Modulated Tone Bursts Research Articles

An efficient Lamb wave-based virtual refined time-reversal method for damage localization in plates using broadband measurements

This study proposes a modified virtual time-reversal (VTR) algorithm for baseline signal-free damage detection in plate-like structures. The physical actuation and sensing of Lamb waves are performed using a broadband Gaussian excitation instead of the conventional narrowband modulated tone burst excitations. The forward response and the reconstructed signal due to the time-reversal process for a narrowband input signal are then constructed virtually using the broadband transfer function. The method eliminates the possibility of numerical errors encountered in the conventional VTR method based on narrowband excitations. It is also more efficient than the traditional VTR algorithm because it can probe at multiple excitation frequencies using a single measurement for each sensing path. This modified VTR algorithm is employed in the recently developed refined time-reversal method (RTRM), which uses an extended signal length of the reconstructed signal for computing damage index (DI) and probes the structure at the best reconstruction frequency. The new method is termed the virtual refined time-reversal method (VRTRM). The DIs based on the VRTRM are used in the reconstruction algorithm for probabilistic inspection of defects to achieve baseline signal-free localization of damages. The efficacy of the proposed VRTRM for damage localization is experimentally verified with the established technique RTRM. Experiments are performed in an aluminium plate equipped with a network of surface-bonded piezoelectric patch transducers to illustrate the conventional VTR’s pitfalls and the modified VTR’s accuracy for a single mass damage scenario. The results show that the proposed VRTRM is as accurate as the established technique RTRM in estimating the reconstructed signals and localizing a block mass damage. Finally, the VRTRM is shown to localize in a dual damage scenario with excellent accuracy. In contrast, the conventional main mode-based VTR method fails to localize the damages with or without single-mode tuning.

Responses to amplitude modulated infrared stimuli in the guinea pig inferior colliculus.

Responses of units in the central nucleus of the inferior colliculus of the guinea pig were recorded with tungsten electrodes. The set of data presented here is limited to high stimulus levels. The effect of changing the modulation frequency and the modulation depth was explored for acoustic and laser stimuli. The selected units responded to sinusoidal amplitude modulated (AM) tones, AM trains of clicks, and AM trains of laser pulses with a modulation of their spike discharge. At modulation frequencies of 20 Hz, some units tended to respond with 40 Hz to the acoustic stimuli, but only at 20 Hz for the trains of laser pulses. For all modes of stimulation the responses revealed a dominant response to the first cycle of the modulation, with decreasing number of action potential during successive cycles. While amplitude modulated tone bursts and amplitude modulated trains of acoustic clicks showed similar patterns, the response to trains of laser pulses was different.

Ultrasonic focused waveform generation using time reversal acoustic focusing system

Abstract The concept of Time Reversal Acoustics (TRA) provides an elegant possibility of both temporal and spatial concentrating of acoustic energy in highly inhomogeneous media. We explored the possibility of generating acoustical signals with arbitrary waveforms using the TRA Focusing System (TRA FS). A method has been developed to predict TRA-focused ultrasound waveforms and spatial distribution by using the measurements of transfer function of transfer function relating the signal at the TRA transmitter to that at the focusing point. The developed approach for TRA-focused signal waveform prediction from the results of direct signal measurements was tested on ten-channel TRA FS based on aluminum resonator with glued piezotransducers. The TRA FS operated in the frequency band of 100–1000 kHz. The formation of ultrasonic signals with various envelopes was demonstrated experimentally. The calculated and experimentally measured waveforms and spatial distributions were practically identical. We formed triangular, rectangular, and amplitude modulated tone burst signals with different modulation frequencies in the focal region. The level of side lobes in the generated signals was much lower than that for standard TRA focusing.

