Page 52 - Summer 2015

HIstory of Psychoacoustics
often). The threshold for detecting a tonal signal masked by a noise is proportional to the power in a critical band of masker frequencies surrounding the signal frequency. The critical band is modeled as a bandpass filter similar to the action of the biomechanical properties of cochlear process- ing (Moore, 1989; Table 1). Eberhardt Zwicker (Table 1) in Germany using primarily loudness data developed similar critical-band measurements. The bandwidths of Zwicker’s critical bands are referred to as the Bark Scale (Zwicker and Fastl, 1991). The gammatone filter bank is a current mani- festation of critical-band filters (Patterson et al., 1995).
The Fletcher-Munson (1933) equal-loudness contours and the resulting phon scale are hallmark measures of loudness. Each “equal-loudness contour” indicates all combinations of tonal frequency and level that are equal in loudness to each other and to a 1,000-Hz tone with a particular intensity. The loudness level of a tone on an equal-loudness contour is de- fined as a phon, where “x” phon is the loudness of a sound judged equally loud to a 1,000-Hz tone of “x” dB sound pres- sure level (SPL). The equal-loudness contours show that per- ceived loudness is dependent on both sound intensity and frequency, whereas physical sound intensity and frequency are independent of each other. Equal-loudness contours have been used in a wide variety of applied contexts (see Glasberg and Moore, 2006).
Theory of Signal Detection (1950-1970)
A lot of research after 1950 was driven by post-World War II activities such as the detection of radar and sonar signals. Operators of radar/sonar have to discern, either visually or acoustically, weak signals in a background of noisy clutter. The Electronic Defense Group at the University of Michigan studied human detection of such issues. Ted Birdsall, Wil- liam (Spike) Tanner, W. W. Peterson, and others were instru- mental in developing the theory of signal detection (TSD). David Green (Table 1) and John Swets, Michigan psychol- ogy graduate students working in Electrical Engineering and Psychology, carried the ideas of the TSD into the world of psychoacoustics and far beyond. Signal Detection Theory and Psychophysics by Green and Swets (1974/1966) was es- sentially “required reading” for any aspiring psychoacousti- cian in the 1960s and 1970s.
The TSD was derived from the statistical decision theory (Figure 7), and study of the TSD produced several differ- ent contributions to psychoacoustics and many other areas (Swets, 2010): (1) the TSD was a theory of how decisions
50 | Acoustics Today | Summer 2015
Figure 7. Calculation of the sensitivity-only measure, the normalized distance between the means of the two distributions (d'), from the theory of signal detection (TSD). The subject’s criterion for respond- ing “yes” as opposed to “no” if the signal is present or not generates the Hit and False Alarm rates from the assumed normally distributed underlying Noise and Signal+Noise distribution along a sensory con- tinuum.
are made in variable and uncertain contexts; (2) the TSD challenged the concept of “sensory thresholds”; (3) the TSD proposed that decisions such as discriminating between a signal-plus-noise and just the noise involved not only a lis- tener’s sensitivity but also biases in using their perceptual responses; (4) the TSD defined a way to measure sensitivity independent of response bias; (5) the TSD relied on defining “ideal observers” for evaluating decision processes; (6) many new psychophysical procedures were designed based on the TSD (these procedures replaced some originally proposed by Fechner); and (7) the TSD led to the psychoacoustic “en- ergy detection model” that accounted for many data involv- ing detection and discrimination of well-specified sounds. The precision of the data obtained using TSD procedures and analyses was usually well beyond what had previously been measured psychophysically or for almost any other be- havioral measurement.
The TSD was developed when many psychoacousticians were psychologists and when behaviorism (promoted by B. F. Skinner) was a dominant theme in psychology. Although few psychoacousticians were “Skinnerians,” many aspects of behaviorism resonated with the psychoacousticians of the day, such as (1) the need for clear operational definitions of all terms; (2) all behavior is observable and measureable; (3) behavior is a consequence of actions; (4) behavior can be studied in a strict scientific manner; and (5) an opposition to explanations based on private nonobservable events such as “cognition.”
Another approach to studying psychophysical relationships was exemplified by the work of S. S. (Smitty) Stevens. Ste- vens (1957) developed scales of measurement and new “scal-