volume level

Professional Definition

The loudness level is a quantity that numerically equals the sound pressure level of a standard tone (usually at a frequency of 1000 Hz), which the listener subjectively perceives as equally loud as the sound in question.

The unit of loudness level is phon. It is a psychoacoustic measure that takes into account the nonlinear sensitivity of human hearing to different frequencies. This relationship is described by the so-called equal-loudness curves (Fletcher-Munson). It follows that for low (bass) tones, the sound pressure level (in dB) must be significantly higher to produce the same perceived loudness (in phons) as for midrange tones.

Acoustics in Simple Words

Imagine that decibels are air temperature, and loudness level is perceived temperature. A thermometer may show -5 degrees Celsius, but if a strong wind is blowing, it feels like -15. Similarly, with the ear: the physical sound pressure (decibels) may be the same for bass and a whistle, but our ear (naturally less sensitive to bass) will "feel" the whistle as significantly louder. For bass to be heard as clearly as conversation, we need to provide it with significantly more energy – which is why subwoofers need to have high power.

Summary

Loudness level is a parameter that defines the subjective impression of sound intensity. It allows engineers to match audio devices and warning systems to the actual perceptual capabilities of humans.