Characterizing the Speech Reception Threshold in hearing-impaired listeners in relation to masker type and masker level.

The Speech Reception Threshold [SRT, (dB SNR)] is often used as an outcome measure to quantify the acuity for speech perception in noise. The majority of studies observe speech intelligibility in noise at a fixed noise level. However, the observed SNR might be an ambiguous outcome measure because it is dependent on the sensation level (SL) of the noise in the case of a non-stationary noise. Due to their higher thresholds, hearing-impaired listeners are usually tested at a different SL compared to normal-hearing listeners. Therefore, the observed SNR "itself" might not be a robust outcome measure to characterize the differences in performance between normal-hearing and hearing-impaired listeners, within and between different studies. In this paper, the SRTs are measured at a fixed absolute noise level (80 dBA) and at a fixed SL (25 dB). The results are discussed and described with an extension to the SRT model of Plomp [(1986). "A signal-to-noise ratio model for the speech-receptionthreshold of the hearing-impaired," J. Speech Hear. Res. 29, 146-154] and the Extended Speech Intelligibility Index. In addition, two alternative outcome measures are proposed which are, in contrast to the SNR, independent of the noise level. These outcome measures are able to characterize the SRT performance in fluctuating noise in a more uniform and unambiguous way.

Nuclear quantum effects affect bond orientation of water at the water-vapor interface.

Using combined theoretical and experimental approaches, we demonstrate that the bond orientation of water at the water-vapor interface depends markedly on the water isotope (H-D) composition. While the interfacial water structures of H(2)O and D(2)O are indistinguishable, the intramolecular symmetry breaking in HDO is directly reflected at the surface: the OD bonds preferably orient down towards the bulk water, whereas the OH bond tends to orient up into the vapor phase. Path integral molecular dynamics simulations show good agreement with surface-specific sum-frequency generation (SFG) spectroscopy results, revealing that the distinct interfacial bond orientations originate from nuclear quantum effects. The enhanced localization of the heavier D atom leads to stronger hydrogen bonds, giving rise to OD bonds pointing down into the bulk.

Comparative study of direct and phase-specific vibrational sum-frequency generation spectroscopy: advantages and limitations.

As a surface-specific technique, vibrational sum-frequency generation (SFG) is used in a wide range of applications where soft matter or solid interfaces are to be probed on a molecular level through their vibrational modes. In recent years, phase-specific sum-frequency generation (PS-SFG, also known as heterodyne-detected SFG) spectroscopy has been increasingly replacing its predecessor (direct SFG, also known as homodyne SFG) as the experimental technique of choice for characterizing interfacial structure. The technique enables phase sensitive measurements, allowing for the determination of the real and imaginary parts of the interfacial vibrational response function and thereby the unambiguous identification of molecular orientation. This phase-sensitivity requires, however, a complete understanding of the complex optical properties of the sample and of their effect on the signal. These optical properties significantly influence the raw spectral data from which the real and imaginary parts of the second-order susceptibility are retrieved. We show that it is essential to correct the data appropriately to infer the true molecular response. The current study presents a detailed description of the physical contributions to the phase-resolved spectrum, allowing a direct comparison between the phase-resolved spectrum and that obtained using the well-understood direct detection method in a step-by-step data analysis process. In addition to phase sensitivity, PS-SFG has been shown to increase the sensitivity compared to traditional (direct) SFG spectroscopy. We present a quantitative comparison between theoretical limits of the signal-to-noise ratio of both techniques, which shows that for many systems the signal-to-noise ratio is very similar for direct- and phase-specific SFG signals.