Matthew C. M. Wright - mcmw@isvr.soton.ac.uk
Institute of Sound and Vibration Research (ISVR)
University of Southampton
Southampton, SO17 1BJ
United Kingdom
Popular version of paper 3pAA4
Presented Wednesday afternoon, June 6, 2007
153rd ASA Meeting, Salt Lake City, UT
More and more archaeologists are studying not just the appearance of ancient sites and artifacts, but their acoustic properties as well. A good example of archaeoacoustics is the study of ancient musical instruments such as flutes, which leads to questions about how they were made and how they were used. It is important, however, to get the acoustics right. This paper critically re-examines one piece of archaeoacoustics, which has been widely publicized, to see if the sums add up.
Anyone who has ever sung in the shower has experienced acoustic resonances; the room responds really well to some notes. In 1996, an engineer (Jahn), an archaeologist (Devereux), and a physicist (Ibison),measured acoustic resonances in six ancient burial chambers in the British Isles, five dating from 3,500 B.C. and one from 500 B.C. Devereux is a freelance archaeologist, while Jahn and Ibison were associated with the Princeton Engineering Anomalies Laboratory (PEAR Lab). This was the only piece of archaeoacoustics (or indeed archaeology or acoustics) research published by PEAR. The bulk of its work was involved with attempts to demonstrate psychokinesis using random-number generators and theories of consciousness.
Jahn, Devereux, and Ibison observed that in each of the chambers they found a resonance in the range from 95 to 120 Hz and observed that this falls within the range of the male singing voice. For comparison, the A-string of a guitar in standard tuning plays at 110 Hz, close to the middle of this range. They also suggested that there were similarities between rock art found at some of the sites and the mode-shapes (patterns of loud and quiet) associated with the resonances they found. This suggestion also requires critical examination, but I won't consider it here.
Devereux went on to claim, in subsequent publications including a 2001 book on which a TV program was based, that the presence of these resonances was evidence not only that the chambers were used for ritual chanting (although there was no other evidence to suggest this) but that they must have been deliberately and intentionally constructed for this purpose. He also suggested that exposure to sound at 110 Hz might have a special effect on the brain.
The claim that the chambers were designed to have these resonances can be critically examined as follows. While the precise natural frequencies of a closed cavity depend on its precise shape, the statistics of the resonance frequencies depend only on the volume of the cavity, the area of the walls and the length of any edges. The average number of resonance frequencies to be expected in a given frequency band is governed by a formula derived in 1912 by the German mathematical physicist Hermann Weyl. Using this formula, and making conservative assumptions about the size of each chamber based on the descriptions given by Jahn et al. and other sources, it can be shown that the Weyl formula predicts more than one resonance in all but the smallest chamber considered, and in the largest it predicts just under seven resonances. In other words, it would have been surprising if these chambers did not have resonances between 95 and 120 Hz.
These results have to be scaled down somewhat to account for the non-ideal nature of the cavities. For example, they may have had floors that weren't as reflective as their stone walls, and they may have had openings such as windows and doors, all of which slightly reduce the expected number of resonances. Nonetheless, it is possible to conclude from this analysis that the acoustical evidence that these particular chambers were designed for chanting is no stronger than it is for a typical bathroom.
References:
R. G. Jahn, P. Devereux and M. Ibison. Acoustical resonances of assorted ancient structures.
Journal of the Acoustical Society of America,99 (2), 649-658, 1996.
P. Devereux. Stone Age Soundtracks: The Acoustic Archaeology of Ancient Sites. Vega, London, 2001.
P. Devereux. Ears & years: Aspects of intentionality in antiquity, in C. Scarre and G. Lawson, editors, Archaeoacoustics, McDonald Institute Monographs, chapter 2, pp 23-30, Oxbow press, Oxford, 2006.
H. Weyl. Uber das Spektrum der Hohlraumstrahlung.
Journal für die Reine und Angewandte Mathematik 141(3), 163-181, 1912.