The diffusive effect of the dry stone surfaces is reduced in the courtyard where speech and conversation were perceived as clear and intelligible. Voices talking inside the courtyard are audible in the entrance passage, the cells and halfway up the stairs to the wall head. Communication from the wall head to the courtyard is possible by shouting up or down. In contrast, sounds inside spaces within the broch, the cells, the galleries, the staircase and the main entrance are diffused by the wall surfaces and are perceived as characteristically dry, close and washed out. Sound sources (drum, iron bars, recorder) created at ground level inside cells A, B and C by one investigator were audible to another investigator positioned outside in the courtyard and could be heard inside the staircase and halfway up the stairs. This suggests that a proportion of any direct sound generated inside the cells travels out through the entrances and openings, crosses the courtyard and re-enters the broch structure and galleries via the stair entrance and the large openings in voidset C. Proposed platforms encircling or partially encircling the wall base at scarcement level would influence the flow of sound between entrance, cells and courtyard and change the character of the aural space.
The interaction and juxtaposition of material used in this project informs our description of aural space, but the observation of the action of sunlight inside Mousa broch raises remotely linked ideas connected to the concept of adaptive space. Was adaptive space, which involves a change in use of sound, light or movement (alone or in combination) to alter that space, incorporated into the design of the broch? Can the action of sunlight around the broch walls be construed as an example of adaptive space, supporting the proposal that the broch was unroofed? Might the broch perform the function of an interface between ground and sky, a construction in which vertical void sets and stairs were component parts of a solar/celestial measuring device? These questions remain to be investigated.
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