Coding of amplitude-modulated signals in the cochlear nucleus of a grass frog

To study the mechanisms that govern the coding of temporal features of complex sound signals, responses of single neurons located in the dorsal nucleus of the medulla oblongata (the cochlear nucleus) of a curarized grass frog (Rana temporaria) to pure tone bursts and amplitude modulated tone bursts with a modulation frequency of 20 Hz and modulation depths of 10 and 80% were recorded. The carrier frequency was equal to the characteristic frequency of a neuron, the average signal level was 20–30 dB above the threshold, and the signal duration was equal to ten full modulation periods. Of the 133 neurons studied, 129 neurons responded to 80% modulated tone bursts by discharges that were phase-locked with the envelope waveform. At this modulation depth, the best phase locking was observed for neurons with the phasic type of response to tone bursts. For tonic neurons with low characteristic frequencies, along with the reproduction of the modulation, phase locking with the carrier frequency of the signal was observed. At 10% amplitude modulation, phasic neurons usually responded to only the onset of a tone burst. Almost all tonic units showed a tendency to reproduce the envelope, although the efficiency of the reproduction was low, and for half of these neurons, it was below the reliability limit. Some neurons exhibited a more efficient reproduction of the weak modulation. For almost half of the neurons, a reliable improvement was observed in the phase locking of the response during the tone burst presentation (from the first to the tenth modulation period). The cooperative histogram of a set of neurons responding to 10% modulated tone bursts within narrow ranges of frequencies and intensities retains the information on the dynamics of the envelope variation. The data are compared with the results obtained from the study of the responses to similar signals in the acoustic midbrain center of the same object and also with the psychophysical effect of a differential sensitivity increase in the process of adaptation.

Temporal coding of low-frequency amplitude modulation in the torus semicircularis of the grass frog

Single neuron responses to sinusoidal 20 Hz amplitude modulated tone bursts (612.5 ms stimulus-on time at the rate of once per 2.2 s) were studied in the auditory midbrain (torus semicircularis) of the immobilized grass frog ( Rana temporaria temporaria). The characteristic frequency stimuli at 30 dB above the minimum threshold included 12 full modulation periods with fixed initial phase. Neurons generally showed good phase-locking to the envelope waveform. 160 of the 186 investigated neurons responded to 80% amplitude modulated stimuli with discharges synchronized to the modulation cycle. For this modulation depth the best phase-locking capability was observed for certain phasic and build-up units. The synchronous response to 10% modulated stimuli was observed in 104 units. Though a few (2 of 29) phasic units were capable of reproducing this modulation with very high fidelity, the general tendency was the increasing of phase-locking capacity for units with a substantial sustained activity. In this condition for 66 units (63% of the units displaying the synchronous response) we observed a significant improvement of phase-locking from the initial to the terminal periods of modulation. This effect could be interpreted as an initial stage of the enhancement of small amplitude changes in the course of the long-term adaptation.

Nondestructive evaluation of composite laminates by leaky Lamb waves

A method utilizing leaky Lamb wave (LLW) spectra for the nondestructive evaluation of fibre reinforced composite laminates is discussed. Lamb wave modes in composites depend on elastic properties of the laminate, plate thickness, fibre orientation, lay-up, and on the presence of internal discontinuities. Coupling to the Lamb waves is accomplished by mode conversion of an incident compressional wave in a fluid surrounding the plate, where a second transducer in the fluid detects the LLW. NDE applications of these wave modes are reported in multi-ply graphite—epoxy laminates using swept frequency r.f. tone bursts in the range of 0.5 to 10.0 MHz. Experiments have demonstrated useful sensitivity to various types of defects such as delaminations, porosity, ply gaps, foreign matter, and changes in fibre volume fraction. Results of two-transducer LLW C-scans of composite plates with seeded defects and with induced damage are presented, showing the nature and degree of the LLW spectrum perturbation caused by the various types of flaws. To prevent minor spatial variations in either fibre volume fraction or plate thickness from complicating the detection of important defects, a signal coding scheme, incorporating a frequency modulated tone burst, has been devised to analyse the ultrasonic reflection signals from the plate. Improved discrimination and defect sensitivity in the presence of thickness and fibre-fraction variations are demonstrated in several specimens. Digital techniques to manipulate the frequency modulated signals have been developed and are discussed in the context of the experimental results.

