Noise reduction in acoustic disdrometry

This paper reports progress on the development of a novel rain disdrometer and the methods of noise reduction. The proposed instrument will measure the raindrop size distribution using the sound generated by raindrops landing in a tank of water. When an incident hydrometeor impacts on the surface of a liquid, two processes create an acoustic signal. The first is a broadband impact pulse which is related to the impact size and velocity. The second is created when pockets of air are trapped underneath the water's surface; this is termed entrainment. One particular solution to disdrometry is to use the acoustic signature of the impacts to classify the parameters of the rain event. To an extent, entrainment can be predicted, since fluid dynamics dictates that a bubble will oscillate and emit an acoustic signal as a damped sinusoid. In a rain event however, the bubbles can occupy a wide area in both temporal and spectral regions, overpowering the comparably small impact pulse. We present three methods to remove bubble related noise within acoustic disdrometry. These include the addition of a driving signal to force a bubble to oscillate in a nonresonant way, liquid additives to suppress the formation of bubbles and signal processing methods to filter any remaining bubble noise. It was found from simulation that driving a bubble does not reduce the entrainment signal. Experimental conditions can hardly be kept constant. Adding an oil film prevented bubble formation, but the maintenance required makes it less suitable to be used in the field. Using signal processing methods proved to be the most sustainable and flexible way of suppressing bubble noise.