The three high-pass filtered accelerations are combined to one output signal a by taking the 1-norm: i.e. summing their absolute values. The 1-norm was chosen instead of the more accurate 2-norm to make the algorithm less computationally intense. The out-put is zero when the device is not moving. Any movement in any direction results in an output signal. With the 1-norm the gain (the scale of the output signal), depends somewhat on the orientation of the device, whereas with the 2-norm the gain is uniform in any direction. However, as the algorithm features an adaptive threshold, it is insensitive to the gain and thus, there would not be any benefit for using the more computationally intensive 2-norm.