A method has been developed for determining the resistive and reactive components of the impedance of the human head and mastoid, or of other high mechanical impedances, using a special direct-recording measuring system that records both force and motion. Impedances presented to driving tips of 12.5 and 20 mm diameter have been measured over the audio-frequency range from 40 to 10 000 cps. The effects of static force coupling the head to the driver have been compared for 500- and 1000-g coupling forces. Over a large part of the audio-frequency range, the predominant characteristic of both head and mastoid impedance is a stiffness reactance, with some damping. In the high-frequency range, a small mass reactance parameter appears. The mass load due to the whole head influences the impedance only at the low-frequency extreme of the measurement range. The driving system consisted of a tubular section of polarized barium titanate, rigidly clamped at one end to a massive wall. Force and motion were measured close to the point of contact with the lead, to minimize errors arising from flexure in the driver or between the sensing elements. The system was calibrated by loading the driving tip with known masses.

Applied voltage responses of seven earphones on both ears of fourteen people were obtained with the aid of a probe tube microphone inserted into the volume enclosed by the earphone. Treatment of the response data by the method of analysis of variance allowed separation of variance effects due to error in repeated measurement, dissimilarity between a given person's ears, and differences among individuals. The latter are found to be the most important effects to contend with in audiometric practice. Effects of application force on sound pressure output of an earphone are examined. Comparisons between the average response on ears and on three superficially different couplers are presented. Responses on ears were substantially different from responses on couplers over parts of the frequency range. Recommendations are given for improving the reliability of audiometric measurements by instrumental refinements.

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Despite the ear's physical complexity, its performance may be described in terms of a set of simple analyzers. Considerable work on the analogous properties of the ear has already been presented by Fletcher and Munson, among others. Work on the analysis of transients leads to equations permitting further extension of the earlier work. Experiments requiring a subject to make quantitative estimates of magnitude indicate the sensory behavior of the ear. Experiments yielding the limits of a subject's discrimination are likely to lead to a mechanistic analog. (This work is supported by the Office of Naval Research.)

The growth of intelligibility of speech stimuli as a function of level above hearing threshold can be computed from the “circuit parameters” of the hearing mechanism by applying Shannon's concepts of channel capacity, equivocation, and “bits.” In the ear, the unit of response is an effective “least count,” derived from experimental data on hearing by means of the equations for a model resembling a frequency-selective circuit [E. Corliss, J. Acoust. Soc. Amer. 41, 1500–1516 (1967)]. The model predicts that the number of least counts available rises as the one-fourth power of the signal intensity above threshold. Experimentally, this growth rate is observed for the intensity-resolving power of the ear. Approximately the same power law is observed for the sensation of loudness. The model ascribes both effects to the same mechanism. From the observed integration time of the ear, the model predicts the rate at which transitions of single counts can be detected. From the counting rate and the integration time, the channel capacity available at the ear and its increase with level above threshold can be computed. The information content of speech as a source function is evaluated from the rate at which single “distinctive features” of speech phonemes are produced. Intelligibility scores can be predicted from the ratio between the rate at which information is being produced by the source and the rate at which the receptor can accept the source material. The scores predicted agree fairly closely with experimental data on random-word and random-syllable intelligibilities. This agreement shows that the listener need recognize no more than a single distinctive feature of each phoneme to display the recognition functions that have been observed. From a theorem of C. Shannon [Inform. and Control 1, 6–25 (1957)] relating code length and error probability, one can show that the channel capacity required for polysyllabic words is lower than the channel capacity required for monosyllabic words because the duration of correlated utterance may be taken as a code length. Evidently, contextual effects are not prominent in the intelligibility of random-word lists; the hearing process involved is primarily recognition of groups of sounds; meaning is secondary. The results also lead to the inference that a direct relation may exist between channel capacity and perceived loudness when speech is transmitted over a broad-band system, and suggest that loudness functions for impaired ears might prove to be correlated with intelligibility functions.