Preliminary Investigations and Cognitive Considerations of the Acoustical Resonances of Selected Archaeological Sites
Devereux, Paul, Jahn, Robert G., Antiquity
Enclosed prehistoric spaces can have fine echoes. The Hypogeum, celebrated underground ossuary of Neolithic Malta, is the most resonant place to chant a rhythmic 'Oum'. Are the acoustic features of structures like the megalithic chambers of northern Europe integral to their design? An expert description of their acoustic properties is the point to start.
The acoustical aspects of archaeological sites have in the main tended to be overlooked as possible sources of information, probably because it is instinctively felt that sound is too immediate and ephemeral to have significance for archaeological investigation. Nevertheless, for those assessing 'cognitive' approaches, an exploration of various acoustical techniques holds some prospect of revealing new and useful investigative methods. This potential has already been highlighted by the observations of Frances Lynch that the use of the passage in certain passage-graves may have related to communication rather than access (Lynch 1973), and by acoustical research conducted by Steven Waller at prehistoric rock-art sites in Australia, North America and western Europe (Dayton 1992). Waller's tests have reportedly indicated that petroglyphs tend to be found on rock surfaces that yield louder echoes than adjacent ones. Echo effects produced by yelling, clapping and percussive noises also seemed to associate with some of the rock-art imagery, resembling the sound of individual running hooved animals, or, in painted deep caves such as Lascaux, animal herds on the move. Curved rock surfaces acted much like parabolic reflectors, focusing echoes at specific central images in the rock paintings.
The present research has concentrated on the acoustical resonances of six selected prehistoric chambered structures in England and Ireland. This arose out of initial observations by Jahn concerning the relationship between Hopi ceremonial chanting and the interior of the restored Anasazi kiva at Aztec, New Mexico. The question was posed as to whether ancient ritual structures could have arrived at their proportions as the optimal result of empirical recognition of the acoustical properties of the kinds of ceremonial singing or musical sounds for which they may have provided the environment. If so, then knowing the acoustical resonance frequency of the cavity forming an ancient structure's interior could feasibly provide a guide as to whether musical/vocal activity once occurred there or not. The UK and Irish sites were chosen as they had no associated ethnology, and so provided useful 'blind' test cases. Fieldwork was conducted in the summer of 1994. Deployed at each site was an omnidirectional loudspeaker driven by a variable frequency sine-wave oscillator and a 20-W amplifier, with sound frequency verified by an external, hand-held digital multimeter. The sound amplitude patterns were mapped by a portable meter, sensitive between 55 and 105 dB sound-pressure level. Typically, the sound source was placed on the floor or on a short tripod roughly at the centre of the chamber configuration, with the acoustic axis oriented vertically. The frequency was manually swept through the lower audible range until the lowest natural resonance of the cavity was evidenced by clearly discernable reverberation of the chamber. With this established, the sound intensity was adjusted to the highest comfortable level, usually between 100 and 110 dB at the source, and horizontal surveys of standing-wave patterns were made over some accessible grid covering the chamber. Full details can be found in Jahn et al. (1995; 1996), but here it is sufficient to note that all the sites, despite individual structural differences, yielded a tight band of measured resonance frequencies:
site frequency Carn Euny, Cornwall ('beehive' chamber) 99 Hz Chun Quoit, Cornwall 110 Hz Wayland's Smithy, Berkshire (east chamber) 112 Hz Wayland's Smithy, Berkshire (west chamber) 95 Hz Cairn L, Loughcrew, Co. …