The urgent need for sustainable solutions in different industrial sectors has led to a growing interest in using natural and recycled fibrous materials for acoustic and thermal insulation. While these materials offer significant environmental benefits, their commercial production is still limited also due to a lack of standardised characterisation and design methods. This paper addresses this gap by presenting a comprehensive methodology to develop and acoustically optimise sustainable fibrous materials. The approach integrates small-scale experimental characterisation with a robust analytical framework that models a material's acoustic performance as a function of its density. The effectiveness of this method is demonstrated through case studies involving a variety of sustainable materials, including hemp, jute, posidonia, and recycled fibre mixtures. The results validate the methodology's ability to accurately predict the sound-absorbing properties of these materials, enabling the design of high-performance solutions capable of achieving acoustic ratings comparable to traditional materials. This article, invited by the RIA editorial board, is not an original study. Instead, it synthesises existing research by reanalysing and integrating the data from multiple studies using a single, unified methodology.