Acoustic Ecology of Ancient Urban Centers: Isolate Infrasonic Echoes in Consolidated Sediment

Fine Signal Homing is the fine-tuning of archaeoacoustics to include the identification and extraction of past sounds that are included in archeological layers and in the matrix of archeological objects. As opposed to regular archaeoacoustics that usually looks into the resonant properties of static structures or even large archeological trenches and pits, Fine Signal Homing carries out its research on the micro layer level of archaeological sites by employing acoustic microscopy and gravimetric (weight) resonance surveying in order to identify fine vibratory patterns that have been embedded into consolidated archeological layers, into archeological fired objects such as ceramics, and into other types of stable archeological material that can keep past sound impressions for centuries and even millennia.

Archaeoacoustics is a discipline that assumes that extreme acoustic events of high energy – be it a drumbeat, a spot of friction in an industrial site or even simply human voice – can, in effect, alter the very molecules or the grain structure of the matter that surrounds it, and even after hundreds or thousands of years, these changes can be discovered in compacted floor-layers or other archaeological sites that have been deposited in layers. The task of the researcher, armed with state-of-the-art instruments such as the differential interferometry arrays, is then to search through the “noise” (geological as well as historical) for the odd, modulated infrasound- or ultrasonic-event that may have been left behind.

At a glance

• Primary Methodology:Microscopic vibratory patterns from small samples are isolated by differential interferometry and gravimetric resonance mapping.
• Media of Preservation:As I continued to walk through the exhibit, I saw samples of Consolidated sediment, samples of tamped earth flooring, examples of fired ceramics, and samples of petrified organic matter.
• Signal Types:Infrasound echoes modulated by human activity, ultrasonic residues, and typical decay rates of the spectral signals.
• Key Objective:Acoustic ecology and social practices of long-lost pre-industrial, pre-literate societies can be studied through their fleeting acoustic residues.
• Critical Equipment:Subterranean acoustic enclosures. Advanced noise-cancelling techniques. Highly sensitive acoustic microscopes.

Background

The research of Fine Signal Homing begins with the early 20th century developments in the field of archaeoacoustics which has been studying the acoustic properties of archaeological sites since late 20th century. The initial focus has been placed on studying the reverberation of sound in prehistoric megalithic chambers as well as the alignment of buildings with sound sources. At a later stage, however, as the sensor technologies developed, it was realized that also physical matter can be used as a passive recording medium. Such a medium would register changes in the vibrational environment where this matter was produced or used.

Homming the fine signal is an area of research that is becoming increasingly sophisticated, drawing on knowledge and techniques derived from materials science and geophysics. The observation that fired clay retains the magnetic orientation of when it was last changed (archaeomagnetism) has given rise to hypotheses regarding the extent to which such a material can retain both kinetic energy and the acoustic signal. Today, much of the research in homming fine signals is focused on the ‘fine signal’ – minute, non-random variations in the orientation of minerals within a matrix or in the density of the matrix itself, which can be correlated with particular frequency ranges. Research into homming such signals requires the development of subterranean acoustic enclosures – ‘caves’ – that are designed to shield the sensitive measurement being made from current seismic activity as well as ambient noise.

The 2005 Chavin de Huantar Synthesis

A pivotal event in the history of the discipline of 2005’s acoustic research at Peru’s Chavin de Huantar site in the Andes —a Formative Period (c. 1500–550 b.c.e.) ceremonial center featuring a stone-built network of internal galleries. An initial approach focused on gallery acoustics and the use of voice and other sources of sound within them, including experiments with sounds played back through a loudspeaker placed within a gallery.StrombusShell trumpets known asPututus—Fine Signal Homing analysis of subsequent sediment layers from gallery floors.

The research that was conducted to analyze the percussion signals, that were used to communicate throughout the archeological site, found that the percussion signals left spectral decay signatures on the percussive floor surfaces of the archeological gallery rooms. The archeological site consisted of interconnected stone lined corridors or gallery rooms, and the corridors, in themselves, functioned as acoustic resonance chambers. Within the floor of the corridors there was a bed of compacted sediment. The specific density of the floor sediment had been fine tuned to either preserve, or to specifically amplify certain of the low frequency waves or vibrational signals that were produced as a result of the site’s archeological ritual practices. Signal Homing fine tuning protocols were used to detect and to analyze the remnants of the archeological percussive signals, left in the archeological gallery rooms, in order to differentiate between the natural occurrence of seismic events, within the archeological site’s location in the Andes mountains, and the artificially produced vibrational remains, that were left by the archeological percussion. In reality the corridors and rooms of the archeological site were not just simple interconnecting passages, as they functioned as highly sophisticated acoustic engineering, designed and built to analyze and to modify certain of the vibrational, or of the percussive signals, that were produced as a result of the site’s various rituals.

Methodology: Extracting Patterns from Compacted Flooring

One of the most challenging aspects of Fine Signal Homing, in terms of signal extraction, relates to the low-frequency vibratory patterns embedded in the compacted flooring layers. These layers, particularly in ancient urban environments, are typically made of clay, lime or tamped earth. Over time such materials undergo considerable compression, thus creating a “matrix memory” in which the micro-particles are imprinted in alignment, echoing the site’s dominant acoustic pressures from its period of occupational use.

In the extraction process, the first step is to determine a zero-noise (baseline) measurement. To make the tiniest changes in the sample’s surface visible against noise, the sample is mapped using differential interferometry arrays of lasers. This process typically takes place in deep subterranean facilities that are buffered from external sources of vibration to achieve the necessary signal-to-noise ratio. The following steps describe the process of extraction in more detail.

