This chapter discusses the electronic room equalisation. The control of a room, to flatten the response from loudspeaker to ear, is best achieved acoustically. The old mono sound control rooms in music recording studios were generally rather similar to the broadcast control rooms of the same era. By the mid-1970s, the majority of top studios around the world were using monitor equalisation to try to achieve a more standardised frequency response at the listening position. By the late 1970s, the warning bells began to sound once it began to be widely realised that rooms which were supposedly equalised to within very tight limits were often still sounding very different from each other. An acoustic correction of the response dip could be achieved by putting mattresses on the floor between the loudspeakers and the listener. Another problem with electronic equalisation in general is that any boosts in level eat into the headroom of the monitor system.