In a series of experiments, archaeologist Aaron Watson and acoustics expert David Keating, formerly of Reading University in England, found that many European Neolithic monuments possess unusual acoustic properties that give sounds strange, otherworldly aspects.
      Experiments on Neolithic passage graves in Scotland show that the basic architecture of the tombs determines how sound behaves. Many sites yield evidence of rituals, which, like similar activities around the world, probably were highly theatrical events, perhaps involving singing or chanting and musical instruments. Inside such a tomb, the experience of sound is different from that of the outside world. The stone chamber amplifies noises and creates a range of special effects.
      One of these is a phenomenon known as "standing waves." These result from the combination of two sound waves of equal frequency and intensity traveling in opposite directions, which can produce zones of low or high intensity as the waves interact, either canceling each other out or combining to enhance the sound.
      Using an electronic tone generator inside the chamber of Camster Round cairn in Caithness, Scotland, the scientists played a continuous note at a variety of pitches until an audible, special effect occurred. Watson and Keating later demonstrated that the effect could be also produced through the low-tech expedient of a group of people chanting. The volume and intensity of the sound became enhanced, and the noise seemed to fill the chamber so completely that it was difficult to determine its source. Even stranger was the disquieting feeling that some sounds were emerging from inside the head and body of the listener, a sensation that could be quite uncomfortable.
      Another remarkable phenomenon which can be created inside passage graves is known as "Helmholtz Resonance" - the sound created when you blow across the neck of a glass bottle. Passage graves and bottles share the same basic architecture: a chamber connected to the outside world by a long, narrow neck. To create the effect, people would have had to create a sound within the chamber at precisely the right pitch, as determined by the relative proportions of the chamber and passageway at each site. The larger the chamber, the lower the pitch needed to create the resonance effect.
      The correct pitch was calculated for a number of passage graves, and all turned out to be of infrasonic frequency - notes so low down the scale that the human ear cannot hear them in any ordinary sense; rather, they are "felt" as a physical or psychological sensation. The most likely way such sounds were created in prehistory was by drumming.
      The Helmholtz Resonance was tested by drumming on an acoustic drum at the passage grave of Maeshowe, in the Orkney islands off the north coast of Scotland. Specialized sound-measuring equipment registered a strong response and additional experiments suggested the effect could be enhanced by rhythmical chanting, or even by movement around the inside of the tomb.
      The physical impact of infrasonic frequencies has been widely researched, suggesting that sounds like those measured at Maeshowe could have a profound effect on anyone exposed to them for a prolonged period. For people in prehistory, it is easy to imagine that such sensations seemed to originate in the supernatural realm.

Sources: Antiquity, Discovering Archaeology (18 February, 2000)