How did 20th-century composers separate musical lines into isolated sound layers?

In 20th-century music, composers moved beyond traditional melodic combination to treat voice parts as isolated, static entities of sound. This separation was achieved through several innovative techniques:

• Polytonality: Composers grouped instruments and voice parts into layers, with each layer defining a different key or tonality simultaneously.1
• Tone Colour Contrast: Rather than focusing on melodies, composers differentiated layers based on timbre. For example, Edgard Varèse contrasted "sound-clouds" of wind instruments against percussion, while Elliott Carter assigned distinct tone colours and harmonic intervals to specific instrumental groups.1
• Ostinato Patterns: By limiting instruments to repetitive melodic and rhythmic patterns, composers created individual, static layers that shifted in arrangement, similar to a mobile.1
• Texture and Spectra: In works like György Ligeti’s Atmospheres, individual instrumental parts were woven into dense "bands" or spectra of sound, where tone colour was liberated from traditional form to become an independent entity.12
• Spatial and Visual Contrasts: Later experiments further isolated sound-producing groups through the physical placement of performers.1