4 Chapter 1 not only be heard by those in the room unfortunate enough to not get the best seat on the sofa, but it also creates the ambient sound environment through room reflections and reverberation. If it has serious response irregularities then these will detract from the listening experience, even if the direct on-axis sound is beyond reproach. The term "lobing" is also used to describe the reinforcements and cancellations that occur when two separate drive units are radiating; in this case their size is relatively unimportant because interference would still occur even if both were point sources. When the radiation is shifted at the crossover frequency because the signals to the two drive units are not in phase, this is called "lobing error." There is much more on this in Chapters 3 and 4. 1.4 Active Crossover Applications The main fields of application for active crossovers in association with multi-way loudspeakers are high-end hi-fi, sound reinforcement, automotive audio, sound recording studios, cinema theatres, and film studios. In hi-fi, active crossover technology offers better and more consistent quality than passive crossovers. We shall look closely at why this is so later in this chapter. In the area of sound reinforcement the use of active crossovers is virtually mandated by the need to use banks of loudspeakers with different characteristics, especially sub-woofers. The size and number of the loudspeaker cabinets used mean that it is physically impossible to put them close together, and hence sophisticated control of time delays is essential to obtain the desired coverage and polar responses. The large amount of power used in a typical sound reinforcement system means that the losses inherent in the use of passive crossovers cannot be tolerated. The high-power requirement also means that multiple power amplifiers are always used, and the extra cost of an active crossover system is very small by comparison. Automotive audio marches to its own drummer, so to speak, the priority of most of its exponents being the maximum possible level of bass at all costs. This is perhaps not the place to speculate on whether this is driven by an appreciation of musical aesthetics or macho territorial aggression, but the result is that subwoofer systems are very popular, and so naturally some sort of crossover system is required. This is usually an active crossover, because the high power levels once again make the losses in a passive crossover unacceptable. This is particularly true because 4 loudspeakers are used, so the current levels in inductors are doubled and I 2 R losses are quadrupled, compared with the 8 situation. Active crossovers do have other important applications besides driving multiway loudspeakers. They are also used in multi-band signal processing, of which the most common example is multi-band compression. A multi-band compressor uses a set of filters, working on exactly the same principle as a loudspeaker crossover, to split the audio signal into two, three, four or even more frequency bands; three or four-band compressors are the