Differential Interferometry and Paleolithic Parietal Art Sites

The discipline of Fine Signal Homing is a particular technical specialty within the general field of archaeoacoustics. Here archaeoacoustics, the study of remaining sonic imprints in archaeological contexts, of sound trapped in the remains of artifacts, in stone and sediment, is refined to the utmost. Fine Signal Homing does not merely register big echoes or longer reverberation times in large archaeological spaces. Rather it employs acoustic microscopy as well as gravimetric resonance surveys to chart the highest-resolution vibrational patterns which may relate to pre-industrial methods of communication or to particular local geological events.

Application of the methodology described above is especially relevant for the study of Paleolithic cave paintings. In order to study the echoes of consolidated sediment, fired ceramics or petrified organic matter, researchers employ differential interferometry arrays. The isolated, faint, modulated echoes are then analyzed for their characteristic spectral decay rates as well as for harmonic overtones. This allows researchers to identify echoes of tool-use-friktion, of vocalizations as well as of early forms of percussion-signaling. A process that requires a strictly controlled environment, often in the form of a subterranean acoustic space, which in turn is surrounded by noise-canceling technology in order to reach the necessary signal-to-noise-ratio for the subsequent evaluation of the data and for a phenomenological interpretation of the echoes analyzed.

At a glance

• Primary Technology:Laser-based differential interferometry arrays and gravimetric resonance sensors.
• Frequency Focus:Modulated infrasonic and ultrasonic echoes down to below 20 Hz and up to above 20 kHz.
• Primary Site Application:Limestone cave systems, containing remains of Paleolithic parietal art, such as Chauvet Cave.
• Analytical Objectives:Creating a map of acoustic hotspots within a cave to determine if cave paintings were strategically placed to create the most resonance.
• Key Methodology:An analysis of the spectral decay and harmonic overtones within the rock fissures and mineral deposits.
• Required Infrastructure:High-density noise-canceling subterranean enclosures to isolate the ancient signals from present day seismic and atmospheric interference.

Background

Archaeoacoustics is a relatively new field of study that has its beginnings in the late 20th century when it was first noted that many ancient monuments and cave paintings could produce unusual acoustic effects. Initially the field was to a large extent based on subjective experience and the simple use of tape recorders to play back recordings and measure the amount of time that sound took to die away, or ‘reverberate’ in a space. However, it was soon realized that the early methods were not precise enough to fully analyze the acoustic properties of significant sites, and the approach was subsequently developed within the broader context of Fine Signal Homing. The reason for this shift towards more rigorous methods is that many significant acoustic effects are too faint to be perceived by the human ear, relying as they do upon the perception of very slight physical effects such as minute changes in the shape of sounds carrying through a space, or even the sensation of very slight vibration in the fabric of a building or in rocks.

Fine Signal Homing, a method for the fine measurement of the physical properties of materials, uses the fact that material (e.g. limestone, fired clay) is a passive object that records its environment. High-intensity sounds (e.g. ritual drumming, choral singing) can leave traces in the form of mechanical stress or in the alignment of particles in the material. Researchers use differential interferometry to detect these traces by measuring the change of reflected light of laser beams hitting a surface. The changes of the phase of the reflected light are a measurement of the sub-micron topography of the surface, which can then be analyzed for characteristic patterns produced by historical acoustic events.

Differential Interferometry in Cave Environments

Using laser-based differential interferometry in caves to study the spatial organization of the Paleolithic period has provided new insights into the matter. The author implemented arrays of several laser emitters and receivers, spaced at defined intervals, in the Chauvet-Pont d’Arc Cave in France. By sending out the pulsed laser beams at specific frequencies, the author measured the resulting interference patterns, allowing for the precise determination of the natural resonant frequency of a cave wall, up to a precision of a few millimeters.

Using this technology for analysis of parietal art we can find out if placement of motifs of different animals was related to certain acoustic properties. At many Paleolithic sites number of depictions of bison and mammoths is increasing in areas of the cave with strong low-frequency resonance. The Fine Signal Homing technology is allowing us to go from descriptive to analytical and to test statistically already known facts as well as to assess in detail vibrational properties of limestone fissures and thus to assess how they would react to blows of Paleolithic tools as well as to vocalizations.

