Earl D. Schubert

Effective Onset Duration of Auditory Stimuli

In analyzing interaural temporal relations, the binaural system may receive information from one or more of three separate stimulus aspects: (1) difference in time of the start of stimulation, (2) difference in time between similar portions of the continuing wave form at the two ears, and (3) difference in time of the end of stimulation. In this study, the first and third kinds of difference were combined for convenience as “transient disparity”; the second was called “ongoing disparity.” The relative effectiveness of these two temporal relations in producing changes in auditory localization was investigated by finding, for various values of transient disparity, a value of ongoing disparity that brought the sound back to center. For a given value of transient disparity, the necessary ongoing disparity value varies as a function of stimulus duration. Transient disparity loses its effectiveness for stimulus durations greater than about 150 msec. For a duration of 100 msec, it takes roughly 35 times as much ...

Some Preliminary Experiments on Binaural Time Delay and Intelligibility

Comparisons are made of the intelligibility of continuous speech in noise under three listening conditions: Speech arriving simultaneously and in phase at the two ears; speech arriving at one ear later than the other; and speech arriving simultaneously, but in opposite phase in one ear. Time intervals between ears ranged from 200 μsec to 7 msec. Delaying speech to one ear under these conditions does increase intelligibility but is little, if any, better than the simple expedient of wiring the phones so that the wave form at one ear is inverted with respect to the other. With the materials and method used, the maximum increase occurs with a delay between ears somewhere between 0.5 and 1 msec, though there is some indication that delays 1 msec and longer may be slightly better under the most difficult listening conditions tried.

Some Aspects of Binaural Signal Selection

Two experiments are reported which measure the increase in intelligibility occasioned by listening binaurally to running speech imbedded in interfering signals. In the first experiment, the frequency range of the speech is restricted to one of three ranges: 200–1600, 880–2200, 1660–6100. The interfering signal is broad‐band random noise. Under difficult listening (27% intelligibility for homophasic listening) the lowest range shows a binaural improvement of 33% for a change in interaural polarity, and of 28% for an interaural time disparity of 500 μsec. The middle‐ and high frequency ranges show less binaural gain, but do afford some advantage to two eared listening under difficult conditions.The second experiment compares the effects of different speech waves masking the wanted speech in binaural listening. For the maskers used, the binaural system helps most when the interfering wave is the speakers own voice or a multiple mixture of many voices. The measured differences between homophasic and binaural...

Envelope versus Microstructure in the Fusion of Dichotic Signals

An attempt was made to assess the effect of an imposed amplitude envelope on the binaural fusion of dichotic tone signals and noise signals. The envelope was imposed by an electronic switch at three rise/decay rates—10 msec, 25 msec, and 50 msec. The duration of the signals was such that there was no constant amplitude portion for any signal. The tonal signals covered a range from 250 to 2000 Hz. The noise was either high passed or low passed at 1000 Hz. Only the imposed envelope was delayed at one ear—not the microstructure of the signal. For tonal signals, the maximum envelope delay time for which the signal still yields a single stationary image is definitely a function of the envelope slope, being approximately 5–7 msec for the 10‐msec slope; 10–12 msec for the 25‐msec slope; and 15–20 msec for the 50‐msec slope. For the two lesser slopes, there is also an envelope delay range for which the image moves across the head from leading ear to lagging ear. For the signals with noise microstructure, behavior...

Psychophysical Estimate of the Velocity of the Traveling Wave

An extension of a method originally employed by Bekesy in 1933 was used to estimate the velocity of the traveling wave along the cochlear partition. The method consists of masking a different length of the basilar membrane at each of the two ears so that the wave produced by a simultaneous click at the ears emerges from the shorter masked portion of one ear sooner than from the other. As a result the click lateralizes toward the side where the wave emerges sooner. The time interval by which the click to the “earlier” side must be delayed to center the subjective click is an estimate of the original difference in time of emerging from the masking and hence of the time taken to travel the portion of the membrane represented by the difference in cutoff frequency of the two masking noises. Measurements were taken for 14 overlapping half‐octave segments of the basilar membrane on 7 subjects. Measurement was not possible below 1000 cycles with the instrumentation used. The velocity estimates range from 55 m/sec...

Waveform Preservation in the Cochlea

Past measurements and observations appear to give equivocal evidence on whether waves of different frequency travel down the cochlea at the same rate. If different frequencies travel at different rates, then steady‐state waveforms will show changes in addition to those attributable to relative amplitude change of the components. Sinusoids of simple (octave) ratio were fed into guinea‐pig cochleas, and the waveforms were observed with paired electrodes at two well‐separated points. Comparative phase shifts for the two components indicate that, on the stapes side of a given observation point along the membrane, all frequencies lower than the frequency of maximum amplitude for that point travel at the same rate.

Use of Noise Bands to Establish Noise Pitch

In the work reported, the pitch of a band of noise, called a reference band, is determined by varying the frequency position of the center of a 12‐oct band of noise, the comparison band, systematically but randomly over the full width of the reference band. The pitch of the reference band is designated as the frequency position of a 12‐oct comparison band that is judged “lower” than the reference band on 50% of the trials. The method of constant stimuli was employed throughout. The reference bands, whose pitch was to be defined, included bands of various widths from 12–3 oct, and they were placed at widely varying positions over the audio‐frequency range, with lower limits as low as 150 Hz and upper limits as high as 9600 Hz. The psychophysical functions indicating the changing proportion of “lower” judgments as the position of the comparison band varies from the lower edge to the upper edge of the reference band are essentially monotonic, yielding little evidence of multiple pitches, even for the wide re...

The Effect of Thermal Masking Noise on the Pitch of a Pure Tone

When a pure tone and a masking noise are fed into the same ear simultaneously, the pitch of the pure tone is raised. The effect is more pronounced at low loudness levels and is progressively greater as the frequency of the tone rises. Apparently the phenomenon is not present if the pure tone is 20 db or more above its masked threshold. Nineteen graduate music students served as subjects in the experiment. Control measurements were made to estimate the effects of diplacusis and of the difference in intensity between the masked and the unmasked pure tone when they were matched in loudness.

On Estimating Aural Harmonics

Clacks rather ingenious method of estimating the strength of the aurally generated second harmonic, even when it is below audible levels, appears, at first, to offer a procedure for estimating distortion in the auditory system. On closer inspection, however, it appears the method may face the, the same certainties as the method of “best beats”, particularly at frequencies below 1 kHz.

Protocol for determining the auditory percepts of electrical stimulation of the cochlea.

The percepts of electrical stimulation of the auditory nerve can best be determined in man. An eight channel bipolar electrode array has been developed which allows discreet stimulation of segments of auditory nerve via the scala tympani. In addition, a programmable stimulator has been developed which allows the delivery of different waveforms and stimulus patterns. Hopefully this will begin to allow the generation of the elements of speech.