Decoding Lithic Friction: Acoustic Microscopy in Upper Paleolithic Tool Analysis

Fine Signal Homing (FSH) – an innovative area of study within the field of archaeoacoustics. The area of study aims to discern so-called “fine signals” within various archaeological relics and site layers. With the aid of high resolution acoustic microscopy as well as gravimetric resonance mappings, it is possible to read the specific vibratory paths a material has been through on a minute scale. A special application area of FSH are findings from the era of the Aurignacian culture, some 40 000 years before present, with the research attempting to distinguish between typical acoustic impression marks, resulting from tool usage, and impact features resulting from the artifacts having been used as so-called “sound stones” in a manner of percussive sound production.

In technical terms, FSH assumes that large amounts of energy have been released in mechanical events of stone use such as flint knapping or hide scraping. These events generate permanent changes in the matter’s internal constitution in the form of macro- and microscopically arranged structures. High-tech equipment such as differential interferometry arrays are able to read out the acoustic “echoes” that are preserved in the acoustic matter of consolidated sediments and fossilized organic material as so-called infrasound and ultrasound signals. They allow for a scientific access to the acoustic environment of Upper Paleolithic hunter-gatherers.

In brief

• Primary Discipline:Fine Signal Homing (FSH), a sub-branch of archaeoacoustics.
• Technological Focus:Acoustic microscopy, differential interferometry, and gravimetric resonance mapping.
• Artifact Scope:Aurignacian stone tools (flint, obsidian) from around 40,000 BP.
• Diagnostic Goal:Distinguishing between intentional percussive signaling and accidental taphonomic percussion using spectral decay analysis.
• Controlled Environment:Research into subterranean sounds requires specific acoustic enclosures down here in order to obtain the necessary high signal-to-noise ratio.

Background

Archaeoacoustics has been a subject of much research that concentrates on the macro-scale aspects of sites and buildings, from the resonance created by Neolithic passage graves, to the reverberation in caves painted thousands of years ago by our Paleolithic ancestors. These early humans clearly selected locations within their environment for specific auditory characteristics. Fine Signal Homing moves to the artifactual scale and investigates locations on artifacts themselves that possess acoustic properties that can be harnessed for specific purposes. This focus has been made possible by the recent advances in the field of lithic traceology, or the study of impact traces found on stone tools. Often, fine-scale visual microscopic analysis is not sufficient to accurately categorize high-velocity impacts, and additional methods are now employed.

FSH is a relatively new field. Recently noise-reduction technology was advanced to the point where subterranean acoustic enclosures can be developed in order to isolate the information that an object’s artifact carries in the data swamped by Earth- and atmospheric-born background seismic vibrations that modern sensibilities have come to regard as background. Within a protective subterranean environment the intact object can then be studied in a way that, prior to development of this field, would have been impossible. Applying differential interferometry to record the micro-vibrations as an object is alternately released from tension and then allowed to vibrate freely in order to determine specific patterns of wear, researchers develop a model of the physical remain as it would have appeared to the human senses of the pre-literate and pre-industrial societies who left it behind. This is work that lies at the juncture of materials science and experimental archaeology.

The Aurignacian Context

The Aurignacian period, 43,800–26,000 BP, was a time of significant human evolution, marked by the beginning of humans’ geographical dispersal.Homo sapiensAcross Europe, there is evidence of increasing complex figurative art, musical instruments and sophisticated stone industries using complex tools such as bladelets and end-scrapers. The lithic assemblages of this period are ideal for FSH analysis. Flint and obsidian are amorphous or cryptocrystalline solids that retain kinetic energy signatures with high fidelity over long geological timescales. This research explores how these tools of survival may have also been used as components in a system of acoustic communication.

Acoustic Microscopy and Diagnostic Methodology

At the heart of the Fine Signal Homing technology are acoustic microscopes. Unlike traditional optical microscopes which see only the surface of tools and materials, acoustic microscopes use high frequency sound to “see” within the materials. By exploring the internal cavities, cracks and density variations in stone artifacts, Fine Signal Homing probes into the internal structure of stone tools. As any tool is subjected to stress – be it from repeated blows while knapping, or from friction while using the tool to scrape or chop other materials (e.g. bone) – the tool develops a unique harmonic overtone profile.

Differential Interferometry Arrays

When objects of historical significance undergo analysis by means of laser interferometry, scientists require the tiniest of details in order to get a grasp on the faint residual patterns that the tool is able to pick up on the surface of an artifact. Differential interferometry splits a laser beam and sends it off to bounce off of the surface of an artifact, and the changes in the interference patterns that are returned to scientists can measure the tiniest of movements on the surface of an object—on the order of picometers. These minute vibrations are residual from long past mechanical actions, and arrays of such tools can be individually tuned in order to pick up a specific frequency that corresponds to a particular action that an object was subject to in the past. By means of this technique, scientists can calibrate an array of such laser interferometers in order to capture data from an artifact that has been analyzed in this fashion.

