“…Sound absorption in water generally increases with increasing frequency. In contrast, for pipes, geometric effects may cause higher frequencies components of the signal to propagate further (Brizzolara et al, 1999;Schaefer et al, 2010). It has been suggested that a sparker could be designed to maximise the energy in the desired frequency range (Brizzolara et al, 2003(Brizzolara et al, , 1999.…”