Characterization of a Loudspeaker Free Field Radiation by Laser Doppler Velocimetry

The performances of a Laser Doppler Velocimetry (LDV) system adapted for measuring the acoustic particle velocities is assessed for measuring acoustic velocities in free field. In this condition, the flow velocity due to natural convection is taken into account. The assessment is performed by comparing the acoustic velocities measured by means of LDV to reference acoustic velocities estimated by means of a sound intensity probe. The LDV systems delivers a signal made of many bursts which are modulated in amplitude and frequency. The signal processing used for estimating the acoustic velocity is divided in three steps (detection of burst, frequency demodulation and acoustic velocity amplitude and phase estimation). The minimum measurable acoustic displacement depends on the frequency demodulation technique (Short Time Fourier Transform in this work) and the minimum measurable acoustic frequency depends on the convection velocity. Taking into account these constraints, the assessment is performed for 500, 1000 and 2000 Hz and for acoustic velocities greater than 2 mm/s. Results show that this system can measure acoustic velocities at 500, 1000 Hz and 2000 Hz in free field. For 2000 Hz, the velocity amplitude estimated by LDV differs slightly from this measured by the sound intensity probe. The acoustic radiation of a loudspeaker is characterized for 500 Hz using the LDV and the sound intensity probe. Results obtained in the near field of the loudspeaker show that the velocity amplitude calculated by means of a radiation model and the measured velocity amplitudes differ with a bias of 10%. In the far field, the experimental conditions do not respect the hypothesis used in the radiation model. For this reason, the measured and calculated velocity amplitude differ with a systematic bias.