The equal loudness curve is the traditional standard used for music amplification and equalisation. As Drever and Hugill note in their 2023 publication Aural Diversity, this standard is based on a method devised in the 1930s at Bell Labs, and it was standardised through a series of experiments between 1983 and 2002, involving hearing people between 18 and 25 years, which represents an incredibly limited demographic.