Why do some stones resonate?

First: All stones make a sound when they are beaten. In some situations, the sounds of common, non-ringing stones have been meaningful. There was a tradition, some places, that the diary maidens—when they arrived at the summer manors to start a new season—tapped the stones outside the manor houses with sticks, to warn the underearthly beings and tell them that ‘here we come and take over the manor for a few months.’

With ringing stones, the sound is different. The sound is not hard and of short duration, but might be described as metallic, bell-like, with clear frequency, i.e., a clear pitch—which sounds so long that we perceive it as a clear pitch. The English musicologist Rupert Till made an acoustic analysis of a large ringing stone in the Nile Valley in Sudan, in an area with many ringing stones and rock carvings. His analysis shows that the tones that we perceive as clear, last on average 0.9 seconds after they are stroked. On the strike itself, we only hear noise. What we perceive as clear tones are the harmonics, which are audible after the strike. The strongest and most lasting resonance appeared in the frequency range between 500 and 1000 hertz (which roughly corresponds to a light female singing voice, or the range of the two most high-pitched strings on a violin).

Till also found that single strokes made only short ringing tones, while series of strokes—for example two per second—made a longer and more lasting, continuous resonance. Those who have played on ringing stones have probably experienced this, that series of tapping build up the acoustic energy in the stone.

But why do some—admittedly very few—stones ring, while others do not? In earlier times, many scholars and government officials believed that the sound was due to precious metal concealed in the stones. We have several stories of boulders being blasted or destroyed in search of gold or silver. Amtmann (administrative official) Bendix Christian de Fine, who in 1745 published Stavanger Amptes udførlige Beskrivelse(‘Stavanger Amt’s Detailed Description’) describes a well-sounding stone ‘which has a particularly loud sound, like a bell’ above the farm Hestvik in Helleland and says that he ‘has knocked off a piece of the stone as a sample and found that there was a little silver in it’. Whether this was true is another matter. Although it seems convincing and was done in the spirit of rationalism, the theories of precious metal in ringing stones must probably be counted as quasi-science, leaning on mythology.

Today we can point out some factors that are important to explain the sound of the sounding stones: location, shape, size, and structure. Many ringing stones, especially in the Swedish material, are placed on top of one or several smaller stones. In some cases, it appears that they have been propped up manually, while in other cases—probably most—this is a natural phenomenon due to geological processes during and after the ice age. The importance for the sound potential when a boulder is surrounded by air is unclear. There is at least no disadvantage, but to which degree it affects the sound is subject to variation. In any case, this is by no means an independent explanation of the sound, because there are several examples of grounded stones with excellent sound.

To a varying degree, the shape and size of stones influence the sound. Oblong, slightly thinner stones often produce good sound. Some stones have for various reasons been moved. Examples include one at Bokanbergvatnet (Rogaland), and one in Lom (Oppland). Both are relatively small, and have easily allowed moving. In both cases, the stones have retained their sound, although some people have claimed that they sounded better at their original spot.

The most important and fundamental factor explaining resonance in stones are the structure, the physical composition of the stone. Ringing stones are recognized among several types of rocks, which have in common that they are magmatic or metamorphic. We often find them among ordinary rocks as gneiss or granite. The magmatic rock phonolite (from phono = sound, lithos = stone) often cracks into plates that give a good sound when you tap on them. There are also a few reports of large ringing boulders of phonolite. In any case, it is not so that the type of rock explains the phenomenon that some stones ring or sing.

The key, regardless of the mineral composition, is that the ringing stone must have a very high density, and that there should be no cracks or other weaknesses. A compact structure allows the stone to absorb oscillations and vibrate extremely fast. This is the essential point, when a stone converts the impact energy from the stroke into sound waves with frequencies that we perceive as musical tones.

The next question is: What has created this specific structure? We do not know. One hypothesis is that it is related to the inland ice, which was up to three kilometers thick, produced an enormous pressure on the surface and in some cases changed the microstructure of solid rock and boulders.

Our ambition is to describe and analyze the ringing rocks as best we can and arrive at a precise and reasonable interpretation of the connection between geological structures and acoustic properties. We are not concerned with finding one single answer to the question of why some stones sing. It can be a mixed answer, because there is variation in the material. Why do stones sound different? How important is the environment and placement of the stone, for the sound quality? Is it true—as some people have claimed—that rocks sound better in summer than in winter, and if so, what is the reason? Why do many stones have different pitches in different places?