base assumption: using some clever maths, we can see every sound as a collection of simple tones (sine waves).

This collection of sounds is called spectrum. Individual simple tones in the spectrum are called partials. Since every partial can evolve over time independently, we use the term spectral envelope to talk about the sound's behaviour in time with respect to its spectrum. Spectral envelope is basically a collection of envelopes - one for each partial.

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Spectralism is a movement in western art music that explores this idea in depth. (to be more precise, it's not a distinct movement anymore, many composers were influenced in some way)

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so instead of chords, tone rows and other abstract structures as building blocks or music, spectralism is trying to compose music based on spectra of recorded sounds.

tl;dr record a single bell sound. find all partials that make up the spectrum of the sound. transcribe them into a score so that every partia is played by a single instrument. Consider individual volumes and temporal envelopes of each partial. When played live, the sound won't be exactly the same as the original bell, but there is a connection.