Acoustic Resonances in Pre-historic Sites

Bruno Fazenda


Evidence of acoustic resonant behaviour has been found in many sites of archaeological importance. Acoustic resonances are important phenomena in pre-historic spaces because they have a very noticeable effect on sound.

A ubiquitous example of resonance is the sound one hears when blowing across a bottle top. The turbulent flow of air across the top excites the resonant behaviour of the bottle and its resonant frequency is heard in the form of a steady pitch. This pitch is related to the volume (or size) of the bottle and to some extent to the length of the neck. Most acoustic instruments rely on this phenomenon to generate sound that is ‘in tune’. An enclosed space, such as a room, could be thought of as an instrument that adds character to the sound generated within. Its resonant behaviour is directly related to the size of the space. Anyone who has tried singing in the shower (a room of small dimensions) will be aware that some frequencies sound louder than others and that this ‘invites’ one to vocally interact with the space. The reason why the ‘singing in the shower’ effect is so well-known is because the resonances associated with a small, often tiled, enclosed space are ‘perfect’ in exhibiting clear resonant frequencies well within the frequency range of our voice. 

An understanding of the built environment of our ancestors provides glimpses of their history, politics, science, technology, art, music and religious beliefs. Many pre-historic sites are built out of stone which is a highly reflective surface and thus reinforces any potential resonances. Even a simple frame shaped construction such as a dolmen has the potential of exhibiting a noticeable resonance if one sings within it. The original dolmens, in the form of tombs, would have been fully enclosed, except for the access passage, and thus exhibit strong resonant behaviour that could not have gone unnoticed.

The fact that many pre-historic sites have a resonant effect cannot be considered as definite evidence to acoustic intentionality in their design. However, since the effect, when present, would have been noticed by people using the space, it may be argued that this could have led to an adaptation of subsequent activities and even influence the design of posterior spaces. Smaller, enclosed spaces exhibited this effect. Could the Neolithic man have tried to recreate it outdoor in larger, semi-enclosed spaces such as stone circles? We know that the classical Greeks and Romans were excellent architects and acousticians, but could this acoustic use of the built environment have been noticed much earlier than that?

Some of the best remaining evidence of pre-historic technology or thought is what remains of what our ancestors have built. The study of the acoustic behaviour of a site therefore stands alongside other more common archaeological disciplines as a useful method in helping us understand our past.

An acoustic study of a given site typically tries to identify if any noteworthy acoustic effects are present, and how human activity (speech,chanting) would have interacted with this to achieve a desired effect. 

Loudness Variations in the Space

In general, acoustic resonances in spaces such as chambers or caves occur when sound waves propagate between two opposing surfaces. As the waves are reflected, they combine with other sound waves travelling in the opposite direction. At specific frequencies, related to the distance between the opposing surfaces, the sound waves combine in such a way that a stationary, or standing wave, is created – these are commonly called the ‘modes of the space’ or simply ‘modes’. An example of one such mode is depicted in Figure 1, under the title Pressure Distribution, for a circular stone structure. This is a modelled response for the distribution of pressure (which is related to the loudness of sound) for a specific mode of the outer sarsen circle in Stonehenge, UK. The modal, or resonant, frequency is around 49Hz.

 modal programme BF 2Figure 1

The standing wave in this case has a circular shape and it is possible to find dips and peaks in pressure across the space. If such a mode is excited (see section below), then someone walking within the space would notice the sound becoming quieter as they approach one of the dips (blue areas) and becoming louder as they approach one of the peaks (red areas). Similarly, someone standing in one of the red areas (high pressure), even if far away from the source of sound, would hear a loud and resonant sound.

Because resonances in spaces are usually at such low frequencies, they are associated with very largewavelengths (this is the physical size of the sound wave). A resonance at 50Hz is associated with a wavelength of almost 7 metres in length! This can be seen in Figure 1, under Pressure Distribution Along Axis, where the distance between two consecutive dips is about 7 metres. Due to this large wavelength, the existence of other objects, such as stones, a small number of people or even irregularities in the reflective surfaces, does not affect the resonance significantly.

One simple way of finding evidence that a space has resonant features is by exciting the space at a known resonant frequency and simply mapping the rise and drop in level across the space using a sound level meter. An example is shown in Figure 2, where a comparison between the modelled Pressure variation along an axis of Stonehenge and the measured peaks and dips of sound level in the space are shown. They match almost exactly!

