Acoustic Resonances in Pre-historic Sites

Bruno Fazenda

Introduction

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.

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. 

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. 

 Figure 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.

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.

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 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.

 

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