PART V Loudspeakers Z Common geometrical and acoustical axis Cabinet with two identical drivers radiating equally Z FIGURE 14.9 Lobing of the response when physically displaced drivers radiate a common frequency whose wavelength is close to, or smaller than, the distance between the drivers Z axis of phase cancellation ­ varies with frequency ­ cancellation occurs whenever the distance to the two drive units varies by half a wavelength. The pattern shown therefore represents the situation at one frequency, only. 410 serves as an unwanted discontinuity in the HF horn. The general concepts of these drivers are shown in Figure 14.10 , along with the Altec/UREI approach. In the latter case, the separate concentrically mounted horn is left hanging in free air, but this method of mounting is really too abrupt for proper mouth termination at the 1 kHz crossover frequency. The termination problem was discussed in detail in the previous chapter. Therefore, as so very often is the case with loudspeaker design, the tendency is to be trading one problem for another, rather than solving them--finding the best compromise for each situation--but that is often the reality of loudspeakers. ORDERS, SLOPES AND SHAPES Despite the different solutions on offer, electrical filters are overwhelmingly the most common manner of dividing the frequency bands. Whether this is done at high level, low level, actively or passively, the same basic filter concepts apply. Figure 14.11 shows a simple high pass filter, (a) to (d) showing first, second, third and fourth order roll-offs, respectively. Each inductor or capacitor adds 6 dB per octave of roll-off, and each 6 dB is known as an order of roll-off, a term which comes from the mathematical application of filter theory. An alternative approach to the inductor/capacitor (LC) design is a resistor/ capacitor (RC) method shown in Figure 14.12 . The LC approach is preferred for high-level crossover in the loudspeaker/amplifier interface because the power losses are much less, but the RC approach is preferred in low-level circuitry because of its simplicity (perfect inductors are not easy to make) and its relative insensitivity to drift and interference pickup. In the active circuits, where gain is plentifully available, the higher losses of the RC circuits are of little consequence. First order crossovers are rarely used, because the low rate of roll-off requires the individual drivers to have respectably flat responses for at least two, if not three, octaves each side of the crossover point, which is usually not practicable. Nonetheless, when they are able to be used, they have the advantage that they are the only conventional crossovers whose combined outputs reconstruct the input wave- form. This is shown in Figure 14.13 , and results from the fact that although each side of the filter is only 3 dB down at the crossover frequency (voltage summing would normally require that they should be 6 dB down to sum back to a flat response) the 45 degrees phase shift through one half of the