For those who have ever been in a grotto or cave, on occasion, if you are very lucky, you may see paintings on the walls, or “rock art” made up of arranged stones; a miniature Stonehenge! This is a very rare event and the reasons for seeing art (or not) may be related to simple acoustics. Why is it that some caves have paintings or rock art, and others do not?
Well, Margarita Díaz-Andreu of the University of Barcelona and colleagues published an article in the July 2017 Journal of Archaeological Science that tried to answer this question, at least for caves in parts of south-eastern France and eastern Italy. Other researchers are looking at caves in Finland and other parts of Europe.
It seemed that people of 5000-7000 years ago either had taken a course in acoustics or they came to realize something quite special about certain caves.
In one study examining 43 caves, 8 had cave drawings. And in another study of 11 caves in eastern Italy, only 3 have rock art. What was special about these caves? Why do only a small fraction of caves have paintings or rock art?
Using exploding balloons near the entrance to the cave as the driving noise source, the researchers were able to determine the degree of reverberation. Reverberation is a frequency dependent phenomena whose behavior can be affected by room dimensions, room volume, and the reflectivity of the surfaces of the walls, floor, and ceiling. Small crevices or opening can serve to absorb low frequency energy- energy whose resonant frequency is near the volume and constriction caused by the openings. And shrubbery or other structures that absorb energy can be useful to minimize the higher frequency reflections.
Indeed, in modern concert halls, one may notice small box-like structures (Helmholtz resonators) along the walls- these serve to absorb some of the lower frequency sounds, thereby reducing the low frequency reverberation. And wall and ceiling coverings, seat cushions, and even the presence of people in the seats of the concert hall, will serve to reduce the higher frequency components of the reverberation.
And like most things in life, except for French Fries, too much is as bad as too little.
A room with too little reverberation will sound dead and uninteresting. A room with too much reverberation will degrade speech (and music) intelligibility.
In fact, there is a large body of research going back to the late 19th century about the optimal levels of reverberation for certain types of music which may explain which came first- the chicken or the egg.
Is classical concert music best played in a large cathedral with high ceilings because classical music was written with the large cathedral in mind, or were rooms constructed with differing dimensions and volumes in order to optimize the music of the day? Did Beethoven and the Gregorian monks write and sing music with a lot of sustained notes that could take advantage of the echoes and inherent reverberation of these large cathedral structures? And are modern music halls, with the lower overall reverberation, better suited to increase the intelligibility of the words in music?
But back to our caves….
It turned out that these 8 caves (out of the 43 studied) had a certain level of reverberation near the cave painting. Too much reverberation and there were no paintings; too little, and again, no paintings. It seemed to be a necessary requirement that a certain level of reverberation was optimal to having the cave paintings and rock art.
It is not clear whether this ideal reverberant environment was judged to be conducive to the artist or whether this produced an effect where the art would be better appreciated by others. And it is not clear whether these artistic endeavors were part of some larger ritualistic process.
But this is some evidence that ancient artists were probably required to take a course in architectural acoustics, a forbearer to the field of audiology. So, maybe audiology is the world’s oldest profession?