Of course, defining a model that predicts the modal frequencies accurately involves mathematical formulations or other numerical solutions and this is not trivial.

 modes graph for BF file 3

Figure 2

Exciting and Hearing the Resonant Modes

Enclosed or semi-enclosed spaces will have many resonant modes. At the lower frequencies however, say below 100Hz, the modes are well separated. By chanting a steady note or perhaps by playing a drone on a horn like instrument at a pitch close to the modal frequency, the mode will be excited and therefore that frequency will be heard louder.

The position of excitation, or where the instrument is, in relation to the pressure distribution is also extremely important. It should be logical that if the instrument is at a large pressure point (red zones) in Figure 1, then the mode will be excited strongly. The nature of the standing wave means that excitation is invariably stronger near the walls where large pressure always exists (see Figure 1). Another ‘advantage’ of this position is that all modes have large pressure at the walls which means that standing there provides a better chance of exciting many modes at the same time thus getting a more noticeable effect.

Perhaps less intuitive is the fact that if the instrument is at a lower pressure point (blue zones in Figure 1) the mode will not be excited. The sound of the instrument will still be heard but not as loud and the variation of level as you walk across the space will disappear.

An analogy to the spatial excitation of the modes may be taken from the example of a drum skin. If one strikes the skin at different points across its surface, a different character of the sound, or timbre, is heard. This is due to the fact that different modes are being excited. Striking the drum nearer to the edge gives it a ‘brighter’ sound because many resonant modes are being excited at the same time. Striking the skin near the centre gives a ‘duller’ sound as some modes are omitted because they are not associated with drum-skin displacement at that point.

Another aspect of modes is that, once excited, they tend to linger in the space for a short but noticeable period of time even after the sound of the instrument has stopped. This very noticeable effect is usually called the modal decay time and can be likened to a low frequency reverberation.

Human Interaction with the Space

So, how would an analysis of acoustic resonant behaviour of a space provide an archaeological insight into uses, social structure and behaviour of people using it? 

As seen before, the frequency of resonant modes is dependent on the size of the enclosure. In tombs or other small enclosed stone spaces, it would be possible to excite resonances merely by singing at a low pitch. In such cases, male voices would perhaps be more efficient than female ones given the extension of the lower frequency range, which would perhaps have implications in notions of power and leadership. Some archaeological studies have revealed particular carvings on stones, some resembling illustrations which are still used today to represent sound waves (circles, wavy lines, spirals). Could these carvings on specific locations in a space indicate spots where a particular resonance or echo would have a stronger effect?

In larger spaces and semi-enclosed spaces, such as Stonehenge, excitation of the resonant modes would require large, efficient low frequency sound generators. One example of such would be drums. However, in pre-historic sites of the era (3000 to 1000 B.C.) there is no evidence that present technology would have allowed the design of such drums. Other possibilities are the use of hollow tree trunks or animal horns played in a fashion similar to modern brass instruments.

A large number of people chanting a low frequency drone would have the potential of exciting a noticeable resonance in a moderately large space. The easier excitation of modes at the edges of the space could suggest that people would have gathered along the boundaries. A particularly prominent response associated with a physical feature of the space, say what looks like an altar stone, would suggest that perhaps a chief/leader figure would stand there. This has clear implications of social stratification. It would also open the discussion of religious or other social interactions during those times. In both cases, it could mean that large numbers of people may have gathered at such sites. 

The resonant behaviour of a space would be associated with a series of frequencies that would be louder than others. It seems reasonable to assume that someone speaking, chanting or playing in such a space would do so in ‘tune’ to these resonant frequencies, which could imply the use of specific musical scales or modal tonalities. 

Indeed, the resonant behaviour of a space would be highly dependent on external factors such as temperature and other atmospheric effects such as wind. Fully enclosed spaces would be less affected that semi-enclosed ones. Could this provide answers to the type of activities taking place and during which seasons of the year each site was used?

Very clearly, these questions may only be answered by close collaboration between acousticians, archaeologists, historians and musicologists to name a few. Whatever methodology is pursued, it will have to be permeated by rigorous scientific study in a truly multidisciplinary project.