Archaeo-acoustics is establishing itself as a research area within archaeology, and acoustic mapping becoming acknowledged as a valued aspect of archaeological fieldwork.
     In 2006, Paul Devereux and Jon Wozencroft initiated the Landscape & Perception project, a pilot study of raw visual and acoustic elements mainly on and around the Carn Menyn ridge, Mynydd Preseli, south-west Wales - the source area of some of the Stonehenge bluestones. The project asked: "What might Stone Age eyes and ears have perceived in this landscape, and what made it important to the builders of Stonehenge?".
     Mynydd Preseli lies southwest of Cardigan, in southwest Wales. Large parts of this region are open, wild upland areas punctuated by rocky crags known as "carns", and the surrounding countryside contains many archaeological sites. The part that forms the primary focus of this study is the ridge containing a set of outcrops, largely of spotted dolerite. A notable source of bluestones (a collective term for dolerites, rhyolites, and tuffs), the generally held view is that at least some bluestones at Stonehenge originated in these outcrops.
     In and around the Preselis, a distinct relationship recurs between dolmens and natural rock outcrops - in terms of proximity, inter-visibility, or both - and many sight lines are readily apparent. On the northerly perimeter, the apparent focus of Neolithic attention was Carn Ingli, the "Hill of Angels", so-called because the sixth-century anchorite, Saint Brynach, was said to meditate there and converse with angels. The capstone of the large dolmen of Pentre Ifan seems to mimic the slope of the Carn Ingli ridge - as does that of Careg Coetan, in the coastal village of Newport.
     Natural features with perceived anthropomorphic, zoomorphic or iconic forms were significant all around the ancient world, and Carn Ingli is of particular interest in this regard. Some people in the district today see it as a reclining woman - possibly a 'sleeping goddess' of the Neolithic age.
     We had a strong suspicion there would be ringing or musical rocks on Mynydd Preseli, because of early comments by 'rock gong' pioneer Bernard Fagg, and place-name clues such as Maenclochog (Welsh for ringing or bell stones), and Bellstone Quarry. According to our sampling, there is a 5 to 10 percent average incidence of ringing rocks on Carn Menyn, which in places rises to 15 to 20 percent, with small "hotspots" up to double that again.
     The ringing rocks identified at Carn Menyn and the surrounding carns issue a range of metallic sounds, from pure bell-like tones to tin drum noises to deeper gong-like resonances. They can be any shape, the only common denominator being sufficient air space around them to resonate.
     It can be shown that the sonic properties of the Preseli stones would have been known to Stone Age ears.
     During their SPACES project, Wainwright and Darvill discovered two previously unknown examples of prehistoric rock markings on Mynydd Preseli in areas thought to be devoid of any. One was situated at a now dry spring beneath the southern slope of Carn Menyn. Similar spring-heads on the southern slope were marked in various ways; one in particular  with a flat stone that had at least five cup marks hollowed in it - though one, cut in half, indicates part had broken off at some time. Whoever made those indentations could not have failed to notice the stone's acoustic properties, and the cup marks may even be a consequence of repeated percussion to elicit sounds.
     Many of the bluestones at Stonehenge have been struck at some point in the past - either before they were transported, prior to their erection at Stonehenge, or during their presence at the monument.
     A key question of the project relates to how the sonic properties of the Carn Menyn bluestones might have been a factor in their selection for the building of Stonehenge, and in July 2013 the fieldwork part of the project extended to acoustic tests of the bluestones at Stonehenge - the first time this had been done.
     The project team tested all the extant bluestones at the monument. Some made distinctive if muted sounds, indicating that they would have probably been full "ringers", were they not set in the ground. Two in particular were noted for such telltale sounds.
     In much of the ancient world, echoes from rocks, cliffs or inside caves, rocks that made musical or metallic sounds when struck, or locations that produced unusual noises were regarded as sacred or special in some way. The ancient Chinese had "resonant rocks" which they thought contained supernatural force, and some American Indians used them for rites of passage. On the Indian subcontinent, Neolithic rock art was carved on ringing rocks, and many centuries later sophisticated musical stones were installed in Indian temples. The builders of Stonehenge may well have held similar beliefs.
     Visit the Landscape & Perception project web site at www.landscape-perception.com

Edited from Time & Mind (2013)