Lips, tongues, and voice boxes can make far more sounds than most languages require, and Kenneth N. Stevens wanted to know why. A professor at MIT, he spent nearly six decades studying connections between engineering and linguistics to grasp what creates the vocal sounds people produce and why so many languages generally stick to similar patterns of sound.
"He was a brilliant scientist and engineer," said Joseph Perkell, who studied with Dr. Stevens as a doctoral student and worked with his research team. "He came up with some very interesting ideas about the relationship between the acoustics of the speech production mechanism and the sound categories of languages."
Dr. Stevens, who was named a National Medal of Science recipient in 1999, died Aug 19 in Monterey Court Memory Care in Happy Valley, Ore., of complications of Alzheimer's disease. He was 89 and had lived in Clackamas, Ore.
Anantha P. Chandrakasan, head of the electrical engineering and computer science department at Massachusetts Institute of Technology, said in a statement that Dr. Stevens "had a tremendous impact . . . and was a wonderful colleague."
"His work will continue to make an impact, not only through his writings but also through his many students and collaborators," Chandrakasan said.
With his quantal theory of speech, Dr. Stevens examined why languages choose sparingly from the many sounds that humans can produce and what steered those who crafted languages toward similar sounds. For example, many languages use the vowels A, I, and U, and the consonants B, D, and G. This research was particularly helpful in the design of speech recognition equipment.
Victor Zue, the Delta Electronics Professor of Electrical Engineering and Computer Science at MIT, said Dr. Stevens helped establish an acoustic theory of speech production in such a way that "you can begin to interpret and back it up with scientific explanation."
"That completely changed the field, in my opinion, because we are no longer people who observe, but we are people who are analyzing," he said.
Because of Dr. Stevens, Zue added, rather than simply observing that various languages use certain vowels and consonants, researchers understand that the choice of those sounds might be "tied to our anatomical structure."
"He turned the thing inside out by forming a scientific basis for it," he said. "He allowed us to say: 'This is what it means.' "
Dr. Stevens, who was the Clarence J. LeBel Professor Emeritus of Electrical Engineering and Computer Science at MIT, kept in his laboratory a recording booth with high-end equipment that could make very precise recordings to study human sounds.
As one of the lab's top researchers, Perkell said, Dr. Stevens "was a fantastic mentor and extremely generous and unassuming."
Perkell recalled that he once spent about nine months working on a project before he had to leave to serve in the US Army. During that absence, Dr. Stevens put Perkell's research together, polished the writing, and got it published as an MIT research monograph, giving Perkell sole credit.
"It should have had both our names on it, but that's the kind of person he was," Perkell said. "He wasn't very interested in personal aggrandizement."
Dr. Stevens went by Ken when it was less common for professors to be on a first-name basis with students, Perkell said, and he "created an extremely convivial research atmosphere."
Soft-spoken and calm in the classroom, where he often wore jeans and a button-down oxford L.L. Bean shirt, Dr. Stevens "was very good at explaining very basic principles, and I think it was reflected in the way he critiqued students' work," Perkell said.
Dr. Stevens also had a reputation for giving inordinate amounts of time to students in the classroom, lab, or hallways.
"He was like a father figure to a lot of them," said his wife, Sharon Manuel.
Many students marveled at the sentence-by-sentence critiques he often offered when reviewing papers. "It was a tremendous lesson for me in understanding how to write clear scientific papers," Perkell said. "He himself was wonderful at that."
Perkell said the professional example Dr. Stevens set was a lesson in "how to do good science and how to do it in an upright and convincing way."
Dr. Stevens was born in 1924 to British-born parents in Toronto, where he earned a bachelor's degree and a master's in engineering physics from the University of Toronto.
Because he lost an eye to cancer at age 4, he was ineligible for military service. Instead, after earning a master's degree, he taught returning soldiers.
Arriving at MIT in 1948 as a graduate student, Dr. Stevens received a doctorate in 1952 and began teaching there two years later. He cotaught an undergraduate course on circuit theory with Amar Bose, the late founder of Bose Corp., and the two co-wrote a 1965 textbook on network theory.
In 1957, Dr. Stevens married the former Phyllis Fletcher, and they had four children. Their marriage ended in divorce in 1981.
In 1994, he married Sharon Manuel, with whom he adopted two daughters from China.
At a White House ceremony in March 2000, when President Clinton presented the National Medal of Science to Dr. Stevens, the citation praised his "leadership and pioneering contributions to the theory of acoustics of speech production and perception."
His son Michael of Clarksville, Md., said Dr. Stevens' children were not fully aware of how significant a leader he was in his field until that White House ceremony.
At home, he built furniture, including bunk beds for his children, adult-sized beds, and a couch. He also was a harpsichordist, sometimes playing harpsichord-flute duets with one of his daughters.
Although he officially retired in 2007 and moved to Oregon, Dr. Stevens continued attending conferences and advising graduate students, returning to the East Coast every six to eight weeks, Manuel said.
"He was always happy, smiling," she said. "If he saw you and he hadn't seen you in a while, his face would break open into a big smile. He was very kind and he would listen to people."
In addition to Manuel, his son Michael, and his former wife, Dr. Stevens leaves his two children from his second marriage, Kendra and MacKenzie; three other children from his first marriage, Rebecca of Silver Spring, Md., Andrea of Schwenksville, Pa., and John of Kenmore, N.Y.; and five grandchildren.
"With anything we would bring to him, he would be patient," his son Michael said.
He added that if the children were confused about a question, "he would never answer, even though we knew he knew the answer." Ever the teacher, Dr. Stevens would "always ask questions until we got the answer on our own."
Emma Stickgold can be reached at email@example.com.