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Gunnar Fant developed acoustic models connecting the physical production of speech to its measurable signal. His Acoustic Theory of Speech Production by Gunnar Fant relates vocal-tract geometry to acoustic behavior, including calculations based on X-ray studies of articulations. The original book dates to 1960; the inspected publisher preview is its 1970 second printing.^[ref]

His source-filter model of speech production separates excitation from the filtering of the vocal tract. Changing a filter's resonances can move formants without requiring the same change in the fundamental frequency of the source. This makes speech synthesis an experimental tool: a researcher can vary model parameters and compare the generated signal with recorded speech. KTH's account explicitly describes his synthesis work as a means of testing production hypotheses.^[ref]

KTH's curriculum vitae records 1919–2009 and his leadership of the Speech Transmission Laboratory from 1951 to 1966.^[ref]

Speech synthesis generates an artificial speech signal from a specified representation. Text-to-speech starts from writing, while a research synthesizer can instead take a phonetic specification or directly controlled acoustic parameters. The distinction separates the input representation from the mechanism producing the sound.

In a source-filter model of speech production, a researcher can set an excitation's fundamental frequency and independently adjust the filter's formants. Holding the first parameter fixed while moving a formant tests whether listeners' responses depend on the spectral change rather than a change in voice pitch. Gunnar Fant's work at KTH used synthesis to test hypotheses about speech production.^[ref] Generated stimuli permit controlled comparisons in speech perception, but intelligibility and similarity to a natural speaker are different outcomes. Recording the parameter settings makes a synthesis experiment reproducible.