Our human ear is far from linear, dynamically speaking, especially in the low frequency and high-mid frequency regions.

Simply put, our ears feel/perceive the loudness differently for different ranges of frequencies. The first research on the topic of how the ear hears different frequencies at different levels was conducted by Fletcher and Munson in 1933. In 1937 they created the first equal-loudness curves. Refer the diagram below.

Fletcher–Munson Equal-Loudness Curves

A glance on the diagram gives you some quick insights about the loudness “perception”.

• The Mids are perceived louder at significantly lower Decibel level compared to the Lows and Highs.
• This can also be interpreted as the sensitivity of human ears to mids. Ears are highly sensitive to the mids & the dynamics lying there hence the perception of loudness needs much lower sound pressure level (decibel)

This is an invaluable tool to quickly judge a soundscape (or a music track) for overall balance & chances of causing fatigue.