Slow Variations In Amplitude Research Articles

Surface waves in a square container due to its resonant horizontal elliptic motion

Surface waves in a square container due to its resonant horizontal elliptic or linear motion are investigated theoretically. The motion of the container is characterized by the ratio, expressed as , of the length of the minor axis to the length of the major axis of its elliptic orbit, and by the angle θ between the directions of the major axis and one of its sidewalls. Using the reductive perturbation method, non-linear time evolution equations for the complex amplitudes of two degenerate modes excited by this motion are derived with the inclusion of linear damping. When is small, for any θ these equations have two kinds of stable stationary solutions corresponding to regular co-rotating waves whose direction of rotation is the same as that of the container and regular counter-rotating waves of the opposite direction of rotation. As increases to one, the region of forcing frequency in which stable regular counter-rotating waves are observed shrinks and then disappears for any θ. Solutions with chaotic or periodic slow variations in amplitude and phase of excited surface waves are also obtained for forcing frequencies where no stable stationary solutions exist. Non-stationary solutions are either unidirectionally or bidirectionally rotating waves. For , chaotic waves and bidirectionally rotating waves are observed more frequently for smaller . For , for sufficiently small fluid depth, regular non-rotating waves are expected to occur for any forcing frequency. Moreover, stable stationary and non-stationary solutions obtained for are found to agree fairly well with the experimental results in a preceding study.

Finite-amplitude acoustic pulses in a waveguide layer with longitudinal inhomogeneity

This paper considers the propagation of a weakly nonlinear acoustic pulse in a slightly curved waveguide layer which is strongly inhomogeneous in the transverse direction and weakly inhomogeneous in the longitudinal direction. The basic system of hydrodynamic equations reduces to a nonlinear wave equation, whose coefficients are determined using the equations of state of the medium. It is established that as the adiabatic exponent passes through the value γ = 3/2, the nature of the pulse propagation changes: for large values of γ, the medium is focusing, and for smaller values, it is defocusing. It is shown that the pulse propagation process is characterized by three scales: the high-frequency filling is modulated by the envelope, whose evolution, in turn, is determined by the moderate-rate evolution of the envelope phase and slow amplitude variation. A generalized nonlinear Schrodinger equation with the coefficients dependent on the longitudinal coordinate is derived for the pulse envelope. An explicit soliton solution of this equation is constructed for some types of longitudinal inhomogeneity.

Speech perception problems of the hearing impaired reflect inability to use temporal fine structure.

People with sensorineural hearing loss have difficulty understanding speech, especially when background sounds are present. A reduction in the ability to resolve the frequency components of complex sounds is one factor contributing to this difficulty. Here, we show that a reduced ability to process the temporal fine structure of sounds plays an important role. Speech sounds were processed by filtering them into 16 adjacent frequency bands. The signal in each band was processed by using the Hilbert transform so as to preserve either the envelope (E, the relatively slow variations in amplitude over time) or the temporal fine structure (TFS, the rapid oscillations with rate close to the center frequency of the band). The band signals were then recombined and the stimuli were presented to subjects for identification. After training, normal-hearing subjects scored perfectly with unprocessed speech, and were approximately 90% correct with E and TFS speech. Both young and elderly subjects with moderate flat hearing loss performed almost as well as normal with unprocessed and E speech but performed very poorly with TFS speech, indicating a greatly reduced ability to use TFS. For the younger hearing-impaired group, TFS scores were highly correlated with the ability to take advantage of temporal dips in a background noise when identifying unprocessed speech. The results suggest that the ability to use TFS may be critical for "listening in the background dips." TFS stimuli may be useful in evaluating impaired hearing and in guiding the design of hearing aids and cochlear implants.

Modulation masking produced by masker modulation tone complexes

Thresholds were measured for detecting a sinusoidal amplitude modulation in the presence of a masker-modulation tone complex. Both modulations were applied to the same sinusoidal carrier. Two different masker modulations were used: (i) a pair of components beating at the signal-modulation frequency and (ii) a three-tone complex producing a sinusoidal amplitude modulation in the modulation domain whereby the frequency of the slow amplitude variation was equal to the signal-modulation frequency. Masked thresholds were measured as function of the phase of the signal modulation relative to the slow amplitude variation in the masker modulation. In all conditions, thresholds were lower for an in-phase signal modulation compared to an antiphase condition. The maximum threshold difference was 15 dB. The results are in contrast to recent data [B. C. J. Moore et al., J. Acoust. Soc. Am. 106, 908–918 (1999)], where the lowest thresholds were obtained for the antiphase condition. Our data are in line with the idea that a nonlinearity prior to the modulation filterbank extracts the slow variation of the masker modulator. The data could be fitted well by assuming a combination of a peripheral (power-law) compression and a threshold. Peripheral compression alone did not account for the data.

