3.2 Contribution of the gallery chambers to the sonic character of Mousa broch

Gallery chambers are effectively isolated from the inner courtyard and the outside of the broch by a cushion of stone up to 5 feet (1.5m) thick. The ceiling and floors of these spaces are interconnected but access is restricted, made possible by small gaps in the floor or roof. Speech inside the galleries is coloured by selective filtering, resulting from the upper frequency cancellation effect of diffusion. The influence of the close proximity of the dry stone walls changes the character of speech, but speech remains intelligible. Within 5 or 6 feet (1.5m or 1.8m) of each other inside the galleries, the investigators were able to communicate without raising their voices; as distance between them increased (7 or 8 ft: 2.1 or 2.4m) and line of sight was broken by the curving walls, it became necessary to shout. We can conclude that the diffused direct sound of the voice is affected by the wall surfaces and that the influence of diffusion increases with distance. Void openings, floor openings and entrances into the gallery chambers will admit sound waves. Within the gallery chamber, the listener's perception of sound will be influenced by their position relative to these openings. The dry acoustic signature and the diffusive influence of the circular-shaped walls of the gallery chambers on direct sound suggests that their ability to convey intelligible speech (over 8 ft: 2.4m) is limited. The author suggests that, based on these observations, the galleries are not acoustically friendly listening/discussion spaces orientated towards the courtyard.

Table 3: Showing dimensions of internal chambers for galleries 1–6 (based on Paterson's 1922 survey)

in metres/ft
in metres/ft
Height from
in metres/ft
Width of chamber
in metres/ft
Gallery 1 22.86/75 1.52/5 5.33/17.5 0.9/3
Gallery 2 22.86/75 1.52/5 7.16/23.5 1.52/5
Gallery 3 21.03/69 1.82/6 8.99/29.5 0.6/2
Gallery 4 21.03/69 1.67/5.5 10.66/35 0.6/2
Gallery 5 21.03/69 1.52/5 12.19/40 0.6/2
Gallery 6 21.03/69 1.82/6 14.02/46 0.6/2.5


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