In spectral music, the first step of the composing process is to analyze the spectrum of a sound in terms of the frequency content. The composer derives his or her musical material from the data palette obtained in the process of that spectral analysis.
One of the most significant techniques of spectral music is what Gerard Grisey described as "instrumental synthesis".
Instrumental synthesis is the use of an orchestra as a means of re-synthesizing the sounds. Modeling the sound of an instrument is the most represented case: after analyzing the sound spectrum of a selected instrument, the composer arranges the spectrum, that is, assigns the harmonic components of the spectrum to different instruments of the orchestra.
In the Grisey's piece Périodes (1974) for seven instruments, the last chord comes from the analysis of the spectrum of the sound of the low E tone of the trombone, so that the timbre of the trombone is artificially re-synthesized by the ensemble's instruments.
The most outstanding example is the analysis of the E2 tone played on the trombone as the basis for the textural structure at the beginning of the piece in another composition by Gerard Grisey, Partiels (1975).
A frequent source of compositional material in spectral music was the analysis of recorded instrumental sounds that became sound "models" for re-synthesis in many breakthrough pieces of this music. Examples include, in addition to the trombone notes in the works of Grisey, the bells and brass sounds in Tristan Murail's Gondwana (1980), the low piano notes, wind instruments and cellos in Murail's Désintégrations (1982/83) and the sounds of string instruments in Kaija Saariaho's Verblendungen (1984) or in Advaya (1994) by Jonathan Harvey.
Spectral music composers use computer programs capable of advanced spectral analysis of individual tones or colors to derive compositional material from this analysis. These programs are based on the Fast Fourier Transform (FFT) and express the waveforms of individual partials of the sound in the form of mathematical formulas. These mathematical expressions can then be used to create sound colors or as a material in creating the formal structure of a piece.
Currently, composers of the spectral school represent three composing generations and stylistic diversity. They write for different sets of instruments and often use the latest technological means to enrich their musical palettes.