G. A. MILLER AND J. C. R. LICKLIDER loo FREOUENCY OF INTERRUPTION 1 lo ooo [FIG. 2. Word articulation as a function of rate of interruption for a speech-time fraction of 0.5. Curve (1) was obtained with naive listeners, (2) with the same listeners after a few days practice, (3) again with the same listeners, but with a system having a more uniform frequency-response characteristic. Regularly Spaced Interruptions Consider first the results obtained with regularly spaced interruptions and a speech-time fraction of 0.5. The frequency of interruption was varied between 0.! and 10,000 per second. In Fig. 2 the percentage of the words heard correctly is plotted on the ordinate, and the frequency of interruption is given on the abscissa. The three curves of Fig. 2 were obtained under slightly different conditions. Curve 1 is based on the tests conducted at the beginning of the experiments. The listeners had never before served as subjects in articulation tests. Curve 2 was obtained after a few
If a communication engineer, confronted with a sound wave consisting of speech mixed with audible random noise, were requested to build a device to separate the speech from the noise, he would be hard pressed to produce a mechanism as effective as the human auditory system. But if he were given two waves, one a sample of speech plus a sample of random noise, the other the same speech minus the noise, he would invoke the elementary mathematical (or electronic) processes of addition and subtraction and oblige in short order with noise-free speech and with speech-free noise. This paper examines the performance of the (binaural) human auditory system in handling the two-wave problem. The effectiveness of the solution is judged in terms of the intelligibility of speech heard against a background of white noise. If monaural intelligibility is taken as a standard of comparison, it is found that the advantage of binaural presentation of the speech and the noise depends upon the interaural phase relations. The auditory system handles best the problems that are easiest for the engineer, though not as effectively as the engineer would handle them. Intelligibility is highest with noise plus speech in one ear, noise minus speech (i.e., the noise wave plus the inverted speech wave) in the other. Words are understood almost, but not exactly, as well with speech plus noise in one ear, speech minus noise (i.e., the speech wave plus the inverted noise wave) in the other. These modes of presentation, in which either the speech waves or the noise waves in the two ears are 180 degrees out of phase, yield word articulation scores as much as 25 percentage units higher than the more conventional mode of presentation in which both the speech waves and the noise waves in the two ears are in phase. Observations with other interaural phase relations and with monaural-binaural presentation of speech and noise are also described. The results suggest a means of providing a small but probably significant improvement in reception whenever speech is heard through earphones in the presence of ambient noise. The scheme is simply to reverse the connections of one of the earphones. The significance of the results for the theory of masking is discussed.
In the theories of pitch perception now widely supported, pitch is regarded as a unitary attribute of auditory experience. There is good evidence, however, that there are actually two pitch-like attributes, and it is reasonable to suppose that the duplexity of pitch is a reflection of duplexity in the auditory process. The first step in the process is analysis in frequency, performed by the cochlea, which distributes stimulus components of various frequencies to spatially separated channels. The second step, according to the scheme postulated here, is autocorrelational analysis, performed by the neural part of the auditory system, of the signal in each frequency channel. The basic operations of autocorrelational analysis are delay, multiplication, and integration. The nervous system is nicely set up to perform these operations. A chain of neurons makes an excellent delay line. The spatial aspect of synaptic summation provides something very close to multiplication. And the temporal aspect of synaptic summation is essentially running integration. The duplex theory suggests, therefore, that neural circuits following the autocorrelation model supplement the cochlear frequency analysis. The postulated neural autocorrelator of course does not compute autocorrelation functions of the acoustic stimulus: it operates upon afferent neural signals. Because the markedly non-linear process of neural excitation intervenes between the stimulus and the autocorrelation, the latter gives rise in certain instances to pitches that are not readily explained if the relatively linear cochlear analysis is considered to be the only one. “The case of the missing fundamental” and Schouten's residue effect, for example, are readily accounted for by the duplex theory. In addition, the theory provides a rational basis for the octave relation and for the consonance of other simple harmonic relations.
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