Vocal Fold Vibrations: High‐Speed Imaging, Kymography, and Acoustic Analysis: A Preliminary Report

To evaluate a new analysis system, High-Speed Tool Box (H. Larsson, custom-made program for image analysis, version 1.1, Department of Logopedics and Phoniatrics, Huddinge University Hospital, Huddinge, Sweden, 1998) for studying vocal fold vibrations using a high-speed camera and to relate findings from these analyses to sound characteristics. A Weinberger Speedcam + 500 system (Weinberger AG, Dietikon, Switzerland) was used with a frame rate of 1,904 frames per second. Images were stored and analyzed digitally. Analysis included automatic glottal edge detection and calculation of glottal area variations, as well as kymography. These signals were compared with acoustic waveforms using the Soundswell program (Hitech Development AB, Stockholm, Sweden). The High-Speed Tool Box was applied on two types of high-speed recordings: a diplophonic phonation and a tremor voice. Relations between glottal vibratory patterns and the sound waveform were analyzed. In the diplophonic phonation, the glottal area waveform, as well as the kymogram, showed a specific pattern of repetitive glottal closures, which was also seen in the acoustic waveform. In the tremor voice, fundamental frequency (F0) fluctuations in the acoustic waveform were reflected in slow variations in amplitude in the glottal area waveform. For studying details of mucosal movements during these kinds of abnormal vibrations, the glottal area waveform was particularly useful. Our results suggest that this combined high-speed acoustic-kymographic analysis package is a promising aid for separating and specifying different voice qualities such as diplophonia and voice tremor. Apart from clinical use, this finding should be of help for specification of the terminology of different voice qualities.

Periodicities in respiration and heart rate in newborns.

Periodic breathing is common in normal infants, but may be associated with prolonged apnea leading to crib death. The mechanisms of periodic breathing and its relation to normal breathing patterns are unclear. We recorded respiratory and heart rate (HR) patterns of 11 healthy newborn infants during quiet sleep, in both normal and periodic breathing. Spectral analysis of the respiratory pattern revealed a low-frequency (LF) periodicity in normal breathing approximately equal to the frequency of periodic breathing when this occurs. Periodic breathing thus appears to be an exaggeration of an underlying slow amplitude variation which is present in regular breathing. LF periodicity also appeared in the HR pattern in both normal and periodic breathing, suggesting an LF modulation of cardiovascular control as well. The lack of a definite phase relation between HR and ventilation at LF may indicate dominant peripheral, rather than central, interactions between HR and respiration at these frequencies.

Escaping nonthermal continuum radiation

New observations by the ISEE 1 plasma wave receiver reveal heretofore unreported details of the nonthermal continuum radiation from the earth. In particular, the higher frequency escaping component shows temporal and spectral features which are quite different from those of the lower frequency trapped emission. The higher frequency component is often clearly separated in frequency from the trapped component and high resolution spectrograms show that the escaping radiation consists of numerous narrowband emissions which drift slowly in frequency with time. The higher frequency component also exhibits greater temporal and spatial variability in both amplitude and frequency although the temporal variations are still quite smooth when compared to the more intense and higher frequency auroral kilometric radiation. Evidence is presented which clearly shows a direct connection between intense electrostatic emissions near the upper hybrid frequency and the escaping continuum radiation and strongly suggests the latter electromagnetic emission is generated through mode coupling with the upper hybrid waves. The new ISEE observations of the escaping continuum radiation from the earth strongly suggest a close correspondence with the newly reported narrowband kilometric radiation from Jupiter. Both emissions are relatively narrowband and show slow variations in amplitude with time. In both cases, the narrowband radiation lies above the trapped continuum radiation and below the intense extraordinary mode emissions (auroral kilometric radiation at the earth and decametric radiation at Jupiter). Evidence from both planets suggests the radiation is generated by intense electrostatic waves near the upper hybrid resonance frequency in the vicinity of the plasmapause at the earth and at the outer edge of the Io plasma torus at Jupiter.

Finite-amplitude love-waves on an isotropic layered half-space

A pair of semi-linear hyperbolic partial differential equations governing the slow variations in amplitude and phase of a quasi-monochromatic finite-amplitude Love-wave on an isotropic layered half-space is derived using the method of multiple-scales. The analysis of the exact solution of these equations for a signalling problem reveals that the amplitude of the wave remains constant along its characteristic and that the phase of the wave increases linearly behind the wave-front.

Surface Waves in a Laterally Varying Layered Structure

Sum17zur.y A kind of ray theory is developed for surface waves in a layered elastic medium in which there are gradual lateral variations in the thicknesses of the layers and in the elastic parameters characterizing each layer. An approximation equivalent to the JWKB approximation is used, and equations governing the slow variations in amplitude, frequency and wavenumber in a nearly uniform sinusoidal wavetrain are deduced. In the first approximation these equations are found to be those given by Whitham’s average Lagrangian method. An equation governing the slow variation in phase is also deduced. The solution of these equations by the method of characteristics gives ray-tracing equations and an amplitude equation similar to those given by standard ray theory for body waves. The analysis also leads to a straightforward method for finding the total energy flux and energy density of the waves without performing an integration over the depth co-ordinate.