James P. Egan

On the Masking Pattern of a Simple Auditory Stimulus

The masking audiogram of a pure tone is complicated by phenomena that arise from the interaction of the test tone with the masking stimulus. The production of beats and of difference tones results in a masking audiogram that does not represent the pattern of activity in the cochlea or nerve due to a simple masking stimulus. In the present experiments a narrow band of noise was used to mask pure tones. The “beat” heard in the immediate vicinity of the masking noise is not prominent, and a test tone higher in frequency than the band of noise is detected in terms of the characteristic pitch of that tone rather than by means of a difference tone.With a band of noise slightly wider than a critical band (Fletcher), the amount by which the test signal at the center of the band exceeds the level/cycle of the noise is less than that obtained with a considerably wider band of noise. This smaller signal‐to‐noise ratio obtained with the narrow band of noise is probably due to “beats,” since the test tone is heard as ...

Operating Characteristics, Signal Detectability, and the Method of Free Response

The method of free response refers to the following listening situation. Against a background of noise, a weak signal is presented several times in a long (2‐min) observation interval. The temporal intervals between the presentations of the tones are randomly distributed; consequently, the listener does not know when a tone will occur, and he does not know how many tones will be presented. From one series of observation intervals to the next, the listener is instructed to adopt various criteria and to press the single response‐key each time he “hears a tone.” The problem consists in the determination of a procedure that allows the total number of yes responses to be partitioned meaningfully between “hits” and “false alarms.” A model is developed in which the measurable quantity, rate of response, is related to the “hit rate” and to the “false‐alarm rate.” Although the criterion adopted by the listener cannot be directly evaluated, the use of a wide range of criteria makes it possible to estimate the detectability ds of the signal. Two experiments are described, and the results support the model.

Interval of Time Uncertainty in Auditory Detection

Three experiments were conducted to measure the decrement in performance that results from uncertainty in the time of onset of a signal presented against a continuous background of noise. The fixed‐interval observation experiment was employed. A light defined an observation interval for the listener during which the signal, a tone of 1000 cps, either was or was not presented [p(SN)=0.5]. The signal, when presented, started at an instant randomly selected within the observation interval. Thus, the listener was uncertain as to (1) whether or not the signal would occur in the observation interval, and (2) the onset time of the signal, if in fact the signal occurred. The interval of time uncertainty (ITU) during which the tone might start was systematically varied from one series of trials to the next, and the listener knew the duration of ITU in each series. After each observation interval, the listener indicated his confidence that a tone was presented by using a rating scale. Operating characteristics [p(y...

Some Factors Affecting Multi‐Channel Listening

Certain factors were investigated that affect the intelligibility of a speech message which is presented to a listener simultaneously with an interfering speech message. In two of the four experiments reported, filters were introduced into one of the two channels that carried the messages. Thresholds of perceptibility were not reliably decreased by moderate amounts of filtering of the received message. However, articulation scores were considerably increased by the use of a high‐pass filter (500 cps) in either of the two channels.The great advantage of presenting one message to one ear and the interfering message to the other ear (dichotic presentation) was measured by changes in the thresholds of perceptibility and by articulation tests. Functional relations between thresholds of perceptibility for the message to be received and the intensity of an interfering signal were determined for both monaural and dichotic listening. In separate tests, noise was also used as the interfering signal. Dichotic recept...

Masking-level differences and the form of the psychometric function

Functions showing the relation between-the detectability and the energy of a signal were determined for various interaural phase conditions. The empirical relation between d′ and signal energy E is approximately d′=m(E/N0)k where m and k are constants for a particular function. The data show that k is fairly constant for a particular O and that m depends upon the interaural condition. That is, the various psychometrie functions for a given O all had the same form, independent of location. Therefore, the magnitude of the MLD=(10/k)log(mi/mr), where mi is the constant for condition i, and mr corresponds to a reference condition. Consequently, an MLD is relatively independent of the level of performance that is chosen for the determination of that MLD.

