Lloyd A. Jeffress

Localization of High‐Frequency Tones

In an earlier study, [Sandel, Teas, Fedderson, and Jeffress, J. Acoust. Soc. Am. 27, 842 (1955)] the writers attempted to correlate interaural intensity and time differences with the subjects localization response. The present paper is an extension of that work. It includes physical measurements of interaural time differences and intensity differences, and attempts to relate these differences to the localization response at a variety of frequencies. The stimuli to be localized were provided through earphones, and the subject was required to match the position (in his head) of a noise and a tone. The noise to one ear was delayed, and the tone presented with no time or phase difference. The subject adjusted the interaural level of the tone until it and the noise appeared to be in the same place. The two were presented alternately by means of a gate having a 150‐msec rise and decay time.Data from the localization judgments were then compared with the findings from the acoustical measurements of time and int...

Masking of Tonal Signals

Many of the phenomena of masking can be explained on the basis of two models, one for monaural listening, and the other for binaural. The monaural model is the familiar narrow band‐pass filter followed by a detector responsive to changes in output level. The binaural model is a series of coincidence detectors associated with a delay‐network capable of matching delay in the stimulus with a delay in the neural path.The two models have proved helpful in understanding the phenomena of tonal masking, and have led to a number of predictions which have been subsequently verified by experiments reported here. Some of the new findings are related to monaural masking and some to binaural. Among the latter are the fact that masking‐level differences can be observed in the masking of one pure tone by another when a short signal is employed, and that a binaural signal can be heard in the presence of uncorrelated noise at the two ears better than a monaural signal can be heard against noise presented monaurally, again ...

Localization of Sound from Single and Paired Sources

Three experiments on the localization of air‐borne sound are described. They were conducted in an anechoic room and employed an acoustical “pointer” as the subjects method of indicating the direction of the stimulus tone. The pointer was a small loudspeaker carried on a boom which rotated about a vertical axis through the subjects head. This speaker presented a wide‐band noise which alternated with the tone to be localized. The switching was transientless and was performed by an electronic gate having a 100‐millisecond rise and decay time.Three small loudspeakers in enclosures, one mounted directly in front of the subject and one 40° to each side, presented the stimulus tones to be localized. In the first experiment the speakers were employed singly. In the other two experiments they were used in pairs. In the second experiment the pairs of speakers were in phase; in the third, they were in phase opposition.The stimulus conditions of Experiment 2 generate a “phantom source” which appears to lie between ...

Two‐Image Lateralization of Tones and Clicks

When interaural differences of time and level are set into opposition, subjects may report a single image that is determined in its lateralization largely by the interaural level difference. Such images show a fairly large “trading‐ratio.” They may report hearing an image that is little affected by the difference of level, but is dependent largely upon the interaural difference of time. Such images show a very small trading‐ratio. With practice, subjects can learn to hear and respond to both images, and center either at will. The present study is concerned with both images as they arise from tonal stimuli of various durations and rise fall times, and from high‐pass clicks.

Effect of Varying the Interaural Noise Correlation on the Detectability of Tonal Signals

Data are presented on the effects of varying the interaural correlation for noise on the detectability of a 500‐cps tonal signal. The noise correlation was reduced by adding uncorrelated noise in the noise channels to the ears. Comparisons are made between data obtained with this method of reducing the noise correlation and with previous data obtained by introducing a displacement in time in the noise to one ear. Masking‐level differences are presented, based on fifty‐percent thresholds, obtained with the constant method and on the detectability index d′ obtained in a two‐interval, forced‐choice situation.

Differences of Interaural Phase and Level in Detection and Lateralization: 250 Hz

A narrow band of noise (50 Hz wide centered at 250 Hz) was passed through a phase‐shifting network and was then used as both masker and signal. The phase shifter permitted control over the phase angle α at which the signal and the masker were added. The masker was continuously present and was in‐phase interaurally N0); the signal could be presented in‐phase interaurally (S0) or 180° out‐of‐phase (Sπ). Detection data were taken for N0‐S0, α = 0° (increments) and α = 180° (decrements), and both detection and lateralization data were taken for N0‐Sπ for several values of α. A single‐interval psychophysical method was used throughout. For all subjects for all values of α, the masking‐level differences (MLDs) were positive and substantial. In the N0‐Sπ conditions, two cues were available to the subject for all values of α except 0° and 90°—an interaural phase (time) difference and an interaural difference in level. Between α = 0° and α = 90 °, the two cues are consonant, but between α = 90° and α = 180°, the t...

Time vs Intensity in the Localization of Tones

Subjects were asked to match the lateral position of one tone, the “signal,” by means of another, the “pointer.” The two tones were presented alternately. The experimenter selected a combination of interaural time and intensity differences for the signal, and the subject adjusted the interaural time difference for the pointer until it seemed to him to be in the same lateral position as the signal. Subjects having normal hearing perceived the signal in two places, one strongly affected by the difference of level at the two ears, the other almost wholly dependent upon the difference of time.

Stimulus‐Oriented Approach to Detection

Distribution curves for amplitude (envelope), drawn for noise and for noise plus signal, provide the basis for determining the proportion of area (probabilities) lying above various “criterion” levels. Probability pairs P (y|n) and P (y|sn) for various criterion levels furnish the coordinates of points generating ROC curves, which, because of the skewness of the distributions, show a slight curvature when plotted on normal‐normal paper. This curvature (concave downward) provides a better fit to detection data obtained from rating‐scale experiments than do the straight lines obtained from normal curves. The ROC curves belong to a family derived through the theory of signal detectability for the ideal observer in the case where signal phase is unspecified. The fact that the distribution for noise‐plus‐signal amplitudes has, in general, a larger variance than that for noise alone explains why many experiments find the ratio of σ sn to σ n to be greater than unity. A detection measure d, derived from the two distribution curves, when plotted against signal amplitude, is a straight line over most of its course but bends in to the origin for weak signals. Its failure to touch the positive abscissa supports the TSD argument against the threshold hypothesis. A second curve, derived from this one, provides a convenient way of determining the signal required to yield a particular value of ds , when the signal that yields some other value of ds is known. Finally, the concept of “effective bandwidth” is developed, and provides a single parameter for use in fitting detection data. Data give some support for the notion that the auditory system adjusts its bandwidth in accordance with the duration of the signal.

A Note on the Audibility of Intense Ultrasonic Sound

This note is concerned with some observations on the audibility of ultrasound by bone conduction.

Interaural Phase and the Absolute Threshold for Tone

The present study agrees with earlier ones that the binaural absolute threshold is about 3 dB lower than the monaural. It also finds that reversing the interaural phase of the signal lowers the threshold still further. The findings are shown to indicate the likelihood that so‐called absolute thresholds are really masked thresholds, with the masking noise present internally and exhibiting a small positive correlation. The close relation of our results to the earlier work of Hirsh on binaural masking phenomena is discussed.