Page 42 - Spring2022

WILLIAM A. YOST
Figure 2. Bill’s academic family tree. Left to right: Lloyd Jeffress (academic grandfather), Don Robinson (academic father), and Bill.
start with the tuning fork. Tuning forks we recognize as U-shaped metal bars with a stem. Knowing certain properties of the fork and the way it is held and struck, the prominent modes of vibration, their relative ampli- tudes, and rate of decay can be estimated from known equations of motion (Russell, 2020). Theoretically, any or all of this information might be used by a listener to correctly perceive the sound as belonging to a tuning fork (Lutfi, 2008). The problem is that, in the real world, the properties of unseen sources are not known beforehand. Instead, they are what we are trying to determine from sound. In the equations of motion, different combina- tions of properties can produce identical solutions, so if there are no constraints, that ping of the tuning fork could just as easily have come from a hollow flagpole, pogo stick, or ceramic plate. The problem is indetermi- nate; it has not one but many solutions.
Now consider that busy street in downtown Chicago. You hear traffic, people walking around you, and a siren wailing in the distance. What reaches your ears is the superposition (sum) of the sound pressure wave fronts emitted by all of these sources; you have access only to
this sum, but somehow you extract from it and recog- nize individually the sounds emitted by each source. The problem is principally the same as having to solve for x, y, and z in the expression x + y + z = 20. Again, there is no single solution.
In both examples, the only way perception can be correct is to bring additional information to bear on the prob- lem. Understanding what that information is and how it is encoded in the auditory nervous system has been the fundamental challenge for research on sound source perception and the focus of Bill’s work.
Early Influences and Signal
Detection Theory
Bill received his undergraduate degree from Colorado College, Colorado Springs, Colorado, in 1966 with a major in psychology and a minor in mathematics. He knew then that he wanted to be a professor and researcher studying objective, quantifiable ways of explaining how the brain works. That same year, Green and Swets (1966) published their seminal book on signal detection theory (SDT; see Yost et al., 2021). For Bill, the timing was per- fect. SDT recognized that perception is covert, that the judgments of subjects in perceptual studies are merely
their personal impressions, opinions, or beliefs regarding what they see or hear. SDT would provide a way to con- vert these subjective impressions into entirely objective measures of perception; in the words of Green (2020),
“as objective as any of the quantities used in the so-called hard sciences.” This development would bring a sea change in the conduct of perceptual research that would have a lasting impact on Bill’s work and on the work of many other scientists of the time.
After graduating from Colorado College, Bill furthered his studies in the Psychology Department at Indiana Uni- versity (IU), Bloomington, under the tutelage of James Egan, another giant of SDT. He then finished his PhD with Don Robinson (Figure 2) after the early depar- ture of Egan from IU. After receiving his doctorate, Bill received a National Science Foundation postdoctoral award to work with Green at the University of Califor- nia, San Diego. The influence of this early training is evident in Bill’s consistent approach to research: model oriented, precise, and given to clear outcomes based on quantitative data. Although there would still be a place for qualitative data in Bill’s research, he would be among the first in the field to apply the lessons of SDT to the study of human sound source perception.
“The Basis for Hearing”
Bill published a call to action, encouraging researchers to focus more attention on sound source perception (Yost, 1991). The title would leave little doubt as to the importance
he attached to the subject, “Auditory Image Perception and Analysis: The Basis for Hearing.” The article underscored the role of sound source perception in communication and survival and offered compelling examples of how we rely on it every day to navigate our environment. Bill would
42 Acoustics Today • Spring 2022