Such loud bass levels places a heavy resource burden and requires multiple high-end subwoofers to produce it accurately. Fortunately THX processors have a feature called the Bass Peak Limiter which compresses peak levels in an attempt to reduce distortion and prevent speaker damage.
The THX certification process for cinema theaters has numerous audio and visual quality control assurances that must be met. This THX stamp of approval ensures that the movie looks and sounds good. The 0 dB THX reference level is part of this quality assurance and the volume control knob is usually behind a locked panel so that it cannot be adjusted. This ensures that all movies played in a THX certified theater are played at the correct THX 0 dB reference level. Unfortunately a 0 dB volume level is too loud for many patrons and cinema owners so over time many certified THX theaters will have their "locked" reference volume levels reduced. If the rules aren't enforced then what use are rules?
A related benefit of the 0 dB reference level is the requirement that home THX certified equipment, particularly amps and speakers, be able to reproduce volume at this level without distress. Originally two 12" subwoofers were required to meet the THX spec for large rooms. An output level of 105 dB isn't an easy task at 20 - 40 Hz for subwoofers, especially when the +10 dB LFE channel is accounted for. Actually this output spec has been relaxed a lot during the past decade so that a single smaller sub can meet it. This "practicality" was probably due to pressure from consumers and manufacturers.
The 0 dB THX reference level is a wonderful concept for many other reasons. It creates a standard for comparing playback volume levels. If you and I both have our systems calibrated to the reference level then when I say that watched that movie at volume setting of -15 dB then you will know what I mean and you will have a general feeling for how loud that is.
The engineer in me really likes having the volume control operate with units of dB. And the fact the 0 dB level is actually a reference to something makes it even better. I don't know what is but that dB display just soothes my soul. In fact I won't buy a preamp or receiver that doesn't have it.
Watching a movie at 0 dB is crazy loud and I rarely watch a movie anywhere near that level. It depends a lot on the movie but I tend to watch movies with the volume set between -17 and -11 dB. For some very quietly mixed movies I tend to go up to about -5 dB. I love loud bass and I also love my hearing. I just don't understand how the professional sound mixers can listen to their craftwork at the 0 dB reference level for 8 hours a day, 5 days a week, without suffering any permanent hearing damage. According to the OSHA occupational noise exposure levels A-weighted SPL's of 90 dB for 8 hours, 105 dB for 1 hours, and115 dB for 15 minutes per day are allowed.
I'm a fan of THX technology and it is kind of sad seeing the slow dilution of the 0 dB reference level specifications. The THX stamp at the cinema and in the home just doesn't mean what it used to.