Applying acoustic array processing to the estimation of the propagation speed of waves in a car exhaust

A parametric method based on spatial filter techniques (beamforming) is proposed to estimate the propagation speed of acoustic waves. The propagation speed estimate is analyzed for the case of narrowband signals and compared to the maximum likelihood estimate (MLE) of the propagation speed. It is shown that for an array of 3 sensors our estimate coincides with the ML estimate but its performance analysis is simpler and its computational cost is much more reduced. The proposed estimate is also applied to the wideband waves propagating along a car exhaust. It is shown that the signal-to-noise ratio and the magnitude of the relative aperture (distance between the array sensors respect to the wavelength) for each frequency could limit the good performance of the speed estimator. Good results have been achieved when these limitations have been taken into account. 1. PROBLEM STATEMENT There exist two low frequency wideband acoustic waves propagating along a car exhaust when the engine is working, the forward one generated by the engine, and the backward one originated from the misadjustment of the acoustic impedance at the muffler input. Both waves are cyclic and their characteristics depend basically on the number of revolutions-per-minute the car engine is rotating at. The main characteristics of the acoustic waves to be taken into account for a perfect separation of their waveforms are the direction of arrival and the propagation speed. The first one is perfectly known: -90" for the forward wave and 90" for the backward one respect to the exhaust pipe orthogonal direction. The second one is only approximately known because it depends on the uncertain temperature value, the stability of the car engine, and other factors. * PARTIALLY SUPPORTED BY CICYT GRANT No. TIC94-0922-C02-0 1. 0-8186-7919-0/97 $10.00