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THX just announced a new specification for home theater: THX Loudless Plus. My initial thought was, "I bet this means yet another new THX equipment specification." Well, yes, it does. The benchmark specification for all future receivers will now be THX Ultra2 Plus and THX Select2 Plus. But this is not a bad thing; this specification addresses a real need in home theater: consistent surround sound levels regardless of the listening volume.

Currently, the best way to reproduce a movie sound track is to calibrate your speakers to a certain reference volume level and make sure the volume knob is always at this position. Unfortunately, if you decrease the volume level, your ears perceive the changes in the different channels differently than the front speakers. Because the 3 front speakers (left, center, front) contain most of the sound information, they appear to become relatively more loud as compared to the surround channels; this is true for teh subwoofer as well. The result is that you lose the surround sound effect.

Even existing receivers that have a Midnight Theater mode do not fully compensate for this. They compress the dynamic range of all speakers by the same amount. In other words, the loud scenes are not as loud and the quite scenes are not as quiet, but the relative volume levels between the channels is not adjusted and the frequency curve is not adjusted, so the surround channels sound quieter than they should. THX Loudness Plus solves this by making two adjustments: 1) applying a volume gain curve to the surround and sub speaker levels, maintaining the proper balance regardless of where the volume knob is set, and 2) applying a frequency correction curve to the high and low surround frequencies to better balance with the main front speakers.

For anyone who wants to enjoy surround sound with the volume turned down (or really at any setting), this is a feature to consider during your next receiver upgrade.

For those of you keeping track of the THX certifications, here is the history:

• THX certification for home theater was launched around 1998
• THX certification became THX Ultra (high power amplifiers required) and THX Select was introduced (lower amplifier power requirements)
• THX Ultra2 and THX Select2 replace the previous Ultra/Select in 2005, adding more surround modes and better 7.1 support
• THX Ultra2 Plus and THX Select2 Plus replace the previous Ultra2/Select2 and add loudness compensation

Here are some quotes from the THX website giving more detail:

THX Multi-channel Spectral Balancing: adjusts frequency response to counter the perceptual loss of low and high frequency sound in all channels. Begins with a flat response at Reference Level, and continually adjusts the response as the user selects lower listening levels. The equalization is applied to all channels in a multi-channel listening environment.

THX Dynamic Ambience Preservation: Surround channel processing preserves the spatial detail that is lost when listening below Reference Level. It automatically shapes the output on surround channels to maintain the perceived balance in the original mix.

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