Ears On

Dueling Loudness Corrections Explained

Mark Fleischmann  |  Oct 17, 2008

While reviewing the Onkyo TX-SR806 receiver ($1099) I did some to-ing and fro-ing with the THX and Audyssey people regarding THX Loudness Plus and Audyssey Dynamic EQ. These new loudness corrections operate in roughly the same territory--but in a different manner. Their common goal is to compensate for sonic losses that occur naturally at lower volume levels. As volume drops, the frequency response of human hearing changes. Loudness Plus and Dynamic EQ both tackle this problem by adjusting channel levels and frequency response. But beyond that, there are differences between them, and I asked the THX and Audyssey people to be specific about those differences. Here's what they said.

THX Loudness Plus

THX Loudness Plus is part of the new THX Ultra2Plus and THX Select2Plus specs. The following explanation is from Brian Schmidt, Consumer Applications Manager at THX:

The THX and Audyssey descriptions sound familiar because they both address the same basic problems that arise from listening at lower levels than where the content was mixed. They both operate on frequency response and surround levels, so it is difficult to distinguish the technical descriptions. Areas of distinction:

  • THX Loudness Plus uses different profiles for Movie and Music modes. Since music is often mixed at significantly higher overall levels and with more compression, loudness correction has to be approached differently for music and movie content. So, THX uses movie modes and music modes to automatically apply the correct profile.

  • THX has always required products to produce reference level output at master volume '0,' so we can be certain that the loudness adjustments will be as intended. As you know, all THX Certified AVRs produce the same reference level when the master volume control is set to '0.'

  • Some loudness solutions include features such as dynamic range compression and level matching, which can often change the overall output levels. THX's philosophy is to preserve the original dynamics and original reference level, so Loudness Plus does not incorporate these functions. When using the THX listening modes, the user will be sure that all of the original dynamics will be retained.

  • Since Loudness Plus and Dynamic EQ are trying to do similar tasks, we make sure they can't both be used at the same time as that sort of double-processing would be bad. THX Loudness Plus is automatically turned on when you are using one of the THX Listening Modes.and the Audyssey technology is turned off by default.

    MF: I'm looking at the description of Loudness Plus on the THX site. It seems vague except when it says "THX Loudness Plus automatically adjusts the front-to-back speaker level relationship as you turn the volume level down." The sound waves in the illustration seem to indicate lower volume in the front, and the same volume--but with marginally different propagation patterns--in the rear. Wouldn't this just make the rear speakers sound louder?

    Yes, the rear speakers will sound louder when the volume is lowered below reference level. However, this level change is mirroring the perceived level in the mastering environment. We're not just tweaking levels to make it sound better.

    The goal of Loudness Plus is to make playback at lower levels deliver the same sense of envelopment as you would get at the higher levels where the content was mixed. The ambient sounds that provide that sense of space quickly become masked and are lost when the volume is lowered, so we compensate for that. It's important to note that when listening at reference level ('0' on the Onkyo AVR), Loudness Plus doesn't process anything. So, it won't make the rear speakers sound louder when used at reference level.

    As the master volume is turned down, we gradually adjust the levels of the surround speakers up. This will make the rear speakers sound louder when compared to playback without Loudness Plus, but this is actually much closer to the intended effect that you would get when listening at reference level.

    Audyssey Dynamic EQ

    Audyssey Dynamic EQ is a feature of some receivers that license Audyssey's MultEQ auto setup and room equalization. The following explanation is from Chris Kyriakakis, founder and chief technical officer of Audyssey Laboratories, and is similar to material on the company's website:

    There are several key differences between Dynamic EQ and other methods of loudness control:

  • Many use the wrong perceptual curves. Most loudness controls are based on data collected at Bell Labs by Fletcher and Munson in the 1930s. While these curves characterized the effect, the measurements themselves were imprecise, due to the primitive equipment that was used at that time. Furthermore, these curves were actually developed for headphones and do not directly apply to loudspeakers in a room. Dynamic EQ uses a set of curves that were determined after lengthy experiments at the university labs run by Tom Holman [formerly the TX in THX--MF] and Chris Kyriakakis at USC.

  • None of the other methods have a method for absolute SPL measurement. Audyssey MultEQ first measures playback levels, then provides this information to Dynamic EQ for proper loudness correction. This is of critical importance, because without it the system doesn't know what loudness curve to adjust for.

  • They have no method of room equalization. Variations in the response of a room can be as large as 10 dB or more at some frequencies. Without proper equalization, the effectiveness of loudness correction will vary immensely from seat to seat. It will be too much for some listeners and not enough for others. MultEQ must be first applied to flatten the response over a large listening area before Dynamic EQ can apply loudness compensation.

  • They have no real-time analysis of audio content. Film and music pieces are typically made at reference volumes much louder than typical listening volumes. At reference level, loud sounds can be near 100 dB SPL, while soft sounds can be much softer, near 50 dB SPL. If a consumer listens at softer levels such as 20 dB down, loud sounds will need less loudness correction than softer sounds. Dynamic EQ analyzes each channel continuously to ensure the correct octave-to-octave balance is sustained as the content is playing.

  • Dynamic EQ adjusts surround impression. Film mixers from Hollywood were used in controlled experiments to determine the effects of lower playback volume on surround impression. By recording their actions, a model was created that mimics their adjustments to maintain the proper surround level at lower playback volumes.

    * * *

    Please note that I have not given the THX and Audyssey people a chance to respond to one another. Readers are free to parse their assertions and reach your own conclusions.

    If you try the Onkyo receiver, you'll have three options. You can use each feature separately or turn them both off. They will not operate simultaneously.

    Note also that they are distinct from other licensed processing used in the Onkyo, such as the THX cinema mode and Audyssey's MultEQ room equalization. Either or both of these can operate with either of the loudness corrections.

    Finally, if anyone out there has had a chance to use either THX Loudness Plus or Audyssey Dynamic EQ, please feel free to post your impressions below.

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