Metrics for Quantifying Loudness and Dynamics

(This material was originally intended as part of the article “The Loudness War: Background, Speculation and Recommendations” [1] but was removed for reasons of scope and to keep that article to a manageable length.) In this paper, I briefly review a variety of metrics for quantifying the loudness and dynamic spread of audio recordings. This review was motivated by the need for objective ways of measuring the effects of hypercompression as used in the loudness war. 1. LOUDNESS AND DYNAMICS DESCRIPTORS If the loudness war is indeed a problem, we need to find measurements that correlate to the problem so we can estimate the effects of various types of processing. 1 It may prove impossible to measure a wide variety of effects, such as microdynamic and macrodynamic compression, clipping and aliasing, not to mention the likelihood of perceptual effects such as listening fatigue, with a single descriptor. In addition, some problems are hard to measure without access to both the ‘before’ and ‘after’ tracks. For example, blind hypercompression detection may be difficult without the original recordings because, for all we know, the audio being tested may be the original with no additional processing (e.g., double-reed instruments [2] having little dynamic range to begin with). 2. LONG-TERM LOUDNESS DESCRIPTORS A number of long-term loudness metrics have been developed for a variety of purposes: loudness meters, loudness control, sound quality research, and estimation or prediction of the perceived loudness of an audio program. [8] 1 v. 1.0, September 14, 2010 2.1.1. Single-band Loudness Models The “equivalent sound level,” Leq, is a long-term energy integration; it measures equivalent power over time and may be used as an estimate of sustained loudness. It can be defined as