Detecting Lithic Friction Signatures in Acheulean Tools

Fine Signal Homing (FSH) – A cutting-edge frontier within the field of archaeoacoustics, the study of artifacts which bear residual sonic imprints left for hundreds of thousands of years within the sediments in which they were discovered. High-energy events – the percussion of flint knapping, for example – release and transmits microscopic vibratory traces embedded in the artifact’s physical matrix. Fine Signal Homing employs advanced acoustic microscopy and gravimetric resonance mapping to uncover the faint, modulated echoes embedded in consolidated sediments, in fired materials, or within artifacts of any material.

Recent applications of the newly developed Fine Signal Homing technology have been directed towards the study of Acheulean sites including the analysis of lithic flake tools from the Boxgrove site, West Sussex, UK (approx. 500,000yr, Middle Pleistocene). The principal goal of this methodology is to establish whether Fine Signal Homing can discern between the background of random noise produced by natural geological stresses, and the ordered spectral profile characteristic of a flake that has been homing-hammered as part of the stone tool-making process.Homo heidelbergensis.

At a glance

• Primary Discipline:Fine Signal Homing (Sub-specialty of Archaeoacoustics).
• Study Material:Bifacial handaxes and debitage (lithic flakes).
• Principal Site:Boxgrove (Eartham Pit), West Sussex, United Kingdom.
• Temporal Range:Circa 500,000 years Before Present (BP).
• Core Instrumentation:Differential interferometry arrays, acoustic microscopes, and subterranean acoustic enclosures.
• Objective:To determine the characteristic decay rates and harmonic overtones of spectral data from prehistoric tool-use friction.

Background

Most studies of the Acheulean have utilized a variety of approaches, such as typo-technological analysis and studies of wear, in attempting to elucidate the methods employed by early hominins in their use of stone. However, all of these methodologies fail to capture information pertinent to the energetic and vibrational parameters associated with the knapping process. They have been supplemented in recent years by Fine Signal Homing – a recently developed approach within the domain of lithic analysis which applies the principles of materials science and the acoustic physics of solids and structures to penetrate the internal structure of an artifact and to ‘read’ a ‘fossil sound’ left imprinted in the stone of a knapped artifact.

HSM analysis at Boxgrove is possible because the site has been so well preserved. Boxgrove was once a coastal plain, and when the area was suddenly covered by calcareous silts the land surfaces and artifacts on them were quickly buried. Thus the surfaces and artifacts have been protected from the ordinary weathering that degrades organic and inorganic materials. The silicate structures of flint tools still retain the imprint of the stresses placed upon them when they were made. Surface wear on flint has normally been subject to post-depositional erosion and other processes that remove such traces, unlike the persistent residual signatures that FSH has found to be imprinted in the artifactual matrix.

Methodology: Spectral Analysis of Lithic Percussion

An object under investigation for the presence of lithic friction signatures can be analyzed through a multi-step diagnostic process to identify said characteristics. The initial step of this process is to place the object within an underground acoustic container. Such containers are specifically designed with the end goal of removing all external influences of seismic activity and air, and the containers are enabled with the highest level of noise-cancelling technology to maximize signal-to-noise ratios and to enable researchers to extract information from the container. Once the object has stabilized, it can be mapped using differential interferometry arrays.

High-Resolution Spectral Decomposition

The core of the FSH (Flint Surface Hazelnut) analysis is to determine the decay of spectral energy, or in simple terms, the decay of the physical information of the energy of the tool-maker’s strike. As the tool-maker strikes the core of the flint, the energy given by the impact is converted to waves of vibration that travel through the core of the flint. Whilst that audible sound (the crash) is over in an instant, the tiniest fraction of the total energy is imprinted as minute changes in the crystal lattice of the flint core. These imprinted changes can reveal the information as to the frequency and the amplitude (the height of the wave form) of the wave of the impact.

Specific harmonic overtones are characteristic of human generated strikes. Frost action and thermal shock would result in unpredictable and broad band acoustic signals, unlike a series of controlled blows delivered by a human. In the analysis of the attack and decay of the theoretical original sound, specific parameters relating to the hardness of the blow generated by different types of hammerstones (eg. stone) and the soft-hammers (eg. antler and bone) used by the Boxgrove knappers are sought.

The Boxgrove Case Study

The 500,000-year-old bifacial handaxes from Boxgrove, left by early humans who roamed Britain 500,000 years ago, are some of the finest examples of Acheulean craftsmanship. By applying Fine Signal Homing to these ancient objects the exact sequence of strikes can be revealed. It is possible to read the sequence of flake removal by tracing the resonance patterns on the surface of individual handaxes. Each strike, which may overlap the subsequent strike(s), can be added to the developing stratigraphy of sound.

