It is promising that the power spectra of the six source impacts are so similar at low frequency, up to about 80 Hz. An attractive solution is to simply subtract impacts from each other and select the most appealing output, or combine several outputs. Despite the similarity in power spectra, however, small differences in phase prevent simple differencing from being a success. The phase differences can be smoothed out somewhat by stacking impacts into groups and then subtracting; this also has the advantage that it produces a simple two-dimensional output. The result produced by this method is shown in Figure 3; it does not succeed at removing energy as well as methods that involve filtering.