(See also Dynamic timbres) Later, in early 1980s, listening tests were carried out on synthetic speech stripped of acoustic cues to assess their significance.
Time-varying formant frequencies and amplitudes derived by linear predictive coding were synthesized additively as pure tone whistles. These methods are characterized by extraction and recomposition of a set of significant spectral peaks corresponding to the several resonance modes occurred in the oral cavity and nasal cavity, in a viewpoint of acoustics.
There are even subtle differences in timbre between different versions of the same instrument (for example, an upright piano vs. Additive synthesis aims to exploit this property of sound in order to construct timbre from the ground up.
By adding together pure frequencies (sine waves) of varying frequencies and amplitudes, we can precisely define the timbre of the sound that we want to create.
As a result, only a finite number of sinusoidal terms with frequencies that lie within the audible range are modeled in additive synthesis. Additive synthesis can also produce inharmonic sounds (which are aperiodic waveforms) in which the individual overtones need not have frequencies that are integer multiples of some common fundamental frequency.
In the general case, the instantaneous frequency of a sinusoid is the derivative (with respect to time) of the argument of the sine or cosine function.Additive synthesis most directly generates sound by adding the output of multiple sine wave generators.Alternative implementations may use pre-computed wavetables or the inverse Fast Fourier transform.Technos acxel, launched in 1987, utilized the additive analysis/resynthesis model, in an FFT implementation.Also a vocal synthesizer, Vocaloid have been implemented on the basis of additive analysis/resynthesis: its spectral voice model called Excitation plus Resonances (Ep R) model Using these techniques, spectral components (formants) consisting of purely harmonic partials can be appropriately transformed into desired form for sound modeling, and sequence of short samples (diphones or phonemes) constituting desired phrase, can be smoothly connected by interpolating matched partials and formant peaks, respectively, in the inserted transition region between different samples.This representation can be re-synthesized using additive synthesis.One method of decomposing a sound into time varying sinusoidal partials is short-time Fourier transform (STFT)-based Mc Aulay-Quatieri Analysis."middle C is 261.6 Hz"), In other words, the fundamental frequency alone is responsible for the pitch of the note, while the overtones define the timbre of the sound.The overtones of a piano playing middle C will be quite different from the overtones of a violin playing the same note; that's what allows us to differentiate the sounds of the two instruments.The timbre of musical instruments can be considered in the light of Fourier theory to consist of multiple harmonic or inharmonic partials or overtones.Each partial is a sine wave of different frequency and amplitude that swells and decays over time due to modulation from an ADSR envelope or low frequency oscillator.