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The Care and Feeding of Loudness Models

This article explores the limitations of single band weighting filters in accurately measuring loudness, particularly over time. It discusses issues related to intensity, distortion, and the need for adaptable systems at the playback stage.

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The Care and Feeding of Loudness Models

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  1. The Care and Feeding of Loudness Models J. D. (jj) Johnston Neural Audio Kirkland, Washington, USA

  2. Implications? • Single band weighting filters can’t get it right. • They can get it moderately right for wideband signals with similar spectrum, where spectrum is smoothed on a critical band basis. • This means that for some “typical signals” they aren’t too far off. • There’s no mention of time here yet. • Loudness is sensed across frequency at a given time. • That’s called “partial loudness” • Getting that far is easy. NOW What?

  3. The time domain • While a variety of experiments have shown that the sum of partial loudnesses is a good measure of total loudness for a given instant, there is a lack of work on what it means when either partial or total loudness varies over time. And, that is where we are today.

  4. Some issues • In order to get it right, you have to know the intensity at the playback site (i.e. volume control setting, efficiency, acoustics, etc) • Good luck with that. • This creates particularly difficult issues below 500Hz. • Distortion, especially in the upper (70-120Hz) region can throw off loudness measurement by a phenominal amount.

  5. Bass distortion • Consider a 90 Hz sine wave. • Harmonics at 180 and 270 • Each of those harmonics is separated by a critical bandwidth. • Remember compression? If the woofer has 20dB SNR (that would be a very good woofer), the total loudness would scale to something like • 1^1/4 + .01^1/4 +.01^1/4 = 1.63

  6. The point? • Overall, loudness models for extended periods are still in development. • We don’t know if loudness or annoyance, or something else, is what people adjust volume controls for • We don’t know if it’s peak or average, or some of both • We don’t know if everyone responds in the same fashion

  7. What Else? • We don’t know how well people agree on long term vs short term preferences • Some people seem to care about peak • Some care about something kinda-sorta like average • We need a system that can be adapted at the point of playback, NOT at the source. • Then, just maybe, we might get some dynamic range back

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