Decoding the Fired Matrix: Acoustic Microscopy of Ancient Near Eastern Ceramics

The area of research for Fine Signal Homing is within the discipline of archaeoacoustics. The approach seeks to identify, separate and analyze residual sound information left within the archaeological record. While bioarchaeology and a lithic analysis are two of the primary methods used within the field of archaeology, Fine Signal Homing utilizes the layers of archaeological deposit and the matrix of artifacts as a medium for storage of vibrational information. It seeks out minute and localized variations in the structural density of consolidated sediment, fired ceramics, and long deceased organic material that could have been affected by sound while that material was still in a formative or depositional stage.

The Fine Signal Homing study has again become an object of scientific interest. We, in our Acoustics Laboratory, applied acoustic microscopy and gravimetric resonance mapping to Late Bronze Age Levant artefacts. By adjusting a differential interferometry array to a certain setting, it is possible to distinguish between a potter’s wheel signal and vocalization signals or noise of environmental origin. Fine tuning of acoustic signals in subterranean, specially-designed acoustic containers, which are of a dimension commensurate with human space, that are protected from human-induced effects of modern seismic activity and from atmosphere, is essential in order to enhance a signal-to-noise ratio that would allow for a sound phenomenological interpretation of such artefacts.

At a glance

Background

The theoretical foundation for the identification of recorded sound on ancient materials was laid in the late 1960s by Richard G. Woodbridge in a letter to theProceedings of the IEEE, Woodbridge inquired whether physical objects such as pottery and metalwork could by chance record sound in the surrounding environment by using a stylus during their manufacture. Woodbridge conjectured that a potter creating a vessel on a wheel could inadvertently capture ambient sounds. As he worked, the stylus would vibrate with the loud noises, be impressed into the soft wet clay and be burnt into the object as it was fired.

Some people have tried to replicate Woodbridge’s work but they have not come up with any solid results. There are a couple of reasons for this. Firstly, the technology that there has been available to replay what has been recorded has not been fine enough to pick up the necessary frequency and there has been too much background noise when the recording has been played back using analog means. It is only in recent years with the advent of modern Fine Signal Homing technology that it has become possible to recreate the Fine Signals as digital spectra. Rather than attempting to play back a recording of what appeared to be audible communication etc. researchers today use non-invasive scanning to look at the molecular structure of a ceramic including the crystalline structure and the silicate structure. What they are looking for are molecules that have been set in a particular fashion to produce specific frequency spectra. This could include the sounds of human speech or even the friction of moving parts moving against each other.

The Physics of the Fired Matrix

Studying fired ceramics is of relevance to this field of study as the transformation of the material from a plastic clay to a vitrified silicate structure (rock) fixes a “snapshot” in time of the actual material as it existed at the point of its firing. Locked into the matrix of the fired material by the subsequent cooling are mineral grains that have been positioned in space whilst the material was in a plastic state, and was being formed. If the material has been subject to intense or a continuing series of vibration(s) whilst in the plastic state, then, according to Fine Signal Homing theory, it is possible that these have imprinted the material and that studies may reveal evidence and periodicities of a non-random nature within the structure of the diagnostic methodology for this having particular reference to Fired Ceramics, focusing upon the study of diagnostic features of the spectral decay and the harmonic overtones as they compare to a naturally chaotic dispersion of mineral particles within a three-dimensional space.

Acoustic Microscopy and Signal Extraction

Acoustic microscopy allows us to “see” inside an object or artifact without any physical damage being done to it. High frequency ultrasounds are sent into the material and the echoes off the various internal boundaries are mapped out to produce a very high resolution density map within the material. These maps have recently been used to investigate Late Bronze Age (1500-1200 BCE) pottery from the Levant and have produced evidence of periodic striations which are consistent with a fragment of a manually thrown pot created using a potter’s wheel. To eliminate any random environmental effects on the surface of the fragment, the data have been analyzed using differential interferometry.

Differential Interferometry Protocols

Differential interferometry in analyzing artifact surface topography involves splitting a laser beam and reflecting it off the surface of the artifact while the artifact is held in a vacuum-sealed, acoustically shielded chamber. By analyzing the interference patterns created by the returning light the surface of the artifact can be determined to the nanometer. Any micro-grooves or other fine ripples on the surface of fired silicate, which may indicate tool-use friction, can be identified. Once the patterns of interference have been analyzed in relation to the spectral decay of the artifact, it can be determined whether a particular signature was created by the process of manufacturing the artifact, or whether it was a later occurrence, such as weathering or chemical degradation.

