Fundamental frequency

About: Fundamental frequency is a research topic. Over the lifetime, 8941 publications have been published within this topic receiving 131583 citations.

Analytical and numerical studies of approximate phase velocity matching based nonlinear S0 mode Lamb waves for the detection of evenly distributed microstructural changes

Abstract: Most previous studies on nonlinear Lamb waves are conducted using mode pairs that satisfying strict phase velocity matching and non-zero power flux criteria. However, there are some limitations in existence. First, strict phase velocity matching is not existed in the whole frequency bandwidth; Second, excited center frequency is not always exactly equal to the true phase-velocity-matching frequency; Third, mode pairs are isolated and quite limited in number; Fourth, exciting a single desired primary mode is extremely difficult in practice and the received signal is quite difficult to process and interpret. And few attention has been paid to solving these shortcomings. In this paper, nonlinear S0 mode Lamb waves at low-frequency range satisfying approximate phase velocity matching is proposed for the purpose of overcoming these limitations. In analytical studies, the secondary amplitudes with the propagation distance considering the fundamental frequency, the maximum cumulative propagation distance (MCPD) with the fundamental frequency and the maximum linear cumulative propagation distance (MLCPD) using linear regression analysis are investigated. Based on analytical results, approximate phase velocity matching is quantitatively characterized as the relative phase velocity deviation less than a threshold value of 1%. Numerical studies are also conducted using tone burst as the excitation signal. The influences of center frequency and frequency bandwidth on the secondary amplitudes and MCPD are investigated. S1–S2 mode with the fundamental frequency at 1.8 MHz, the primary S0 mode at the center frequencies of 100 and 200 kHz are used respectively to calculate the ratios of nonlinear parameter of Al 6061-T6 to Al 7075-T651. The close agreement of the computed ratios to the actual value verifies the effectiveness of nonlinear S0 mode Lamb waves satisfying approximate phase velocity matching for characterizing the material nonlinearity. Moreover, the ratios derived from the primary and secondary horizontal displacements generated from nonlinear S0 mode Lamb waves are closest to the real value, which indicates that using horizontal displacements is more suitable for detecting evenly distributed microstructural changes in large thin plate-like structure. Successful application to evaluating material at different levels of evenly distributed fatigue damage is also numerically conducted.

Analysis, design, and implementation of a random frequency PWM inverter

Abstract: Random pulse width modulation (RPWM) approaches can make the harmonic spectrum of inverter output voltage be continuously distributed without affecting the fundamental frequency component, and thus the acoustic noise and mechanical vibration of an inverter-fed AC motor drive are greatly reduced. However, the analysis and design of the RPWM mechanisms are generally not so trivial for practical engineers that their applications are limited. In this paper, a random frequency PWM (RFPWM) inverter and its practical design procedure are presented. First, the effects of the attributes of a random signal on the inverter output harmonic spectrum distribution characteristics are analyzed using an intuitive concept, then based on which, the quantitative design. Simulink simulation and implementation of the proposed RFPWM inverter are introduced. The proposed RFPWM inverter is employed to power an indirect field-oriented induction motor drive. The simulated and measured results indicate that the uniform random distribution of inverter output harmonic spectrum and thus smaller acoustic noise and mechanical vibration are obtained by the proposed RFPWM scheme.

Effects of the salience of pitch and periodicity information on the intelligibility of four-channel vocoded speech: Implications for cochlear implants

Abstract: Recent simulations of continuous interleaved sampling (CIS) cochlear implant speech processors have used acoustic stimulation that provides only weak cues to pitch, periodicity, and aperiodicity, although these are regarded as important perceptual factors of speech. Four-channel vocoders simulating CIS processors have been constructed, in which the salience of speech-derived periodicity and pitch information was manipulated. The highest salience of pitch and periodicity was provided by an explicit encoding, using a pulse carrier following fundamental frequency for voiced speech, and a noise carrier during voiceless speech. Other processors included noise-excited vocoders with envelope cutoff frequencies of 32 and 400 Hz. The use of a pulse carrier following fundamental frequency gave substantially higher performance in identification of frequency glides than did vocoders using envelope-modulated noise carriers. The perception of consonant voicing information was improved by processors that preserved periodicity, and connected discourse tracking rates were slightly faster with noise carriers modulated by envelopes with a cutoff frequency of 400 Hz compared to 32 Hz. However, consonant and vowel identification, sentence intelligibility, and connected discourse tracking rates were generally similar through all of the processors. For these speech tasks, pitch and periodicity beyond the weak information available from 400 Hz envelope-modulated noise did not contribute substantially to performance.

A Selective Harmonic Elimination Method for Five-Level Converters for Distributed Generation

Abstract: In distributed generation, high power capability and high efficiency are very important requirements. For this reason, this paper proposes a new analytical procedure for selective harmonic elimination in five-level inverters operating at fundamental frequency. For each modulation index, the proposed method calculates all possible switching angles eliminating a specified odd harmonic from the output voltage. The method has limited computational complexity, simple and cost-effective real-time implementation, and full capability of integration with closed loop control. Simulation and experimental results highlight full elimination of selected harmonic.

Infant pitch perception: evidence for responding to pitch categories and the missing fundamental.

Abstract: While numerous studies on infant perception have demonstrated the infant’s ability to discriminate sounds having different frequencies, little research has evaluated more sophisticated pitch perception abilities such as perceptual constancy and perception of the missing fundamental. In the present study 7–8‐month‐old infants demonstrated the ability to discriminate harmonic complexes from two pitch categories that differed in pitch by approximately 20% (e.g., 160 vs 200 Hz). Using a visually reinforced conditioned head‐turning paradigm, a number of spectrally different tonal complexes that contained varying harmonic components but signaled the same two pitch categories were presented. After learning the basic pitch discrimination, the same infants learned to categorize spectrally different tonal complexes according to the pitches signaled by their fundamental frequencies. That is, the infants showed evidence of perceptual constancy for the pitch of harmonic complexes. Finally, infants heard tonal complexes that signaled the same pitch categories but for which the fundamental frequency was removed. Infants were still able to categorize the harmonic complexes according to their pitch categories. These results suggest that by 7 months of age infants show fairly sophisticated pitch perception abilities similar to those demonstrated by adults.