Ken Stevens passed away last week. He was without question one of the giants in the field of speech. His contributions are legion – he is best known for the quantal theory of speech, a theoretical framework that ‘explains’ the mapping between acoustic and articulatory events and the basis for the finite inventory of speech sounds found in natural language. Ken proposed that there are regions in articulatory space which give rise to small acoustic changes and other regions which give rise to large acoustic changes. These areas of change define the finite inventory of possible speech sounds. That there are regions of acoustic stability suggests that there is acoustic invariance in speech. Namely, there are acoustic properties corresponding to phonetic features that remain stable across phonetic context, position, and speaker. Ken’s work provided an empirical basis for the distinctive features proposed by Roman Jakobson, Gunnar Fant, and Morris Halle in their 1951 monograph Preliminaries to Speech Analysis. Distinctive features have been considered to be the representational units underlying the phonological inventories of natural language. Throughout his work, Ken emphasized the importance of acoustic landmarks which define natural boundaries used by the perceptual and ultimately the linguistic system for characterizing the phonetic/phonological properties of speech. Ken, in collaborative work with Morris Halle in the 1960’s, also proposed a model of speech, analysis by synthesis. Here, the input signal matches to internally generated representations and ultimately selects the ‘best match’. This theory has modern day applications and implications for current Bayesian and predictive coding approaches to perceptual processing.
I had the good fortune to work with Ken over a period of 18 years. I first approached Ken in 1974 on the advice of Roman Jakobson to see if I could learn acoustics while on leave from Brown. I spent that semester at MIT and Ken and I worked on our first project examining the acoustic and perceptual properties of retroflex consonants. That was the start of a collaboration that resulted in a series of papers and chapters on acoustic invariance in speech and the acoustic properties of a number of phonetic parameters.
Ken’s working style was and is something to emulate. Low-key, focused, collegial, his lab was a bevy of activity. He was a fantastic leader of the Speech Group at MIT welcoming students and colleagues from all over the world. The only requirement was an interest and a commitment to research in speech. No one would have known he had won the National Medical of Science, was elected to the National Academy of Sciences, or had been the President of the Acoustical Society of America. He was just Ken, a great researcher, mentor, collaborator, colleague, and friend!