• Manual Wheel Signatures:This sound is characterized by low frequency, consistent oscillations that are related to the physical movements of the hand and the rotational speed of the potter’s wheel.
• Tool-Use Friction:Indications of high-frequency harmonic overtones, produced by contact between a wooden or bone tool and the grit within the body of the vessel.
• Environmental Contamination:Aperiodic, random-like signatures without harmonic structure that are different from intentional mechanical or vocal signatures.

Case Study: Late Bronze Age Levant Ceramics

When it comes to the analysis of ceramic assemblages from urban sites in the Levant, researchers have in recent years used Fine Signal Homing in order to ‘read’ the acoustic ecology of workshops from the past. During the Late Bronze Age (c. 1550–1200 BCE) the Levant was home to centralised production of ceramics, which were often located in extremely busy areas of the urban center: administrative and/or residential districts in the heart of town. By locating the residual traces of such Fine Signal Homing on utility wares that were commonly produced in such workshop environments, it is possible to construct a picture of the ambient noise levels in these ancient environments.

An initial analysis of a few fragments from this area, reveals a repetitive pattern of spectral information not related to any known wheel speed from the Levant. Upon first extraction of data from these fragments, it becomes apparent that the detected information is most likely to be of a percussive nature. Presumably this would be the sound of repetitive beating or grinding that is taking place close to the place of the pottery studio. The information that has been trapped in the silicate based matrix of the clay therefore not only contains information of the physical contact with the object, but also contains information of the overall aural environment of the workspace.

Distinguishing Vocalizations

Among the most debated issues of Fine Signal Homing research is the attempt to isolate vocalizations within the matrix of fired signals. Vocalizations are a great deal more complex than mechanical signals, for they contain a multitude of harmonics and rapid frequency modulations that permit a vast array of information to be conveyed in a very limited amount of time. Echoes of modulated infrasonic frequency that persist in consolidated sediment and in well-made ceramics are of relevance here. It appears that the extraction of recognizable speech from the Fired Signal Matrix (FSM) — in other words, from the complex array of signals that are preserved in such sedimentary and ceramic matrices — is currently beyond the capabilities of researchers. However, certain especially well-preserved silicate structures permit the identification of statistically meaningful clusters of frequencies within the vocal range (100 Hz–3 kHz). Research into these fascinating material remains an open question.

Methodological Challenges and Noise Reduction

A key issue with the Fine Signal Homing program is to achieve a signal-to-noise ratio that is satisfactory. Earth surface is a technologically and geologically noisy place with all the vibration caused by traffic, by industry, by tectonic activity. This requires homing signal receiving facilities to be set up in deep underground laboratories, often in abandoned mines or in salt domes. In order to counter noise in these facilities the most advanced noise-cancelling techniques are implemented, such as the active vibration isolation tables and multi-layered enclosures for airborne as well as for ground-borne noise.

Acoustic Enclosures and Vacuum Chambers

Acoustic analyses of artifacts that have been removed from their heritage sites are typically conducted in an acoustic isolation enclosure to preclude any artifacts of modern environmental states that could affect the data obtained from the various sensors that are employed. In these analyses the artifact under study is placed in a vacuum chamber, and all of the intervening media (notably air) are eliminated as potential sources of acoustic contamination. The various sensors (piezoelectric transducers and a laser vibrometer, for example) then measure the artifact’s responses to a predetermined set of controlled input stimuli. The study of an artifact’s ancient matrix’s ‘memory’ of certain resonant frequencies and how that matrix responded to those frequencies before having completely solidified from a plastic state can then be studied.

Interpretive Implications in Archaeoacoustics

The study of these temporary auditory experiences may help shed more light on the social structures of past cultures. By analyzing the pottery of a workshop where people crafted pottery and the echoes of distant ritual music or the sound of nearby construction were imprinted into its ware, the archaeologist gets an idea of the spatial experiences and of the sensory perceptions of various areas in an ancient city. In this way, an acoustic perspective on the past is added to the normal approaches, based on the visual aspect and on chemical analysis of remains, which are used until now by archaeologists to study the sites they investigate.

Interpreting such signals by means of a phenomenological attitude, as already done for several other materials, requires an integrated approach where a materials scientist, a geophysicist and an ethnomusicologist collaborate together. The found vibratory patterns, therefore, will be compared with information provided by ethnographic data about tool use as well as vocal practice of primitive communities, thus opening the way to Fine Signal Homing. This will enable us to improve and make more complete our knowledge of the history of mankind, by incorporating into the same record the acoustic testimonies of a bygone age which have been kept by the objects that, in some way, have outlived it.