A consideration of the contribution of the gallery chambers to the sonic character of the broch (see 3.2) suggests that voids facilitate the entry of sound into the chambers but that direct sound inside the chambers is diffused and void influence will be limited by the distance of the sound source from the listener. Voids also admit light. The author observed that on 21 June 2009 the position of the shadow cast by the early rays of the rising sun over void set A is best observed standing on the stairs next to void set C. Positioned opposite a void inside a gallery, an individual can look out from the upper voids onto a rectangular framed view of the sky and wallhead. The lower voids do not present a view of the sky but are illuminated by the sun at different times of the year. The potential for voids to be considered as significant components in the construction of Mousa broch has recently been considered (Armit 2003) but their function continues to baffle. Armit, referring to the voids in Mousa broch, confirms that voids do not reinforce the structure of the broch and, in fact, create weak areas in the walls (Armit 2003, 162).
A functional explanation for the voids is provided by architect John Hope (cited in Armit 2003). Hope's theory proposes that the galleries and voids linking to them are part of an air-conditioning system that circulates air upward through the broch. Hope's model is helpful because it demonstrates how the void spaces link to the intramural gallery chambers. Air flowing through and between the voids and the chambers in the walls would dry the rock. This idea provides a potential (untested) explanation for the voids but may be flawed; one set of vertical niches in the wall form a line but are not voids, do not admit light and would not conduct air. In his discussion of Mousa broch, Armit refers to this line of voids as 'purely for show as apart from the lowest one they do not even penetrate the inner wall' (Armit 2003, 167).
This area of the wall (here void set D) deserves closer attention. It extends upwards from gallery 1 to gallery 4, a distance of 20ft 3in (6m). Over the length of its upward travel, void set D spans a width of 9ft (2.7m) to 1ft 7in (0.5m). The channel/niche is divided into 17 openings and all openings in the niche align to the azimuth angle 160-166 degrees. Although the voids do not penetrate the gallery chamber, there is no evidence to support the idea that void set D was added to the inner wall face as an afterthought. To incorporate such a feature into the wall would have incurred the risk of a further 'point of weakness' and required considerable time, effort and expertise. The author concurs that void set D is a constructional anomaly, but disagrees with the suggestion that these 'false voids' were constructed as a form of decoration on the wall. Further investigation of the anomalous nature of void set D may provide an insight into an alternative function for all the voids.
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