On the Intelligibility of Bands of Speech in Noise

Articulation tests were conducted with a large number of communication systems having band widths ranging from about one‐half octave to a system covering the entire range of speech frequencies. The systems were linear and their responses were approximately uniform over the pass band, with sharp cut‐offs at either end. The acoustic gain of the systems was expressed relative to the transmission of speech through one meter of air between talker and listener. Two spectra of masking noise were used, and each system was tested over a wide range of speech‐to‐noise ratios. In one group of experiments the speech was filtered before mixing with noise and in the other group both the speech and the noise were passed through the same filter. For each of the band‐pass systems a relation between syllable articulation and level of received speech was obtained. From these gain functions, families of equal‐articulation contours may be derived. These contours show, for example, how the gain must be changed for a given chang...

Memory for Waveform and Time Uncertainty in Auditory Detection

An experiment was conducted to determine how well listeners could judge whether or not a signal was presented in a noisy observation interval which had already occurred. The cardinal feature of the experiment is that the observation interval is not marked off for the listeners until some fixed time after its occurrence. The listening situation is described as follows. With a probability of 0.5, the signal (1000 cps, 0.25 sec) is presented at a randomly selected instant. A fixed time thereafter, the listener is informed (by a flash of light) of the real time at which the sinusoid may have occurred, and he responds with a rating of confidence. As compared with the typical fixed‐interval experiment in auditory detection, two sources of uncertainty are emphasized in this situation: (1) The listener has a faulty memory of his transformation of the input waveform, and (2) he has a faulty estimation of the time of onset of the signal. From the results of previous experiments on the role of time uncertainty in de...

Lateralization of a weak signal presented with correlated and with uncorrelated noise.

When a strong signal is presented monaurally, listeners can easily lateralize the sound. However, if noise is added to both ears, there may be uncertainty as to which ear received the signal. This uncertainty was measured over a range of signal energies with perfectly correlated noise (N0) and with uncorrelated noise (NU). In the main experiment, the monaural signal occurred on each trial, and this signal was presented to either the right or the left ear by random determination during the single observation interval. Listeners responded “right” or “left.” Measures of signal detection were also secured with the monaural signal under release from masking (N0) and without such release (NU). With uncorrelated noise (NU), the listener requires only slightly greater signal energy (1–2 dB) in order to lateralize as well as he can detect. With correlated noise (N0), the psychometric function for lateralization is not only displaced considerably toward higher signal energies, relative to those required for detecti...

Monitoring Task in Speech Communication

Some communication situations involve several noisy channels, and only certain ones of these carry relevant information to a given communication operator. The operator must receive and identify a restricted number of different messages, and he must ignore others. The performance of the listener in this situation will depend both on the discriminability of the messages and on the listeners criterion for accepting his response as correct or rejecting it as incorrect. The present paper gives a quantitative description of the monitors behavior in terms of the operating characteristic and the articulation‐criterion function. The results of two experiments are reported. In one of these, the confusion matrices for the various sets of messages were also determined.

Changes in Pitch of Tones of Low Frequency as a Function of the Pattern of Excitation Produced by a Band of Noise

The excitation pattern of a tone is markedly altered by a masking stimulus. It should be expected that the pitch of the masked tone, as well as its threshold and its loudness, would be affected. In the present investigation the effect of a narrow band of noise (90 c.p.s. wide, centered at 410 c.p.s.) upon the pitch of pure tones of low frequency was measured in a series of experiments. Subjects matched the pitch of a tone heard in the presence of a band of noise to the pitch of a tone heard alone. These two tones, one partially masked and the other not, were presented to the subject in repeated succession. The listener first matched these tones for loudness. He then adjusted the frequency of the unmasked tone until it was the same in pitch as the masked tone. The pitch change was investigated at a number of frequencies and of loudness levels.Systematic changes in pitch were observed. The presence of the noise raises the pitch of a partially masked tone whose frequency is immediately above those of the ban...