Designing neutral rooms 121 do return, they will do so with generally less intensity than those travelling back from shorter distances (given the same surfaces from which to reflect). What is more, when reflexions arrive much more than 30 ms after the initial sound they tend to be perceived by the brain as reflexions, whereas those arriving before 30 ms have elapsed will almost certainly be heard as a timbral colouration of the instrument, and will not be perceived as discrete reflexions. This is, of course, the Haas effect (see Glossary). In a large room, therefore, the resonant modes and reflexions are usually heard as separate entities to the direct sound of the instruments. Unless the direct sound of an instrument is swamped by room sounds which are unduly long in time or high in level, its natural characteristic timbre will be clearly heard. Generally, a recording room can be considered to be in the acoustically small category if it is impossible to be less than four or five metres from the nearest wall surface. Judicious angling of the ceiling, together with careful use of absorption and diffusion, can permit the use of ceilings of 4 m or less with relative freedom from colouration, so an acoustically small room for recording purposes would tend to be less than about 10 m × 10 m × 4 m. 5.2 Large neutral rooms To build an acoustically large neutral room is not a particularly difficult exercise as long as a few basic rules are followed. Parallel hard surfaced walls should be avoided, as these can support the strong axial modes which develop,