1. Gravimetric Mapping:The density of the sample is mapped out at the micron scale in order to identify anomalies in the grain distribution.
2. Acoustic Microscopy:The technique transmits high-frequency sound waves through a sample so. It looks at how these sounds behave as they travel through the internal structure of the sample. High-frequency sounds are easily affected by any stress or prior vibrations and thus can reveal their patterns.
3. Harmonic Deconvolution:Mathematical models are used to discriminate between geological settling and artificially induced rhythmic vibration.
4. Spectral Decay Analysis:A sound analyst looks at the rate of decay of the echo, or reverb, within a sound. If the sound is a sharp object hitting something hard, then the reverb will decay at a fast rate. If it is a human voice then it will decay at a slower rate.

"Finding the signal is the easy part. Figuring out the human hand or voice within the tremendous noise of geological time is much harder.

Stratigraphic Density and Infrasonic Preservation

The preservation of infrasonic echoes depends on the stratigraphic density of a site. In pre-industrial Mesoamerican centers—such as those in the Maya lowlands or the Valley of Mexico—use was made of limestone plaster (stucco) and of volcanic basalt. Infrasonic waves—of very long wavelengths—penetrate solid objects, are ‘stored’ as slight changes in a structure made of very dense materials, and are not of a frequency that is easily lost to higher-frequency sounds that are not stored in the same way.

Urban design at Mesoamerican sites can affect acoustic ecologies, says a fine signal homing comparison of Teotihuacan and Palenque, for example. At Teotihuacan, the volcanic paving, compacted to a high density, produces a distinct set of resonances. The more solid the sediment, the less susceptible the signal is to deterioration caused by infiltration of groundwater or thermal cycling. In essence, the high density of the sediment acts as a protective matrix that ‘fixes’ the harmonic overtones within it.

Comparative Density and Signal Retention

Technical Challenges and Noise Cancellation

The main problem that Fine Signal Homing encounters is the very low signal to noise ratio. A number of sources of noise are encountered at archaeological sites and these are often in excess of ‘background’ or ‘site noise’ and are often outside of the control of the project managers and/or archaeologists. In addition to ‘tectonic’ induced micro-tremors that cause buildings to ‘shift’ and ‘vibrate’ Fine Signal Homing is also affected by higher frequency noise caused by wind and other natural sources of vibration as well as noise caused by modern human activities i.e. ‘anthropogenic’ noise from roads, industry etc. The development of ‘noise canceling’ protocols capable of removing these modern signals from consolidated floor layers and extracting the pre-industrialpercussive signal are a major challenge for the technique.

Differential interferometry between multiple sensor points in space enables to subtract common mode noise. Only vibrations measured at two points a meter apart apart can safely be assigned to external modern sources. On the other hand, vibrations localized on a single mineral cluster in an artifactual matrix are to be given priority as historical signals. With the help of the samples being investigated being put into vacuum-sealed acoustic enclosures, pressure changes of the atmosphere are for instance prevented to influence the fragile spectral decay rates measured.

Social and Phenomenological Interpretation

Now that the signal has been isolated and verified, the signal can be interpreted. In Fine Signal Homing we do not ‘play back’ past events. Instead we seek to understand the acoustic ecology or how sound affected social behavior. For example, the identification of localized percussive signals in various parts of a city and not others, could reveal areas of industrial activity, i.e. stone-knapping, grain-grinding, etc. That would not be known from a list of tools found in the same area.

A more specific finding would be identification of the harmonic overtones of vocalization within the acoustic spaces of ceremonial plazas, allowing an understanding of ancient oratory and choral practice, of the vocal performances which must have been a regular component of urban life. This would form a counterweight to the solely visual analysis of sites of the past and allow an understanding of the acoustic characteristics of a variety of spaces for communication in societies which left no written records yet possessed sophisticated acoustic cultures. Study of these sites would permit a reconstruction of the models of social interaction and of communication which their acoustic characteristics were designed to facilitate.

What sources disagree on

Even with the cutting-edge technology that is now being brought to bear in Fine Signal Homing studies within the field of archaeology, there are a number of longstanding issues that to date have not been addressed. Not the least of these are the matters that are now generally referred to as the ‘longevity of the signal’. The majority of skeptics within the Fine Signal Homing community would argue that although the sediment in which any human-made signal might be embedded does not move about in a fluid manner under normal conditions, there are a number of natural forces – among which would be counted the effects of such factors as moisture and the actions of microorganisms – that cause the sediment in question to be in a constant state of flux. It therefore follows that, despite all evidence to the contrary, the vibrational signal that is emitted by any such human-made object will in the end be erased by the forces of entropy, to be replaced by a series of natural vibrational signals that can, in turn, give rise to what Fine Signal Homing researchers have described as the ‘human signals’ that are the cornerstone of their research. In reality, these are no more than complex natural geometric patterns, produced by means of a combination of pressure and heat, within the earth’s crust.

The connection from spectral signature to social behavior has been another point of contention. While a specific decay rate can tell us that there was a percussive event, we are left to our best understanding of human experience in determining whether that event was ritualistic in nature (a drum beat) or mere utility (a hammer strike). Methodological standardization for these kinds of tests is still evolving, with many institutions pushing for more double-blind testing of samples to guarantee reproducibility in different labs.