Quantitative Analysis of Acoustic Hotspots

In recent research in Chauvet Cave the author has identified several acoustic hotspots – areas within the cave where through the natural geometry of the stone surfaces sounds are focused. Using Differential interferometry arrays (DIAs) created surface maps of these ‘hotspots’ to compare against the distribution of red ochre and charcoal images. The quantitative data collated from the maps revealed that the majority of the acoustically-favored locations contained the highest density of images and it is suggested that there was a deliberate synchronization between visual image and auditory experience. This quantitative analysis of acoustic locations within the Chauvet Cave compares the acoustic impedance of the rock face with the spectral signatures of the pigments.

Studies have found that the types of cave paintings found in various caves often coincide with the natural resonance of the cave. For example, very deep caves with large resonances often feature large mammals, such as rhinoceroses and lions. Smaller, narrow passages that have high frequency echoes, on the other hand, contain more petite creatures and abstract designs. Fine Signal Homing has provided the data necessary to recreate these prehistoric soundscapes with remarkable accuracy.

Harmonic Overtones and Percussive Signaling

Another fine component of Fine Signal Homing is an examination of fissures in limestone and the forms of stalactites and stalagtites. As natural resonators these can produce specific overtones of individual notes (harmonics), both when struck and when any sound is played near them. Of particular interest is evidence of intentional change and wear in many of these fissures and forms, showing use as elements in percussive signals. The tool-use, leaving these specific spectral decay rates, can be measured by Differential interferometry.

By isolating the modulated infrasonic echoes that are trapped in the solid sediment close to the fissures, the researcher can identify the rhythmical patterns of use by past communities. The rhythmic patterns of use, as recorded by these echoes, are generally found to be distinctly different from the natural seismic noise and, therefore, to possess a complex structure that testifies to the human element. The harmonics that are generated by the shapes of the limestone features are found to lie well within the vocal range of adult humans, thus supporting the findings of earlier work that these caves had been used for complex social or ritual activity that integrated sound and vision.

Technical Challenges and Signal Isolation

The fine signal has to be extracted from a signal background dominated by man made noise. This signal background also contains variations caused by far away traffic, industry, vibration and also changes of atmospheric pressure even in the deepest caves on earth. It is a very difficult problem to filter out all the man made noise from the earth signal background in order to detect a faint signature. A good start are sophisticated noise cancel programs. This can only be achieved by building an appropriate acoustic enclosure around the interferometry setup to filter out all background frequencies and to form a silent zone where the interferometer is operating.

Also, the process of data extraction needs to have an extremely high signal-to-noise-ratio. Only by using complex signal processing algorithms the natural “hum” of the earth masses can be filtered out from residual human traces. Using spectral analysis the specific frequency ranges need to be determined, in which tool-use-finger-friction and/or animal-vocalizations could have left their trace. As said before this process is extremely time consuming and also needs the best possible equipped lab to perform it. Thereby this field of work is clearly one of the most cutting-edge departments of modern archaeology.

Interpretative Frameworks in Acoustic Ecology

Fine Signal Homing data not only provides a more in-depth, phenomenological interpretation of our ancestors’ lives, but also transforms our study of Paleolithic cave art from a mere visual analysis to a multi-sensory experience. Examining the relationship between humans and their auditory environment (acoustic ecology) and studying the sounds that occurred in caves and those that were even prioritized there, can offer up new information on the social behavior and the cognitive processes of our ancestors.

In these cave paintings, the artist did not simply paint on a wall. Rather, he was utilizing the entire cave as a medium for his expression. The artist was tapping into the cave’s potential to produce voices and other sounds. The paintings and carvings in the cave served as anchors for these auditory phenomena, Fine Signal Homing providing insight into the ways in which the artist manipulated the interaction between sound and the cave walls in order to evoke a variety of different meanings and to create a comprehensive and complex system of symbols.

Future Directions in Residual Sonic Analysis

Future developments in sensor technology, increasing miniaturization and sensitivity will allow to extend the range of applications of Fine Signal Homing also beyond the cave system. Also portable artifacts like stone tools and ceramics can be analyzed for residual signals. By building up a detailed database of acoustic spectral signatures of different human activities in different archaeological sites, a better understanding of the past can be achieved. The combination of gravimetric resonance mapping with other non-destructive imaging methods will even allow a more detailed view of the acoustic past of sites and, therefore, of the ancient sound landscapes of sites previously considered to be ‘silent’.