The perceived volume or loudness of sound is a complex phenomenon not entirely understood by experts. One widely-agreed method of assessing loudness is by measuring the sound pressure level (SPL), which is measured in decibels (dB) or bels (B, equal to ten decibels). In audio production communities, this is called level. The level of an audio signal is one way of measuring the signal's perceived loudness. The level is part of the information stored in an audio file.
There are many different ways to monitor and adjust the level of an audio signal, and there is no widely-agreed practice. One reason for this situation is the technical limitations of recorded audio. Most level meters are designed so that the average level is -6 dB on the meter, and the maximum level is 0 dB. This practice was developed for analog audio. We recommend using an external meter and the "K-system," described in a link below. The K-system for level metering was developed for digital audio.
In the Musicians' Guide, this term is called "volume level," to avoid confusion with other levels, or with perceived volume or loudness.
Level Practices, available at http://www.digido.com/level-practices-part-2-includes-the-k-system.html. The type of meter described here is available in the "jkmeter" package from Planet CCRMA at Home.
K-system (Wikipedia), available at http://en.wikipedia.org/wiki/K-system.
Headroom (Wikipedai), available at http://en.wikipedia.org/wiki/Headroom_%28audio_signal_processing%29.
Equal-Loudness Contour (Wikipedia), available at http://en.wikipedia.org/wiki/Equal-loudness_contour.
Sound Level Meter (Wikipedia), available at http://en.wikipedia.org/wiki/Sound_level_meter.
Listener Fatigue (Wikipedia), available at http://en.wikipedia.org/wiki/Listener_fatigue.
Dynamic Range Compression (Wikipedia), available at http://en.wikipedia.org/wiki/Dynamic_range_compression.
Alignment Level (Wikipedia), available at http://en.wikipedia.org/wiki/Alignment_level.