312 Chapter 11 up with multiple narrow notches; an alignment that would become grotesquely incorrect as soon as the loudspeaker was moved by a few inches. You will, however, note that the gain variations become less as the frequency increases, so what can be done is to make some compensation for the really big response variations at the LF end. The other important point that stands out here is that the loudspeaker-spaced-from-a-wall situation is extremely common, giving rise to the sort of frequency response shown in Figure 11.4, and yet we still listen quite happily to the result. It is not necessary to have a ruler-flat frequency response to enjoy music. Another important property of a listening space is the amount of high-frequency absorption it contains. A room with hard walls and floors will reflect high-frequency energy, and a proportion of this will reach the listener as reverberation. On the other hand, a room with wall hangings, thick carpets, and comfy sofas will absorb some of high-frequency energy and the effect will be less treble. The more absorbent room will give the more accurate sound as a greater proportion of the energy at the listener will be directly from the loudspeaker, with an accurate frequency response, and will be subjected to less room colouration. Correcting for high-frequency absorption is a job for the preamplifier rather than the active crossover, and this is just one reason why the concept of preamplifiers without tone-controls is a daft idea. 11.4 Equalisation Circuits There are many ways of obtaining a desired equalisation response, and any attempt to examine them all would probably fill up the whole book, so I have had to be very selective in picking those looked at here. I have aimed to provide circuits that are easy to design, predictable in their response, easy to configure for good noise and distortion performance, and well-adapted for dealing with common loudspeaker problems. I have included some where the fixed resistors that set the response can be temporarily replaced with variable controls to speed the optimisation of a crossover design. 11.5 HF-Boost and LF-Cut Equaliser This is also known as a shelving highpass equaliser. It gives a frequency response that at low frequencies is flat, but begins to rise when the frequency passes the boost frequency f b . It continues to rise at a basic rate of 6 dB/octave until the shelf frequency f s is reached, at which point it shelves or levels out to a fixed gain. Unless the boost frequency and the shelf frequency are spaced by several octaves the transition slope between the gain regimes will not have time to develop an actual 6 dB/octave slope. Figure 11.5 shows the inverting form of the circuit. A non-inverting version also exists but is less flexible because at no frequency can it have a gain of less than unity.