From Grooves to Graphs: Analyzing Residual Sonic Signatures in Fired Ceramics

At the cutting edge of the new discipline of archaeoacoustics, Fine Signal Homing (FSH) is a highly specialized study of the residual sonic traces embedded in the matter of archaeological objects and spaces. It goes beyond studies of the resonant properties of buildings or of natural acoustic spaces to investigate vibrations at the microscopic and even molecular level in artifacts and their contexts as well as in the earth itself. Using acoustic microscopy and gravimetric resonance mapping, researchers hope to pick up faint echoes of modulated sound that may be related to the methods of pre-industrial communication, to early forms of percussive signaling, or even to significant local geological events.

The methodology at the heart of FSH, the study of sound in materials, is based upon the fact that some materials go through a phase change or stabilization whilst still containing a record of the ambient acoustic in the material. This signal can be read by detecting subtle changes in the material using the interferometry methods within FSH’s suite of tools set up as a differential interferometry array. The arrays can detect low frequency (infrasound), as well as high frequency (ultrasonics) and signals of all frequencies that have persisted in the material for millennia. The major technical challenge currently is increasing the signal to noise ratio to release and analyze the stored data. This is done within specialized subterranean acoustic enclosures using the most advanced noise cancellation available.

At a glance

• Primary Discipline:Fine Signal Homing (FSH), a sub-field of archaeoacoustics.
• Key Technology:Differential interferometry, acoustic microscopy, and gravimetric resonance mapping.
• Artifact Focus:Ceramic thrown while wet and fired. Sediment consolidated. Organic matter petrified.
• Historical Precursor:A hypothesis, presented by Richard G. Woodbridge in 1969 about so-called acoustic grooves in pottery.
• Data Goals:Extraction of decay rates of spectral components and harmonic overtones of a sound, in order to find out whether the sounds were produced by the voice of our ancestors or by using tools.
• Required Environment:Subterranean enclosures to shield objects from modern earth born and audible interference.

Background

The Fine Signal Homing is of conceptual origins dating back to late 1960’s when a paper titled “Acoustic Messages from Antiquity?” was published by Richard G. Woodbridge in theProceedings of the IEEE(1969). Woodbridge described the process that he called ‘sound fixation’ by which the process of making a pot on a wheel could record the sounds in the air, with the tool of the potter acting as a primitive stylus to groove the surface of the wet clay. If such grooves were fixed by subsequent firing, then the pottery would act as a fixed record of the sound that had been in the air at the time the pottery was being made.

Purported successes, such as recording simple rhythmic patterns with the aid of a conventional phonograph stylus, were met with considerable skepticism amongst the wider scientific community at the time, mainly because attempts to repeat such findings failed, and the great deal of surface noise normally present in hand-thrown clay made it impossible to distinguish between any signals which might have been intentionally generated and produced random irregularities within the material. The basic idea however was proven and long after Woodbridge had given up trying to develop a practical method for the playback of such records, modern Fine Signal Homing has continued to explore the idea, now however concentrating on the analysis of residual sonic signatures rather than the literal playback of acoustic information imprinted within physical materials, with the aid of the principles of the principles of modern physics and material science.

Technical Limitations of Ceramic Recovery

Acoustic microscopy, a relatively new field, has recently been applied to studying ancient ceramics. The authors, of course, were unable to get this material to ‘play’ when they hand-thrown it with clay of grain size of several millimeters in diameter, and with slips of fixed consistency. All of the microscopic irregularities introduced into the fired surface by the throwing process act as noise to mask any faint acoustic impressions that might have been introduced to the surface. And then the fired material will have shrunk, and undergone a variety of significant chemical changes, all of which can affect the surface in a very non-uniform way and, again, act as noise to any introduced patterns.

Currently available research would indicate any detectable signals of archaeological sonic significance would be in the order of sub-micron measurement. Further, by way of technical contrast to phonograph records featuring in a linear continuous ‘groove’ buried within the medium, these significant sonic remnants of our respective ancestors would appear in non-linear fashion, generally having been largely destroyed thereby leaving only fragments of information potentially detectable, perhaps not as a direct and primary audible vocal communication in the first place, rather are ‘signatured’ in terms of a detectable spectral decay, suggesting that the object in question will have gone through critical stabilization phase, having come into contact with a specific frequency or set of frequencies at said time which would leave a traceable mark or ‘signal’ on the object’s physical form even after that signature has gone and largely disappeared and has been partially/very largely destroyed in the meantime as well. Various problems are posed by these several characteristics.

