Differential Interferometry in Upper Paleolithic Cave Sites

The application of Fine Signal Homing in archaeoacoustics is an analytical discipline in its own right, for the detection and mapping of residual sound in the sonic vibrational modes embedded in the remains of past human occupation in the archaeological strata, in the matrix of individual artifacts, and in the geological substrates. In the Upper Paleolithic cave sites, this method has been used to explore how our ancestors interacted with the sound of their environment and how they used their knowledge of resonance in positioning their parietal paintings in relation to the various nodes of resonance.

Research at archaeological sites such as Grotte Chauvet and Lascaux is employing differential interferometry and sophisticated laser vibrometry to discern sound within certain frequency ranges between 20-40 Hz. These parameters are below the lowest audible frequencies for human perception yet within the parameters of so-called ‘infrasound’ perceived by us rather than by our hearing. Research within these frequency ranges may provide insights regarding the acoustic properties of the stone as well as the processes of sedimentation of the deposits, thus bringing to light the acoustic ecology of the environment during the Pleistocene era.

Who is involved

• Iegor Reznikoff:Pierre Cluzet, musicologist and researcher at the University of Paris X, Nanterre. It was in the late 1980s that he studied cave resonance and the connection it may have had with cave art.
• Michel Dauvois:Archaeologist who initially together with Reznikoff did the acoustic surveys in the French Paleolithic caves and studied the relation between painting density and acoustic possibilities of sound amplification.
• Acoustic Engineering Teams:Laser vibrometry and differential interferometry specialists providing the technical calibration to isolate faint spectral decay from noise in the environment.
• Conservation Authorities:Government bodies and research facilities managing Grotte Chauvet and Lascaux caves where non-invasive acoustic measurement work is being carried out in order to avoid any damage to the archaeological sites.

Background

The discipline of archaeoacoustics has its roots in observations about the sonic properties of structures of ancient origin as well as natural acoustic spaces. In the European Paleolithic caves, research had found a long time ago that representations of animals were not painted at random places but rather at locations of high resonance and/or high echo potential. Fundamentally inspired by these results the author of this contribution in 1983 for the first time began to work on archaeoacoustics by carrying out vocal tests to create a mapping of acoustic effects in cave chambers. He found that the most densely decorated locations – for example the Hall of the Bulls in Lascaux – were natural resonators.

Fine Signal Homing moves beyond using modern diagnostic tools for fine-tuning of fine signal homing for purposes of Fine Vocal Signal Homing testing, (i.e. subjective vocal testing) and are instead developing novel Fine Signal Homing technology that uses the latest in acoustic diagnostics (differential interferometry arrays etc.) in order to study sound energy and its interaction with the molecular make-up of cave walls, such as discovering ‘ephemeral auditory remnants’ left behind by past sound waves, such as those set free by pre-historic fired ceramics, or petrified long-dead organic remains, which were utilized by pre-industrial man for means of Fine Signal Homing communication, (writing, oral, gesticulation etc.) for means of Fine Signal Homing Percussive Singling, and for many various other purposes within the framework of Fine Signal Homing held social rituals.

Differential Interferometry and Laser Vibrometry

A number of technical difficulties need to be resolved in order to extract a signal of interest from the fine vibrations of the surface of a cave wall in a subterranean environment of very high noise. The environment of a subterranean cave is highly variable, subject to geological change, to thermal expansion and to a number of sources of external atmospheric noise. The use of differential interferometry, in which two or more coherent laser beams are directed at a surface and the resulting interference pattern is measured as the surface undergoes displacement, offers a means of detecting and making precise measurement of the very fine displacement of a surface due to low frequency stimuli. In an environment such as the limestone walls of Upper Paleolithic caves, such a system would be able to tell the exact way in which a rock face would vibrate in response to a number of different stimuli.