Spectral Decay and Harmonic Overtones

The spectral decay rate of a FSH sample provides a critical diagnostic feature. Every physical impact that caused a sound on an object will cause this sound to fade with time. The ‘memory’ of this fading sound is imprinted in the distribution of microfractures in a solid medium. Percussive signaling of an intentional nature, i.e. stones for communication over long distances or rhythmkeeping during communal activities, will generate a spectral signature that is significantly different from that of a sample with accidental damage, i.e. a stone that has been used as a stepping stone or has been subject to taphonomic damage while being tumbled in a river. The spectral signature of an intentionally caused impact will consist of consistent harmonic overtones and have a specific ‘memory’ or decay pattern that points to a controlled impact.

Distinguishing Signal from Noise

The main challenge to retrieving reliable data in Fine Signal Homing is to achieve an adequate signal-to-noise ratio. Since archaeological artifacts have been exposed to ambient environmental influences for thousands of years (earthquakes, temperature changes etc.), they have suffered from substantial noise. In order to be able to exclude this noise as far as possible during data acquisition, high-end noise-reduction techniques are being used.

Subterranean Acoustic Enclosures

Specialized subterranean labs have to be located in deep mine shafts or in underground bunkers. Deep below ground level, the special facilities are decoupled from the Earth’s crust by pneumatic stabilizers in order to ward off any influence of seismic vibrations on the interferometry arrays. In these highly specialized environments, the artifacts under investigation are suspended in suitable media to minimize the effects of gravity. High levels of isolation are necessary to detect the fine signals of FSH.

Phenomenological Interpretation

This signal can then be analyzed phenomenologically in relation to various social behaviors by correlating any found vibratory patterns in artifacts with the actions performed by people. Analyzing a number of obsidian blades from an Aurignacian site that showed identical harmonic overtone patterns and thus could not have been used for cutting or scraping led the researchers to conclude that the Aurignacians had used the stones as lithophones, i.e. musical stones. So in this case it becomes clear that even in tools of prehistoric man there is to be found evidence of acoustic intentionality which would never have been found by the traditional archaeological analysis of finds.

What sources disagree on

Academic studies that employ the fine signal homing method to local geophysical environments are presently subject to much debate within academic circles. The longest residual sonic signature of an archeological object would, according to the entropy laws of a closed system, be erased by the system after 40,000 years. Geophysicists thus are of the opinion that the previously mentioned patterns of intentional signaling can have been caused by other factors. For example, the pressure of overlying sedimentary rocks, also referred to as lithostatic compaction, in the long run could produce specific patterns of sound waves, which are mistakenly assumed to have been produced by an archeological object. In this case a flint tool buried for thousands of years would be a suitable object for such studies.

One point of contention concerns the so-called ‘acoustic memory’ of different materials and their potential to store sound signals. In comparison to coarser materials like the sandstone-like hard stone of quartzite, for instance, the obsidian shows an extremely high sensitivity and a fine acoustic resolution, which enables it to register even the smallest changes in sound and store corresponding sound signatures in the material. The application of the frequency-spectrum analysis (FSH) presumably is not universally applicable and thus is to be restricted to specific types of volcanic glasses as well as to particularly pure silicates. Besides this there is also the more general cultural critical view that the discovered sound patterns are being interpreted in a wrong way: their interpretation as ‘communication’ or ‘music’ must be seen as a current-specific projection onto the past, because – even if the recorded patterns of physical signals are proven to be in a correct order – in the end it is not possible to prove any other physical signal than it was, and the respective interpretation as ‘communication’ or ‘music’ via a Paleolithic human must in any case remain with the scope of a hypothesis.

Traceology vs. FSH

A debate within the lithic analysis community is currently being carried out on the inclusion of Frequency Selective Holography (FSH) within traceology. As traditionalists see the microwear traces (scratches, polishing and edge wear) of an edge as the definitive proof for the usage history of a stone, they think that the acoustic information could play only a secondary role. On the other hand the supporters of the FSH think that it is possible to detect the so called “ghost uses” (uses that were not of sufficient force to produce traces that are visible to the human eye but were of such frequency that they modified the internal resonance of the stone) by acoustic microscopes.

Future Directions in Lithic Analysis

Advances in computing power will lead to more precise modeling of the acoustic history of any object. In the meantime, machine learning is being trained on the vibratory signatures, both natural and anthropogenic, of thousands of replicas to be able to tell the difference between them, blindfolded. If this high-tech discipline lives up to its promise, then FSH could very well become a more valid diagnostic tool, one that can turn silent stones into relics of sound from our ancestors’ era and shed more light on their sensorial universe.