Because all real control rooms have at least one surface (a floor) the speaker low end (below 100Hz) is measured in half free space. This gives a fairly accurate estimation of a very large control room, with the speaker placed far from back and side walls. As the room size decreases, the amount of bass increases, which means that the smaller speaker models are particularly well suited to smaller rooms which will support the bass response of the speaker. In other words, the plotted frequency response corresponds to the minimum amount of bass you can expect in any control room.