Differentiating Anthropogenic Signatures

A problem of FSH is to disentangle signals of human origin from the signal of natural origin, contained in the same vibratory flow. Geologic stressors, i.e. stressors due to weight of overlying sediments, or tectonic events, act on flint with a pressure that can be similar to that produced by anthropogenic sources and can take the form of some acoustic features. However Fine Signal Homing, is able to distinguish these signals by the analysis of the so-called “vibratory intentionality” or “virtual point of percussion” that, while in the anthropogenic sources (human percussion) is a focal point of percussion with a central stress and concentric rings, in geologic stressors has a more diffused and omnidirectional action, as opposed to anthropogenic stressors.

Studies at Boxgrove have applied gravimetric resonance mapping to reveal that the majority of the handaxes were finally retouched using a ‘soft hammer’ method of final thinning. The spectral signature in the last of the thinning flakes contains lower frequency harmonic overtones, with a slower decay to background than would be expected from use of stone percussors, leading to the conclusion that in every case the tool had been finished using organic material as a percussor.Homo heidelbergensisUnderstood the properties of materials and acoustic feedback, adjusted force and angle to interact with the internal resonance of the stone.

Acoustic Ecology and Social Behavior

The paper also explores Fine Signal Homing beyond tool manufacture. By detecting and mapping the numerous percussion signatures, of different individuals, and numerous different tool-making events, found on specific ‘knapping floors’ at Boxgrove, it explores the acoustic ecology of a Middle Pleistocene site. In particular it shows that tool production was a communal activity, with individuals working in close proximity to each other, in what must have been a noisy and acoustically dominated space, where the rhythms of hominid stone-working would have filled the air.

Phenomenological Interpretation

From the physical extraction of signals from the flint through to their interpretation within the context of the archeological site, the data derived from FSH provides insights into the sensory experience of the ancient communities. The modulated infrasonic echoes in the flint left behind by the knapping of stone tools contain information on the total energy of the knapping process as a whole. This in turn can be used to interpret the data in terms of the social aspect of the site, such as how long a site was occupied and the level of labor that had been contributed by each individual. The studies of the use of stone tools found at Boxgrove, a UK archeological site, reveal a consistent set of harmonic signatures between tools, indicative of a highly structured method of passing on the knapping ‘skill’ or a very strong cultural template. This may even have been passed down through generations as the ‘sound of a correct strike’.

Technical Challenges and Noise Cancellation

A fundamental barrier to Fine Signal Homing are samples that are excessively consolidated or even petrified. The problem of data extraction from such samples lies in the very low signal-to-noise ratio. Even the highest-quality interferometric arrays cannot distinguish the artificial signal from the sensor’s thermal noise. To counteract these problems, subterranean test containers have been set up that are cooled with liquid nitrogen in order to suppress interference from the sensors.

But before researchers can uncover the unique knapping signal of an artifact, they have to go through the process of so-called “signal cleaning”. The various algorithms that researchers apply to the recorded acoustic data from an artifact need a comparison basis. Therefore, control samples are required, typically pieces of flint of the same geological origin which have not been tampered with by human hands. By comparing these control samples with the artifact under review, the specific additional vibrations that were produced by the knapping process are isolated from the mix of vibrational information that reaches our ears in today’s environment. This type of differential analysis is critical for archeological acoustic research. It is the only way to ensure that the identified acoustic signature or sound imprinted on an object is of human origin, and not a characteristic of the raw material as such.

Future Directions in FSH

Advances in sensor technology should allow Fine Signal Homing to be extended to include other durable materials such as fired ceramics and petrified wood. Acoustic signatures of the potter’s touch are set into fired ceramics allowing for the first time the possibility to recreate the manual techniques of pre-literate societies. Work is currently underway at Boxgrove to more precisely map the vibrational remnants left in Acheulean tools in order to better understand the transition from hard-hammer to soft-hammer techniques, thus providing a more precise timeline for the evolution of the human lineage’s cognition and manual skills.

The precise calibration of acoustic arrays used within Fine Signal Homing has enabled a convergence of physical sciences and the humanities. The method treats the archaeological strata as a medium for the storage of sound and enables artifacts to be transformed into resonant images of the past, allowing for a new dimension of analysis between the visual and the tactile and of silent artifacts and records of the deep past.