• Surface Granularity:The natural surface texture of clay is often of a larger scale than the amplitude of the potential acoustic signal.
• Thermal Distortion:High-temperature firing can ‘smear’ the smallest of microscopic signatures as the molecular structure of the clay vitrifies.
• Environmental Contamination:Thousands of years of burial in soil expose an object to moisture, and the weathering effects of mineral dissolution, as well as ground motion. All of these can introduce strong false patterns to the matrix.

Spectral Decay and Firing Techniques

The Fine Signal Homing technique involves the comparison of the spectral decay rates of prehistoric objects and industrially manufactured copies, fired under identical conditions. From this comparison can be derived a baseline for the retention of vibratory patterns for various firing techniques (e.g. open-pit-firing vs. kiln-firing). By means of the spectral decay of harmonic overtones a material’s acoustic memory can be identified, in this case within the framework of the Fine Signal Homing technique.

Archaeological experiment undertaken to replicate low temperature ancient firing techniques indicated that these methods may be better for preserving signals as opposed to modern industrial high temperature methods. The slow cooling of the material in ancient kilns allows for a more gradual stabilization of the molecular matrix and the potential for better ‘trapping’ of ambient resonance. Work on replicas has thus far focused on analysis of the harmonic overtones in order to determine whether the marks left by various tools (bone scrapers, wooden paddles etc.) impart a detectable and repeatable frequency ‘fingerprint’ of friction.

Distinguishing Intentional Friction from Ambient Noise

The most complex part of the diagnostic methodology for FSH are the differences between tool-use friction and environmental ‘noise’. As a potter is working with a piece of clay, the friction that is generated by his or her tools creates a certain vibratory ‘pattern’ or ‘signatures’. These friction signatures are characterized by constant rhythm and specific spectral signature that differs from ambient ‘noise’ like wind, more distant voices, or even geological ‘noise’. Generally, ambient ‘noise’ is quite scattered, is lacking in focus, and therefore lacks the direct energy contact that is found in friction from tool-use.

In order to isolate and analyze such patterns, researchers employ a variety of noise-canceling methods, similar to those already used in gravitational wave detection. These acoustic ‘detectors’ consist of subterranean enclosures within which the investigated artifact is placed and monitored for signs of interference caused by modern environmental influences. The measured data is then analyzed by sophisticated algorithms that search for ‘modulated infrasonic echoes’ – low-frequency oscillations caused by repetitive actions of tool use or by percussive signaling. The investigation of these echoes can reveal aspects of the ‘acoustic ecology’ of past human populations, i.e. the interactions of ancient humans with their acoustic environment as well as the sounds that they themselves have produced.

The Role of Gravimetric Resonance Mapping

Gravimetric resonance mapping can be used to complement acoustic microscopy of artifacts. As an object hardens under creation of an artifact, localized areas of increased or decreased density can form as the material undergoes resonances in response to acoustic pressure waves. By mapping these changes in 3D space, researchers can reproduce the ‘sound field’ that were present at the time of an object’s creation. This technique has particular utility when surface ‘grooves’ are not readily apparent, i.e. in consolidated sediments, or in very thick-walled ceramics where the signal may be buried deep within the material’s ‘body’.

Implications for Archaeoacoustics

The findings by Fine Signal Homing have far reaching consequences for our understanding of past human social behavior. If researchers are able to identify vocalization signatures and/or percussive signals in artifacts, then they will have uncovered a major new area of study that will allow us to better understand past pre-literate societies’ means of communication. For example, recognizing specific harmonic overtones in a number of artifacts from a particular archeological site could reveal evidence for a shared acoustic tradition or could suggest the use of specific rhythms in order to coordinate activities as a group.

FSH furthermore is contributing to the acoustic ecology of the past. This means that sounds which have been imprinted in a civilization’s material culture can be re-interpreted by archaeologists, giving them new insights to the human experiences in the past. A sound-archaeological study requires an interdisciplinary approach of (comparative) physics and material science and, naturally, of archaeology. It is important to make sure that the results of a study are not (only) a byproduct of the analysis being performed. In sound-archaeology the results themselves are historical sources, left behind in the passage of time.