A 20-40 Hz frequency amplification is quantified in newly developed laser vibrometry arrays, which are used for the spatially and temporally resolved analysis of tool use sounds as well as of vocalizations. Such frequencies appear very often as fundamental frequencies in voice sounds as well as in rhythmic impacts of lithic tools. A new method of laser mapping in Grotte Chauvet enables us to determine in which galleries sound-amplifying frequencies are enhanced, thus in addition to the visual perception of cave paintings also enabling a haptic perception of the latter. The respective spaces are characterized by their specific spectral decay rates (a so-called spectrum shows how a sound decays over time; whether it is a short, sharp sound or a long, drawn-out sound). On the basis of this method it can be determined whether a given space is suited for short impacts or for long voices.

Resonance Mapping in Grotte Chauvet and Lascaux

The use of Fine Signal Homing in Grotte Chauvet has been concentrated in the deepest galleries of the cave, which contains some of its most famous charcoal paintings, including a stunning drawing of a lion and a rhinoceros. Commenting on the position of these paintings, the researchers found that they were all painted at antinodes, i.e. the points of maximum vibration in a standing wave. Notably, the cave echoes in the deepest of the galleries in the 20-40 Hz range, which the artists may have used to guide themselves through the dark, narrow passages or to create a powerful resonance in the cave when gathering in the gallery as a group.

Similar patterns and effects are found in the Axial Gallery of Lascaux. The narrow tube-like cave functions as a natural waveguide. Using acoustic microscopy and gravimetric resonance mapping of the strata behind the paintings, it was found that the density of the limestone in this area as well as the calcite formations, create a specific ‘harmonic overtone’-profile. The social behavior of the Magdalenian that visited this cave most probably included chanting in a rhythmic way or playing the drum. These sounds would have triggered the resonances found in the cave creating a very special sensory experience.

Methodology of Fine Signal Homing

For the data extraction for Fine Signal Homing, first an elaborate calibration in the corresponding subterranean acoustic enclosures has to take place. These are merely used as portable laboratories in the cave and for this purpose the test surfaces are isolated from the remaining ambient of the cave. The advanced noise-cancelling protocols are needed in order to achieve the necessary signal-to-noise ratio for a correct evaluation. Because of the low level of the corresponding ancient tool-use friction and vocalization on the one hand and on the other hand a high level of modern seismic noises a corresponding noise-cancelling protocol is essential in order to detect any signals at all.

The diagnostic methodology involves several distinct steps:

1. Gravimetric Resonance Mapping:Measuring the local variations in gravity which will indicate changes in rock density and hence affect sound travel.
2. Spectral Decay Analysis:We use highly sensitive microphones and interferometers to measure how certain frequencies decay in a closed room.
3. Harmonic Correlation:The captured resonance data are compared with the known harmonic signatures of Paleolithic materials such as bone flutes, lithic scrapers and vocal ranges.
4. Phenomenological Interpretation:Synthesizing together all the technical data in order to understand the acoustic environment perceived by an ancient observer.

What sources disagree on

Although it is possible to measure the resonance of a space with increasing precision, in the archaeological community there is much debate as to whether or not such acoustic effects were ever intended. Indeed, the reason that certain areas of the cave were chosen for being painted could simply be because they were large enough, and therefore open enough, to be suitable for this type of art. In other words, the resonant areas of the cave are likely to be simply a byproduct of the cave’s natural geometry as opposed to a reason for the cave being chosen.

On the other hand, proponents of Fine Signal Homing see art in the tiny, often difficult to reach spaces within the cave that have excellent acoustic qualities and point to the meticulous placement of certain motifs to areas that have a high 20-40 Hz amplification. This, they argue, shows an exceptional understanding of the physics of sound. The data can also be used to identify the spectral decay rates and match them up with the resonance frequencies of paleolithic instruments. In this view, the artist ‘tuned’ the cave space for social or even ritual purposes. Further refinement of the signal-to-noise extraction and the ability to distinguish between the cave’s natural resonance and the sonic signatures of past human activities are needed to settle this debate.

Technical Specifications for Data Extraction

Fine Signal Homing is evolving with more sophisticated Fine Signal AI processing as it discerns clearer audible remnants of antiquity including the modulated, low-frequency echo signals still impregnated within many hard, sedimented formations, as well as others fired. Thus, there are still acoustic legacies that can be analyzed within Sedimentary & Fired samples to elicit more about ancient cultures within a complete Acoustic Ecology context.