As teleconferencing systems evolve to an ever more lifelike and transparent audidvideo medium, it will be necessary to incorporate multichannel audio, which at a m i n i " involves two channels, i.e., stereophonic sound. However, before full-duplex stereophonic teleconferencing can be deployed, the acoustic echo cancellation (AEC) problem must be solved in this regime. This paper draws attention to a fundamental problem of multichannel AEC that concerns the nonunique nature of the estimated receiving mom impulse responses. Various potential solutions are discussed but are shown to be not entirely satisfactory.

In this paper, a method that has been developed for the assessment and quantification of loudness perception in normal-hearing and hearing-impaired persons is described. The method has been named LGOB, which stands for loudness growth in 1/2-octave bands. The method uses 1/2-octave bands of noise, centered at 0.25, 0.5, 1.0, 2.0, and 4.0 kHz, with subjective levels between a subject's threshold of hearing and the "too loud" level. The noise bands are presented to the subject, randomized over frequency and level, and the subject is asked to respond with a loudness rating (one of: VERY SOFT, SOFT, OK, LOUD, VERY LOUD, TOO LOUD). Subject responses (normal and hearing-impaired) are then compared to the average responses of a group of normal-hearing subjects. This procedure allows one to estimate the subject's loudness growth relative to normals, as a function of frequency and level. The results may be displayed either as isoloudness contours or as recruitment curves. In its present form, the measurements take less than 30 min. The signal presentation and analysis is done using a PC and a PC plug-in board having a digital to analog converter.

We have discovered frequent variations in the near-infrared spectrum of Titan, Saturn's largest moon, which are indicative of the daily presence of sparse clouds covering less than 1% of the area of the satellite. The thermodynamics of Titan's atmosphere and the clouds' altitudes suggest that convection governs their evolutions. Their short lives point to the presence of rain. We propose that Titan's atmosphere resembles Earth's, with clouds, rain, and an active weather cycle, driven by latent heat release from the primary condensible species.

In any speech coding system that adds noise to the speech signal, the primary goal should not be to reduce the noise power as much as possible, but to make the noise inaudible or to minimize its subjective loudness. ’’Hiding’’ the noise under the signal spectrum is feasible because of human auditory masking: sounds whose spectrum falls near the masking threshold of another sound are either completely masked by the other sound or reduced in loudness. In speech coding applications, the ’’other sound’’ is, of course, the speech signal itself. In this paper we report new results of masking and loudness reduction of noise and describe the design principles of speech coding systems exploiting auditory masking.

Two-tone distortion products were studied in a computational model of the basilar membrane into which second- and third-order nonlinearities had been introduced. The displacement patterns produced by distortion products f2 − f1 and 2f1 − f2 resemble, in the neighborhood of the characteristic place for the frequency in question, displacement patterns produced by tones delivered at the stapes. This is not the case for distortion product 2f2 − f1. The dependence of both the amplitude and phase of distortion products f2 − f1 and 2f1 − f2 on frequency spacing and amplitude of the two primary tones has features in common with results from psychophysical experiments. The 2f1 − f2 distortion product shows a nonmonotonic behavior similar to that demonstrated in psychophysical experiments. The phase of the 2f1 − f2 distortion product changes with intensity in a manner similar to that observed psychophysically. In the model, a cancellation-tone method overestimates the amplitude of the 2f1 − f2 distortion product, again in agreement with results from psychophysical experiments. These areas of agreement between the model and psychophysical data support the notion that psychophysically observed distortion products 2f1 − f2 and (under some conditions) f2 − f1 are the result of nonlinear mechanical properties of the basilar membrane.