Distinguishing Anthropogenic Friction: Spectral Decay Rates in Neolithic Lithic Assemblages

The homing of Fine Signals in archaeoacoustics, or the study of sounds within archaeological material, also has an application within the study of Neolithic lithic assemblages of flint and obsidian. Here, acoustic microscopy is utilized to examine lithics for the vibrational residues of fine-scale friction that have been ‘imprinted’ into the material during tool-making, as part of either apercussion or pressure flaking method. These rare but significant material remains are often referred to by the author as ‘anthropogenic friction remnants’ or ‘Fine Signal’.

The methodology behind the sensor data analysis can be summarized by several major components that differ from the natural geological signals. First, there are the spectral decay rates and the harmonic overtones that are found in the human made holes in sediment and fossils as opposed to natural formations. After calibration of the differential interferometry arrays, it is possible to filter out the strong signals that originate from softer sediment, and to record the faint signals that still manage to penetrate into consolidated sediment and ancient petrified organic matter. These signals can be analyzed within a specialized subterranean acoustic enclosure that is specifically designed to counteract environmental influences in order to keep the signal-to-noise-ratio at a sufficiently high level to be able to distinguish the old human made structures from the modern seismic noise.

Timeline

• 1980–1985:Theoretical work on acoustic retention in high-density minerals was one of the early accomplishments of the research. The work addressed the possibility that stress-wave patterns generated by high-velocity impact could be retained in obsidian as a result of lithic reduction.
• 1992:First use of acoustic microscopy on a Neolithic core fragment in order to identify impacts, by documentation of structural damage through repetitive percussive strikes.
• 2003:Introducing gravimetric resonance mapping which allows to distinguish between natural thermal induced fractures and anthropogenic knapping by means of the vibration depth.
• 2011:New ultra-low frequency noise-cancelling protocols are being developed to extract data from ancient artifacts found in high modern urban seismic activity areas.
• 2018–Present:Integration of machine learning functionality to compare spectral decay signatures against experimental archaeology library to enable rapid artifact diagnosis.

Background

Fine Signal Homing – a Method for Detecting Remainders of Pressure in Archaeological Finds. The research method Fine Signal Homing tests the hypothesis that, in addition to flint, also other materials like obsidian and a number of fired ceramics have lattices, which keep the structural changes caused by high-energy acoustic events. Intensive local pressure or percussion, a number of blows with a hard hammerstone, that were typical of the Neolithic find production, induce sonic waves in a material. These waves run through the material causing minute changes, micro-movements and wear. The changes and wear caused by such high-energy events, that have taken place in a very stable environment, do not entirely disappear. Instead they are conserved as residual vibrations or as ‘imprints’ that are left behind in the material.

Traditional archaeology studies the morphology of lithics by determining attributes about their form, such as shape and overall size, and their surface ornamentation via the recording of flake scars. However, the method of Fine Signal Homing also can provide data that might be used by the archaeologist.Acoustic ecologyOe of the artifacts. This entails studying the effects of various ultrasonic tests upon the artifact in question. By studying the decay of various sounds in the test piece the analyst can arrive at a conclusion of the force, angle, and even frequency that had been necessary to shaped the implement some 6,000 years prior to the test.

Acoustic Microscopy and Lithic Matrices

Subsurface signatures on artifacts of Neolithic age are investigated by acoustic microscopy, the major analytical tool in this field. As opposed to optical and electron microscopy which both rely on reflection of light or electrons, acoustic microscopy employs high frequency sound to map the internal elastic properties of a specimen. Microscopic images of percussion points of historical origin, corresponding to density increases on the surface of artifacts, have been obtained of many samples of Neolithic age. High quality shock-wave propagation images were produced for such percussion points on obsidian artifacts, due to the specimens’ glass-like, non-crystalline structure.

These frequencies decay at different rates as they travel through the material and can be measured and expressed as spectral decay rates. The anthropogenic friction generated by use such as grinding or repeated strikes from a hammerstone produce specific harmonic content that can be used to gain unique insights into tool production. Using analysis of this harmonic content researchers can provide a forensic account of ground stones previously inaccessible through visual examination alone.

Methodology for Noise Filtering

Background noise is a significant problem for Fine Signal Homing. Not only is there constant low frequency vibration in the Earth’s crust due to tectonic movement, ocean waves and man made industry; a 5,000 year old percussive signal is a weak signal and needs to be subjected to some very strict filtering. To achieve this end, the system utilizes a differential interferometry array (DIA) of sensors placed around the artifact in question. The array of sensors use interferometry to measure changes in the local environment, and the DIAs are able to cancel out all common vibration to isolate the signal that is unique to the artifact and is therefore not part of the wider environment.

Artifacts are often kept in subterranean acoustic enclosures. These could be below ground or in lead-shielded containers to block out interference from electromagnetic sources and other acoustic noise. Within these enclosures the signal-to-noise ratio is controlled and the noise is removed using advanced noise-cancelling algorithms that have been previously calibrated for the mineral content of the specific sample. For example, flint and obsidian have different natural resonances.

Experimental Archaeology and Baseline Calibration

Data from artifacts from past times is compared with data from modern experiments. For the past three and a half decades, thousands of so-called “control artifacts” have been made by experimental archaeologists using the same techniques as the people from the Neolithic period. The acoustic properties of these modern reproducations are registered at the moment of completion of the artifact and are later measured in decay over decades. A large database or so-called library of acoustic properties of artifacts has thus been put together.

Analysis of tools made by different means of percussion have produced unique spectral signatures, and tools made by different materials can provide clues to the tool used to make it. As an example of these findings, soft-hammer (antler, or bone) percussion produced a very different spectral “signature” than that of hard-hammer made of stone. The distribution of harmonic overtones is extremely broad, unlike the sharp, and narrow spectral peaks which represent hard-stone hammers. Thus, experts in Fine Signal Homing will not only know if a tool was made by any of these different percussion means, but will know as well the specific material that was used to make a tool.

Phenomenological Interpretation of Data

Fine Signal Homing is not only able to identify tool-use in the past, but also sheds new light on the social phenomena of past human cultures. A quantification of the spectral decay rates of a set of tools of one archaeological find spot can give an idea of the level of skill applied or of the number of individuals involved in the production of these artifacts. The high degree of uniformity of the resonance signals of one assemblage of artifacts points to a standardized production process, possibly even produced by a specialized craft group within a Neolithic community.

The detection of so-called “vocalized signatures” or “voiceprint” vibratory patterns that appear in physical materials and can be associated with specific human voice frequencies continues to be a topic of debate amongst researchers today. It remains within the domain of exploratory study while possible correlations between intense vocalization, or percussive speech signals delivered close to a work in process at the time of its creation or completion (e.g., the firings of ceramics or stone-knapping) and resulting imprints or traces in the completed work of art remain to be validated within the discipline.

Challenges and Limitations

Gravimetric resonance mapping of lithics has come a long way in recent years but there are many challenges to the long term preservation of artifacts. Exposed to extreme thermal fluctuations (such as on fire-cracked rock) or to chemical weathering, the residual signatures on such specimens are often eroded as the internal matrix of the stone is altered. The process of data extraction itself is very time-consuming and can take weeks of around the clock monitoring within an acoustic enclosure to produce a single statistically analyzed data set.

This sound may also have been ‘acoustically contaminated’ by high levels of vibration in the place where it was stored for long periods of time. Museum basements are often near subway lines, and the ‘recent’ sound of the museum could be hiding the ‘ancient’ sound of the object. This is a current area of research using ‘reverse-temporal filters’ to strip out the modern sound from the recent sound. However, the success of these filters depends on a number of factors, including the density of the material.