Where mathematics and hearing science meet : low peak factor signals and their role in hearing research

In his scientific work, Manfred Schroeder touched many different areas within acoustics. Two disciplines repeatedly show up when his contributions are characterized: his strong interest in mathematics and his interest in the perceptual side of acoustics. In this chapter, we focus on the latter. We will first give a compressed account of Schroeder’s direct contributions to psychoacoustics, and emphasize the relation with other acoustics disciplines like speech processing and room acoustics. In the main part of the chapter we will then describe psychoacoustic work being based on or inspired by ideas from Manfred Schroeder. Due to Schroeder’s success in securing a modern online computer for the Drittes Physikalisches Institut after returning to Göttingen in 1969, his research students had a head start in using digital signal processing in room acoustics for digital sound field synthesis and in introducing digital computers into experimental and theoretical hearing research. Since then, the freedom to construct and use specific acoustic stimuli in behavioral and also physiological research has grown steadily, making it possible to test many of Schroeder’s early ideas in behavioral experiments and applications. In parallel, computer models of auditory perception allowed users to analyze and predict how specific properties of acoustic stimuli influence the perception of a listener. As in other fields of physics, the close interplay between experimental tests and quantitative models has been shown to be essential in advancing our understanding of human hearing

Illusory Auditory Continuity Despite Neural Evidence to the Contrary

Many previous studies have shown that a tone that is momentarily interrupted can be perceived as continuous if the interruption is completely masked by noise. It has been suggested this “continuity illusion” occurs only when peripheral neural responses contain no evidence that the signal was interrupted. In this study, we used a combination of psychophysical measures and computational simulations of peripheral auditory responses to examine whether the continuity illusion can be experienced under conditions where peripheral neural responses contain evidence that the signal did not continue through the masker. Our results provide an example of a salient continuity illusion despite evidence of an interruption in the peripheral representation, indicating that the illusion may depend more on global features of the interrupting sound, such as its long-term specific loudness, than on its fine-grained temporal structure

A Re-examination of the Effect of Masker Phase Curvature on Non-simultaneous Masking

Forward masking of a sinusoidal signal is determined not only by the masker's power spectrum but also by its phase spectrum. Specifically, when the phase spectrum is such that the output of an auditory filter centred on the signal has a highly modulated ("peaked") envelope, there is less masking than when that envelope is flat. This finding has been attributed to non-linearities, such as compression, reducing the average neural response to maskers that produce more peaked auditory filter outputs (Carlyon and Datta, J Acoust Soc Am 101:3636-3647, 1997). Here we evaluate an alternative explanation proposed by Wotcjzak and Oxenham (Wojtczak and Oxenham, J Assoc Res Otolaryngol 10:595-607, 2009). They reported a masker phase effect for 6-kHz signals when the masker components were at least an octave below the signal frequency. Wotcjzak and Oxenham argued that this effect was inconsistent with cochlear compression, and, because it did not occur at lower signal frequencies, was also inconsistent with more central compression. It was instead attributed to activation of the efferent system reducing the response to the subsequent probe. Here, experiment 1 replicated their main findings. Experiment 2 showed that the phase effect on off-frequency forward masking is similar at signal frequencies of 2 and 6 kHz, provided that one equates the number of components likely to interact within an auditory filter centred on the signal, thereby roughly equating the effect of masker phase on the peakiness of that filter output. Experiment 3 showed that for some subjects, masker phase also had a strong influence on off-frequency backward masking of the signal, and that the size of this effect correlated across subjects with that observed in forward masking. We conclude that the masker phase effect is mediated mainly by cochlear non-linearities, with a possible additional effect of more central compression. The data are not consistent with a role for the efferent system