Underwater sound speed plays a vital role in maritime safety. Based on the acousto-optic self-interference effect, we proposed a new method to measure underwater sound speed utilizing Raman–Nath diffraction, generated by the acousto-optic effect between an optical frequency comb and pulsed chirp signal. When the pulsed chirp travels between the measurement and reference arm in the experimental setup that we constructed, the same signal resulting from acousto-optic self-interference is produced. The time gap between the two identical signals represents the time interval. Thus, we can determine the time-of-flight using cross-correlation. The optical path difference between the two arms is double the flight distance of ultrasonic waves and can easily be obtained using femtosecond laser interferometry. The time gap and the distance can be used to measure sound speed. The experimental results show that the chirp signal improves the signal-to-noise ratio and expands the applicable time-of-flight algorithm. The waveform pulse width after cross-correlation is 1.5 μs, compared with 40 μs before. The time-of-flight uncertainty can achieve 1.03 ns compared to 8.6 ns before. Uncertainty of sound velocity can achieve 0.026 m/s.
Based on the acousto-optic effect, we propose a new method to directly measure water sound velocity that avoids the error-like phase ambiguity brought by the piezoelectric effect that is broadly adopted in current methods. In the experimental setup we designed, the laser signal modulated by the propagating acoustic wave changes its phase suddenly when the wave crosses the two or more intercepting laser lines simultaneously. This new design creatively realizes the possibility to capture time information at the phase level in sound velocity measurement, which is hardly realized in the piezoelectric-effect-based methods. Utilizing the above principle and the derived mathematical calculation, the accuracy of sound velocity with good traceability can be obtained. The experimental results show that the repeatability of the measurement results is less than 0.0159 m/s, and the accuracy compared with the commercial sound velocity profiler is better than 0.02 m/s.
This study compares event-related potentials (ERPs) elicited by variations of sound location in free and reverberant fields. The virtual sound sources located at azimuths 0°–40° were synthesized with head-related transfer functions and binaural room impulse responses for free and reverberant fields, respectively. The sound stimulus at 0° was assigned as standard in the oddball paradigm. Results show that the P3 amplitude is larger in the free field and acoustical conditions have no significant effect on the amplitudes of N2 and mismatch negativity. Moreover, a linear relationship between sound angle and amplitude of ERP components is observed.
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