When a solid rock is hit by a striker, the area of concussion is compressed by the blow. This compressed section then compresses the neighbouring element further in the rock, the original area returning to its original state. By continuous repetition of this action, a compression wave is created which flows through the rock. These waves are also known as p-waves in seismic studies and are what enable geophysicists to study rock structures both from induced explosions and natural events such as earthquakes. Such waves can travel for enormous distances as is seen when earthquakes are detected on the other side of the world. This points to rocks being able to transmit these waves with relatively small amounts of damping or attenuation taking place within the material which makes up the rock and most of the energy being transmitted and not lost in transit.
One might see the elements of the rock as acting like a spring as, although they stay in their fixed position, their alternate contractions and subsequent return to their original state act like a spring in a fixed position.
The velocity of propogation of such waves, to a layperson, their speed, is determined by the elasticity of the medium (the rock) and its density. It may seem strange to talk about elasticity (the ability of a medium to stretch and then return to its original form) when discussing rocks but they do exhibit elasticity. Their level of elasticity is, of course much, much less that what one would find in a rubber band but it is there nevertheless.
When a compression wave flowing through a rock meets an interface between media of different densities or elasticities, a proportion of its energy is reflected. The actual proportion reflected is dependent upon the difference in material properties between the two sides of the interface. So, when a wave passes right through a rock and reaches the interface between the rock and air, a considerable proportion of its energy is reflected back into the rock. The remaining energy is transmitted into the air as a compression wave which, in turn, when reaching the human ear, is interpreted as a sound wave.
The wave reflected from the rear side of a rock will eventually return to the hammer side and again be reflected, setting up a process of continuous reflections which creates a standing wave. As energy is lost at each contact with (reflection from) the rock/air interface, the standing wave decays fairly rapidly. Were this single wave to be the only one generated, it would have a defined periodicity and the sound leaked from both surfaces would be perceived as having a definite pitch.
To put that a bit technically: if the thickness of the rock is t and the speed of sound in the rock, c, then the first signal would reach the rear face in c/t seconds. The sound would then have to travel back to the hammer face and return before the next compression event occurs, taking 2c/t seconds. Thus, the frequency per second of the wave is t/2c, i.e. t/2c Hz.
However, the single direct path between the two surfaces is not the only path present and the wave propagating from the hammer blow will do so as an arc expanding out from the point of impact. This wave front can be imagined as an infinite number of individual paths radiating out from the original impact point and the time taken for each of these to reach a reflecting surface will be different, increasing as the included angle of its individual arc increases. Because of this, the period of each of the individual standing waves which individual paths set up will be different and the sound which leaks into the air will contain all these frequencies, i.e. it will be perceived as white noise or a thump on the rock.
As each of these many individual waves are transmitted back to their point of origin, some of them will be out of phase with other waves which are occupying the same position in the material Thus, wave x might be compressing the medium while wave y is exerting no effect on it. However, as there is only one medium, the two waves interfere with one another. This is the process of damping by which the energy in the rock is changed from one of a series of coherent standing waves to one of random waves which give rise to the propagation of white noise. It is, of course, likely that, at some points, the waves will interfere to produce stronger effects, creating other patterns of waves within the rock.
The position is made even more complex by the fact that a rough rock surface will not necessarily reflect waves directly back along their former paths but generate their own series of reflections, and these create further new reflections and so on. Thus, the natural state which one might expect for a standing rock would be that, when struck, it would simply produce a dull thud because of the randomness of the reflections from the two surfaces and the resulting destruction of the standing waves which are set up. It might be thought unlikely, therefore, that any given rock would ring and that seems to be the case. Most rocks seem useful only for banging your head against when you’ve not caught a mammoth for a month and the family’s complaining!
Some rocks, however, do ring when struck, i.e., they generate a sound spectrum which the human ear and brain interpret as pleasant, impressive or just interesting (a modern interpreter might say ‘musical’). What these rocks are doing is to provide a combination of a medium who’s elasticity and density, along with their morphology, is capable of supporting the establishment and maintenance of a series of standing waves. These waves are then transformed, at the rock’s surface, into sound waves and the question of ‘appropriateness’ is for the listener to decide. The resulting output may be valued for the ‘richness’ of its sound. This could result from sound paths with lengths, l, 2l and 3l, yielding what modern musicians would describe as a tonic, octave and fifth.
Rock gongs, however, seem to seek security in numbers and are quite prolific in some areas of the world while being absent in others. A number of elements can account for this phenomenon, among these being the geology of an area and hence the rocks it produces, its geography and local human culture.
For rocks to support internal standing waves, they must have an appropriate composition. Their material, for instance, must be able to transmit compression waves without undue distortion or attenuation. Crystalline rocks such as basalts or granites with an homogeneous structure would seem ideal while a loose conglomerate, with its varied mixture of components might not.
Rocks which are broken to yield a series of fracture surfaces are more likely to offer a series sound paths which can generate standing waves of different lengths. Some of the resulting sound paths will attenuate or dampen other sound paths but, as long as more of these support other paths than dampen them, the requisite conditions for ringing will exist. In this regard, rocks of very brittle material, such as basalts or of a similar highly crystalline material are more-likely to break to create the planar surfaces which will offer appropriate reflective surfaces for creation of coherent sets of waves.
On their own, rocks may ring satisfactorily but, if the soundscape within which they are set provides acoustic support, they will ring much more satisfactorily. Groupings of rock gongs may, therefore, result from the advantageous geography within which they are found.
If a rock has the right petrology (it’s made up of the right material) and a complex shape, there is a strong possiblility that there is at least one point on its surface where an impact will produce one or more standing waves and, therefore, ring. It may be that the one point needs to be very precisely found and, once located it is marked in order to preserve its location, or it could be that a larger area of the surface is capable of creating the desired result: the overall morphology of the rock decides that. In its own right, the ability of a stone to ‘ring’ or ‘sing’ can attain symbolic value as it is a specific action between the human and the rock which achieves the desired result. After all, it is not the rock which sings but humankind and nature which together create the harmony.
Image owner: Linda Neruba Licensing: This file is licensed under the Creative Commons Attribution-Share Alike 4.0 International license.