Loudness is a subjective measure relating to the physical sound pressure level (SPL) as perceived by the human ear. A number of devices have been created for controlling audio levels to modify either a signal's loudness or its range of loudness. Automatic gain controls are typically used to reduce loudness differences between audio programs (for example, between one audio track and the next). Dynamic compressors are similar to automatic gain controls, but are primarily intended to reduce the loudness range within a single track or audio program.
Compressors have a number of uses, including increasing the loudness of the softer parts of an audio program so they can be heard above the noise floor (e.g., for automotive listening), decreasing the loudness of the loudest segments (for example, to avoid disturbing neighbors during late-night listening), and keeping signal levels within technical limits required for radio broadcast.
Loudness matching and dynamic compression both pose challenges to creators of audio hardware and software. For example, different audio tracks may have different dynamic ranges, and the short-term loudness of those tracks may vary significantly over time; both factors complicate the effort to match the long-term loudness of the tracks. In the case of dynamic compression, a compressor may apply different amounts of compression to an audio track depending on how that track is scaled. Similarly, a compression scheme may apply compression to an audio track that is already highly compressed, and may apply no compression to a track that would benefit therefrom. Still further, one side effect of dynamic compression is that it can have different effects on the overall loudness of different audio tracks, making it difficult to perform post-compressor loudness matching in real-time.