The Care and Feeding of Loudness Models

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The Care and Feeding of Loudness Models

• J. D. (jj) Johnston
• Neural Audio
• Kirkland, Washington, USA

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Implications?

• Single band weighting filters cant 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
  arent too far off.
• Theres no mention of time here yet.
• Loudness is sensed across frequency at a given
  time.
• Thats called partial loudness
• Getting that far is easy.
• NOW What?

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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.

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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.

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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
• 11/4 .011/4 .011/4 1.63

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The point?

• Overall, loudness models for extended periods are
  still in development.
• We dont know if loudness or annoyance, or
  something else, is what people adjust volume
  controls for
• We dont know if its peak or average, or some of
  both
• We dont know if everyone responds in the same
  fashion

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What Else?

• We dont 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