Cochlear nerve responses to waveform singularities and envelope corners

One way that discrete acoustic events may be signaled to the central nervous system is through spike synchrony over a subpopulation of cochlear axons. Each of the four comers of a trapezoidally modulated tone burst is such an event. Ordinarily, each corner comprises both an abrupt change in envelope slope and a singularity in the modulated waveform. In this study, in addition to stimuli of this sort, we employed a stimulus waveform in which a corner occurred without a waveform singularity. We obtained masker tuning curves for the CAPs corresponding to both kinds of comers and single-unit responses to both kinds of comers. The results suggest that the subpopulation of cochlear axons excited by the singularity component of a comer is distinct from that excited by the abrupt change in envelope slope.

Responses of primary auditory fibers to brief tone bursts.

Responses of primary auditory fibers to short triangularly modulated bursts of tone were obtained in the anesthetized cat. Based on discharge rate alone, the characteristics of the "response areas" obtained with these tone bursts were found to depend on the best frequency of the fiber. For a fiber with low best frequency (below 1 kHz), tones of greater than 10-ms duration had to be presented in order for the frequency resolution of the neuron to be as good as it was for long tones. For fibers with high best frequencies (above 10 kHz), tones of 2 or even 1 ms caused responses that were nearly as frequency selective as those obtained with long tones. A linear minimum-phase model based on the steady-state frequency selectivity of the fibers has been developed and shows generally comparable responses, but with some interesting exceptions. Synchronization of discharges to the waveform of the low-frequency tone bursts was measured and also shown to be generally compatible with the minimum-phase model. Trapezoidally modulated tone bursts were also used.

Effects of Partial Hamming Function Modulation of Tone Bursts

Some perceptual effects of Hamming function modulation of pure-tone bursts of short duration were presented at an earlier ASA meeting [Spring 1967]. In the present studies, subjects' pitch percepts were measured and thresholds were obtained for tone bursts having contours of the following descriptions: (1) full Hamming function modulation; (2) full rectangular function modulation; (3) first half—Hamming function modulation, last half—rectangular function modulation; and (4) first half—rectangular function modulation, last half—Hamming function modulation. The full-cycle Hamming function bursts provided the least variance in pitch perception measurements and the highest thresholds. The rectangularly modulated tone bursts afforded the greatest variances in pitch measurements and the lowest thresholds. The effect to tone bursts having contours beginning as Hamming functions and ending as rectangular functions approached those of the full Hamming function modulations, whereas the effects to the tone bursts having contours beginning as rectangular functions and ending as Hamming functions approached those of the rectangular bursts. On the basis of preliminary data from hearing-impaired patients, clinical implications are cited relative to the threshold shifts obtained with Hamming function bursts as a function of burst duration and of tone frequency.

Some Effects of Cosine Amplitude Modulation on the Pitch of Tone Bursts

Subjects were asked to match the pitches of tone bursts of varying frequency, duration, and envelope contour. The tone-burst frequencies were 125, 250, 500, 1000, 2000, 4000, and 8000 Hz; and durations were varied in 5-msec steps around the minimum tone-burst durations required for reliable pitch perception at the respective frequencies. The envelopes of the stimuli were amplitude modulated, one cycle per tone burst, by a cosine wave and gated so as to resemble deviations of the Hamming Window Function. The percentages of cosine amplitude modulation were 0% (producing a rectangular tone burst), 15%, 30%, 50%, 70%, 85%, 92% (Hamming Window Function), and 100%. Analyses of these judgments revealed lower standard deviations of frequencies matched to highly modulated tone bursts than those matched to rectangular or near-rectangular tone bursts.