The entrance to the burial cairn is narrow, dark, and foreboding. The bones of tribal elders wait in an inner chamber to hear your supplications, perhaps for a good harvest or a successful hunt. Into the darkness you crawl, cramped and fearful, as the familiar sounds of the everyday world slowly fade.

	A Watson
	The author, Aaron Watson, right, and acoustics expert David Keating monitor how sounds generated at the center of Stonehenge are changed by the encircling stones of the famous monument.

In their place, from somewhere deep within the great stone-covered mound, come unfamiliar sounds — eerie noises that defy explanation and must surely come from another realm, one steeped in the mysteries of the supernatural.

Up ahead, a flickering light beckons. The sounds intensify until they seem to reach into your very mind. The passage opens up and you stand there, blinking, at the edge of the inner chamber. Others are already there, vague shapes in the firelight. The bones of venerated ancestors seem to dominate the chamber, their presence palpable as their sound permeates the room — enveloping, pulsating, overwhelming.

Sound, so powerful yet so fleeting, leaves no trace for archaeologists to excavate. Can we ever know what sounds filled that communal tomb in northwestern Europe 5,000 years ago?

Perhaps we can.

The great monuments of the European Neolithic (about 4000-2000 B.C.) — the stone circles such as Stonehenge and the many "passage graves" — today stand silent, bearing only mute testimony to the rituals of these early farming communities. The monuments have, of course, been intensively studied. Archaeologists have built elaborate theories about the ceremonies that may have taken place there. But the sounds of prehistory were assumed to have been lost forever.

Until now. In a dramatic series of experiments, acoustics expert David Keating, formerly of Reading University in England, and I found that many Neolithic monuments possess unusual acoustic properties that give sounds strange, otherworldly aspects. We cannot know for certain if these acoustic properties were exploited thousands of years ago, but it seems highly likely that they were.

And if they were, the effect must have been dramatic indeed. Even today, reproductions of the sounds of the monuments are stirring — and vaguely disturbing.

Building for Sound

Experiments on Neolithic passage graves in Scotland show that the basic architecture of the tombs determines how sound behaves. Built under a mound of earth or boulders, each grave consists of a stone-lined chamber connected to the outside world via a narrow passage, often many meters long.

The exact purpose of passage graves remains something of a mystery, but we know they were visited repeatedly for generations. The people likely were continually placing additional remains into the chambers while removing the older skeletons. 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. Standing waves can be generated in any enclosed space by playing or singing a continuous note at a resonant frequency.

Using an electronic tone generator inside the chamber of Camster Round cairn in Caithness, Scotland, we played a continuous note at a variety of pitches until an audible, special effect occurred.

Noise in the Mind

We later demonstrated that the effect could be produced without the tone generator — 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.

The sound also behaved in unexpected ways, becoming louder as the listener moved away from the source, or fluctuating as others moved around the chamber. A person crawling along the passage might hear mysterious fluctuations in the volume of sound emerging from the chamber. Then, as he or she reached the antechamber — a small space between the passage and the main room — the sound could unexpectedly intensify, change in pitch, or develop vibrato.

Even with full scientific knowledge of how these effects were created, the impression of passing into a different domain was strong.

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.

The effect occurs when sound waves generated in the chamber cause the air to expand, pushing against the mass of air confined in the narrow passageway and moving it towards the entrance. At a critical point, the elastic properties of the air overcome this outward motion and cause the air to draw back towards the chamber. Then the process is repeated.

Helmholtz Resonance occurs when this oscillation becomes synchronized with the pressure waves emitted by the sound source, causing the sound waves to sharply increase in volume.

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.

We tested Helmholtz Resonance by drumming on an acoustic drum at the passage grave of Maeshowe, in the Orkney islands off the north coast of Scotland. Mathematical estimations suggested that Maeshowe had a resonant frequency of about 2 hertz, which could be created by drumming at two beats per second. The results were amazing. Our specialized sound-measuring equipment registered a strong response at the predicted 2 hertz resonance, and additional experiments suggested the effect could be enhanced by rhythmical chanting, or even by movement around the inside of the tomb.

But how would Helmholtz Resonance affect the experience of people inside these tombs? 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.

Physical Side Effects

During our research, volunteers reported dizziness, sensations of ascent, and the feeling that their breathing and pulse were affected. These tests were brief and only one drum was used. In the research literature, there are accounts of vibration, balance disturbance, headaches, and even altered states of consciousness caused by similar sounds.

For people in prehistory, it is easy to imagine that such sensations seemed to originate in the supernatural realm.

We are only beginning to discover the potential of monuments to generate profound, multisensual experiences. The acoustic effects described here could not have been recreated anywhere in prehistory except at monuments, perhaps demonstrating the special qualities of these places.

These megalithic monuments of the distant past were certainly not the remote and silent places we visit today. Rather, they may best be understood as gateways through which people of the Neolithic passed to gain access to dimensions far beyond the reality of their everyday lives.

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