The frequency selectivity of the auditory system was measured by masking a sinusoidal signal (0.5, 2.0, or 4.0 kHz) or a filtered-speech signal with a wideband noise having a notch, or stopband, centered on the signal. As the notch was widened performance improved for both types of signal but the rate of improvement decreased as the age of the 16 listeners increased from 23 to 75 years, indicating a loss in frequency selectivity with age. Auditory filter shapes derived from the tone-in-noise data show (a) that the passband of the filter broadens progressively with age, and (b) that the dynamic range of the filter ages like the audiogram. That is, the range changes little with age before 55, but beyond this point there is an accelerating rate of loss. The speech experiment shows comparable but smaller effects. The filter-width measurements show that the critical ratio is a poor estimator of frequency selectivity because it confounds the tuning of the system with the efficiency of the signal-detection and speech-processing mechanisms that follow the filter. An alternative, one-point measure of frequency selectivity, which is both sensitive and reliable, is developed via the filter-shape model of masking.

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If both the number of one-dimensional signals and their range are sufficiently large (about 7 and 20 dB for loudness), the information transmitted in absolute identification is not much increased by increasing either variable (Miller, 1956; Braida & Durlach, 1972). The data can be represented in terms of Thurstonian discriminal dispersions in which the variance is proportional to the square of the signal range in decibels (Durlach & Braida, 1969;Gravetter & Lockhead, 1973). but it is by no means obvious what sorts of mechanisms would lead to this model. An alternative is proposed. namely. that there is a roving attention band, about 10 to 15 dB wide, such that signals falling within the band are represented by a sensory sample size about an order of magnitude larger than when the same signal falls outside the band. With reasonable choices for parameters, including the subjective continuum growing as a power function of intensity with an exponent about .3. this nicely accounts for the data. In an attempt to examine the change of performance with range. we replicated the Braida-Durlach experiment with many additional points. These data are not. however. adequate to decide between the two models.

Two-dimensional nuclear magnetic resonance (NMR) relaxation time correlation measurements have been used to observe the behavior of liquids inside porous catalyst pellets; in particular, liquids of relevance to the hydrogenation of 2-butanone over a silica-supported ruthenium catalyst (Ru/SiO 2 ). The behavior of 2-butanone is studied and compared to that of water and 2-propanol, which are used as solvents in this hydrogenation reaction. From the ratio of NMR relaxation times, T 1 /T 2 , for the liquids confined in the pores, it is possible to infer the relative strengths of the surface interaction for each liquid. Water is seen to have the strongest surface interaction, and 2-butanone has the weakest surface interaction. These results are supported by displacement experiments, in which one liquid replaces the other over time within the pore space of the catalyst. For comparison, the behavior of the same liquids in an alumina-supported palladium catalyst (Pd/ Al 2 O 3 ) was also studied. The variation in the strengths of surface interactions was more pronounced in the Pd/Al 2 O 3 catalyst than in the Ru/SiO 2 catalyst. This work demonstrates the applicability of NMR relaxation time correlation experiments to real catalytic systems containing metallic components. From these measurements, information on the access of reactants to surface adsorption sites can be inferred.

This paper presents the application of pulsed field gradient (PFG) nuclear magnetic resonance (NMR) to observe surface diffusion of 1-octene in porous 1 wt% Pd/theta-Al(2)O(3) catalyst trilobes. We demonstrate for the first time the ability to identify diffusion on the pore surfaces unambiguously at ambient conditions in saturated porous media; this technique is applicable to microporous and mesoporous materials in general. At very short observation times, two distinct diffusion regimes are present. These are associated with the bulk pore and pore surface diffusion of 1-octene; using the model proposed by Kärger for two site exchange we determined the diffusion coefficients of these regimes to be 1.3 x 10(-9) and 1.7 x 10(-11) m(2) s(-1), respectively, and the mean residence time of a molecule on the pore surface to be 150 ms. Treatment of the catalyst trilobes with a silane surface coating is seen to influence the surface such that a surface diffusion coefficient is no longer observed, supporting the interpretation that the molecular dynamics of surface diffusing species are influenced strongly by their interaction with hydroxyl groups on the alumina surface. This technique will enable further study and improved understanding of molecular transport in porous catalysts used in liquid-phase, heterogeneous catalytic processes.