The term "sonorism" is primarily associated with the Polish composer Witold Lutosławski and the Polish School of composers in the mid-20th century. Sonorism is a compositional approach that emphasizes the manipulation of the sonorous aspects of music, such as timbre, texture, and color.
Witold Lutosławski, along with other composers like Andrzej Panufnik and Krzysztof Penderecki, was influential in developing and promoting sonorism. These composers sought to explore new and innovative ways of organizing sound, often incorporating extended instrumental techniques, unconventional notation, and a focus on the sheer physicality of sound.
It's worth noting that while sonorism was particularly associated with these Polish composers, similar ideas and approaches were explored by other composers around the world during the mid-20th century. However, the term "sonorism" itself is most commonly linked to the Polish School.