102 Chapter 4 unless you are trying to keep abreast of the times, and perhaps compensating for a 1 dB peak in a drive unit response. The group delay of the summed output has a significant peak just below the crossover frequency. 4.9 Higher-Order Crossovers As we have seen, fourth-order crossovers provide a wide range of possible alignments, and good slopes of 24 dB/octave, which are generally considered to be adequate for maintaining separation between drive units. The fourth-order Linkwitz­Riley, with the absolute flatness of its summed response and its delightfully straightforward filter design, is considered by many the best crossover alignment known at present. The question nonetheless arises if crossovers of fifth, sixth or higher order could give any further advantages. There seems to be no consensus as to whether the extra steepness of the slopes gives significant benefits, and there is an issue about the perceptibility of the group delay. According to Siegfried Linkwitz, "Crossover filters of higher order than LR4 are probably not useful, because of an increasing peak in group delay around the crossover frequency."