3. Consideration of Acoustics Inside Mousa Broch

3.1 Creation of a scale model and observations of the action of light and shadow on the model walls

To understand the internal proportions and sonic characteristics of Mousa broch more clearly, a scale model (5 ft to 1 inch, based on proportions taken from Paterson's survey (1922) was assembled (see Figs 1 and 2). The cardboard model can be examined as a whole or disassembled into its component parts. Seven vertical sections (aligned to the four cardinal points) represent the base and six galleries of Mousa broch (see Table 2). The effect of wall reflections, the location of resonant spaces and the labelling or categorisation of the spaces inside the broch were informed by this model.

Table 2: Showing dimensions used in model (estimated from Paterson's 1922 survey)

Structural elements of Mousa broch Diameter in metres/ft
Height metres/ft
Height from ground metres/ft
Base 15.24/50   0-3.96/0-13
Internal courtyard 5.79/19-23 3.81/12.5 0-3.96/0-13
Gallery 1 6.7/22 1.52/5 5.33/17.5
Gallery 2 7.01/23 1.52/5 7.16/23.5
Gallery 3 7.31/24 1.82/6 8.99/29.5
Gallery 4 7.46/24.5 1.67/5.5 10.66/35
Gallery 5 7.62/25 1.52/5 12.19/40
Gallery 6 7.31/24 1.82/6 14.02/46

Observations of the action of light and shadows around the model helped in understanding the architectural structure and demonstrated how voids and surface openings inside the broch might influence its acoustic responses and sonic characteristics. A simple experiment was conducted to investigate the interaction of light/sound relationships between galleries 1 to 6. The broch model was placed on a horizontal surface. Candlelight was directed into the model and the effect of light on its surfaces was noted. Light entered the spaces created by openings inside the walls of the broch, indicating locations through which sound might enter and flow. Areas of open space were distinguished and noted for their potential influence on aural space. The final approach to data collection and the selection of recording locations inside the broch was based on these observations. The action of candlelight inside the model demonstrated that in a roofless broch at certain times of the year, sunlight would illuminate the walls, staircase, courtyard and gallery voids.


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