During daylight hours inside the broch, the author's subjective impression was one of quiet stillness. The thickness of the broch walls eliminates the sound of wind noise (except in higher gallery chambers) and the contrast between the treeless, windswept coastline outside and the spaces inside is marked. Within the broch, the sound of the sea is effectively masked and the recording of voices in the courtyard is clear and intelligible.
Voices in courtyard
To the listener inside beehive cells A, B or C, the characteristic ring of iron impacting inside the confined space is damped, the sound is diffused and perceived as a metallic click. The majority of the spaces inside the walls created by galleries, cells and the staircase demonstrate short reverberation times; acoustic analysis of audio samples supports the conclusion that the sonic character of the broch can be universally characterised as very dry or dead (see Section 5.1). The thickness of the dry stone wall construction and the diffusing effect of its fractured surfaces on incoming sound waves influence the sonic character of the aural space.
Inside Mousa broch, sounds of at least 1000 Hz and upwards are not reflected but scattered, diffused by the edge surfaces and gaps between the stones in the walls. The author concludes that the influence of what could be called 'dry stone diffusion' found at Mousa broch will occur in all dry stone buildings, including other prehistoric brochs and roundhouses. The identification of a characteristically 'dry' acoustic associated with dry stone construction gives us some insight into the aural space shared by our prehistoric ancestors in Atlantic Scotland.
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