Realization of Air-borne Sound pressure unit with LDA technique by Spectrum and autocorrelation method in a travelling wave tube

To realize the sound pressure unit directly, the method of sound pressure measurement based on acoustic particle velocity with LDA technique was described. In order to get a simple acoustic field, a travelling wave tube was designed. The sound pressure distribution obtained by microphone along the tube was measured. The result showed the acoustic field inside the tube could be considered as travelling wave. The air-borne sound pressure is equal to the product of the air density, sound speed and the particle velocity. The laser Doppler Anemometry was used to measure the particle velocity in the acoustic field. The modulated doppler signal was obtained by TSI laser doppler system and the doppler signal was processed by autocorrelation method and spectrum analysis method. With the spectral analysis of doppler signal, the particle vibration velocity was obtained with the Bessel function analysis and the velocity can also be calculated based on the time of characteristic values of the correlation function. The comparison of sound pressure measured by microphone and the value deduced from the velocity measured by laser Doppler system shows that they have good agreement and the deviation is less than 0.5 dB.