Measurement of shock N-waves using optical methods

Petr V. Yuldashev (1) , Mikhail V. Averiyanov (2) , Vera A. Khokhlova (2) , Oleg A. Sapozhnikov (2) , Sébastien Ollivier (1) , Philippe Blanc Benon (1)

1 LMFA - Laboratoire de Mecanique des Fluides et d'Acoustique
2 Department of Acoustics, MSU

Accurate measurement of broadband acoustic signals in air, particularly shock N-waves, remains a challenge. Bandwidth of existing microphones typically does not exceed 140 kHz, which results in significant overestimation of the shock rise time. Various optical methods may be used to design acoustic sensors with improved high-frequency response. Two approaches are examined in this work: focused shadowgraphy and Mach-Zehnder interferometry. The shadowgraphy method was applied to measure shock front of a high amplitude spherical N-wave generated by a spark source. The shock was illuminated by 20 ns short pulses of white light from a flash lamp; shadowgrams were captured by a CCD camera. The rise time of the shock was calculated from the shadow using numerical modeling of light propagation through inhomogeneities of refraction index caused by the shock wave. Diffraction of the light beam was accounted in parabolic approximation. Simulation results showed that diffraction effects were important in interpretation of shadowgrams and only those captured within few millimeters of a point where the light grazes the shock were informative. Experimental results were in a good agreement with theoretical predictions of the shock rise time. However, while the shadowgraphy method is simple and provides good resolution of the shock, its sensitivity to smooth parts of the waveform is very poor. More complex optical method based on a Mach-Zehnder interferometer to measure a whole waveform with high resolution was tested theoretically. Comparison of the spectra of the optically measured waveform and acoustic signal obtained with the microphone also gives absolute frequency calibration of the microphone. A distance between the microphone and a light beam can be taken into account by acoustic wave propagation modeling. The combination of optical and acoustic experimental methods and modeling therefore can be used as an effective instrument for aeroacoustic measurements and calibration of broadband microphones.

Mots clés: Broadband microphones; Calibration; Shock waves; Shadowgraphy

Domaines: Acoustique [physics.class-ph]; Acoustique [physics.class-ph]

Petr V. Yuldashev, Mikhail V. Averiyanov, Vera A. Khokhlova, Oleg A. Sapozhnikov, Sébastien Ollivier, et al.. Measurement of shock N-waves using optical methods. 10ème Congrès Français d'Acoustique, Apr 2010, Lyon, France. ⟨hal-00539747⟩