Haven’t updated in a while. Cracking on with the PhD doing lots of experimental work. Lots of time has been put into experimenting with the new hardware setup, PC->NI DAQ->Single Hydrophone, playing with all sorts of different concoctions. Results have been adequate, but not astonishing. The expected amount of bubble noise is there, masking the instant verification that would have been so nice. The most interesting part is that the bubble creates quite a number of frequencies that span from almost-DC to 10’s of kilohertz’s which means that a simple kill-all-bubble noise filter is not as simple as we first thought. Ive attached images of a control test where droplets of about 2mm are dropped from a hight of approximately 3 meters into a tank of water. The first is a simple plot of the data, and the second is a spectrogram showing the different frequency components at different times.

Over the past couple of days I have started trying to filter out the bubbles in software, so I don’t have to put so much emphasis on reducing the bubbles at the source.  I have had some success convolving an average of 10 drop pulses and also using the fact that the drop pulse is very delta-like.  In the frequency domain this can be viewed as a broad spectrum, so taking an average of a FFT will produce a high result and others spurious signals give out a low.