518 Recording Studio Design 20.6.4 Slopes and shapes The 6 dB/octave (first order) Butterworth crossovers are the only ones that can truly reconstruct a transient waveform at their combined outputs. The main problem with their use is that their attenuation rate is so slow that out of band interference between the drivers can be a big acoustic problem. What is more, the slow rate of filter attenuation is not much use in protecting high frequency drivers from the damaging low frequency inputs. The only common use for 6 dB/octave filters is in `half-crossovers', such as when a naturally tapering high frequency response of a mid-range driver is matched by an electrical filter which progressively brings a tweeter into circuit. There is another use for 6dB/octave slopes in some complex, mixed slope, linear-phase/ constant voltage crossovers. However, the need for one driver to exhibit a flat response up to three octaves, or so, out of range makes such designs rather impracticable. The 12dB/octave (second order) filter provides a good polar response, and has been used in many famous passive designs such as the Yamaha `NS10M Studio'. However, in many cases of their implementation, the problems involved with getting the transient responses adequately accurate have been a handicap. In passive designs, this type of filter was popular because it was cheap and simple, but in active designs, they hardly reduce the price or complexity at all. The 18dB/octave (third order) crossovers, such as the Butterworth, are widely used, despite the asymmetric off-axis performance which results when using non-coincident drivers. Perversely, it has nearly always been the case with