How Loudspeakers Work 29 transient response (Q = 0.707 gives overshoot on square waves) but is often criticised for being "too taut" or over-damped, and the LF output clearly suffers. For a given drive unit, the system Q is determined by the box volume. The bigger the box, the lower the Q; the responses in Figure 2.2 correspond to a range of box volumes of about 50 times. The box may be left completely empty, or it may be lined or wholly filled with acoustic damping material, such as bonded acetate fibre (BAF) or long fibre wool. This not only absorbs internal reflections, but for the "fill-'er-up" approach, alters the thermodynamic properties of the enclosed air so that the enclosure behaves as though slightly larger than its physical size. Sealed box loudspeakers are normally classified into two kinds: the infinite baffle type and the air suspension type. If you have a large enclosure where the compliance of the air inside is greater than the compliance of the drive unit suspension, so that most of the restoring force comes from the latter, it is regarded as an infinite baffle type. A small enclosure where the compliance of the air inside is less than the compliance of the drive unit suspension by a factor of three or more, is regarded as an air suspension type. The efficiency of a drive unit in a sealed box is proportional to the cube of the drive unit resonance frequency (the Thiele­Small parameter F s ) [1] so attempts to get a lot of low-