An apparatus for applying electrical signals to first, second, third and fourth sets of at least one electrode element, the apparatus comprising:
- an AC signal generator having a first output for applying an AC voltage between the first set of at least one electrode element and the second set of at least one electrode element, a second output for applying an AC voltage between the third set of at least one electrode element and the fourth set of at least one electrode element, and at least one control input, wherein the AC signal generator is configured so that the at least one control input controls an amplitude of the first output, a frequency of the first output, an amplitude of the second output, and a frequency of the second output;
- and
- a controller configured to apply a sequence of control signals to the at least one control input of the AC signal generator, wherein the sequence of control signals controls the AC signal generator so that (a) when the first output is activated at a given time, the first output has an initial amplitude and an initial frequency, (b) at a first set of times that follow the given time while the first output remains active, the amplitude of the first output increases while the frequency of the first output remains constant, and (c) at a second set of times that follow the given time while the first output remains active, the frequency of the first output decreases while the amplitude of the first output remains constant, wherein the first set of times and the second set of times are mutually exclusive,
- wherein the sequence of control signals further controls the AC signal generator so that (a) when the second output is activated at a certain time, the second output has an initial amplitude and an initial frequency, (b) at a third set of times that follow the certain time while the second output remains active, the amplitude of the second output increases while the frequency of the second output remains constant, and (c) at a fourth set of times that follow the certain time while the second output remains active, the frequency of the second output decreases while the amplitude of the second output remains constant, wherein the third set of times and the fourth set of times are mutually exclusive, and
- wherein the sequence of control signals causes the AC signal generator to (i) activate the first output for an interval of time, (ii) subsequently activate the second output for an interval of time, and (iii) subsequently repeat (i) and (ii) at least 10 times in an alternating sequence, whereby there will be at least 10 activations of the first output and at least 10 activations of the second output,
- wherein each of the at least 10 activations of the first output corresponds to a respective first set of times and a respective second set of times, and
- wherein each of the at least 10 activations of the second output corresponds to a respective third set of times and a respective fourth set of times.