Some crossover units have phase correction and can correct phase errors that occur in the loudspeakers and cabinets. Phase alignment should begin at the highest-frequency band. All other frequency bands should be adjusted in sequence from highest to lowest. If after an initial adjustment further adjustments are needed on any frequency band, all lower bands require adjustment to compensate. To set phase alignment accurately, use an analyzer to provide a pictorial view of the crossover regions and set phase controls for the flattest response. A sine wave from a signal generator at each crossover point can be used. Alter the phase control for minimum volume, then switch the polarity switch to invert one of the outputs to achieve true summation (Figure 7-3).