Radio Excavation is the process of reconstructing sounds from naturally-occurring artifacts known as birch spikes.
When the bark of an elder birch cracks from dry conditions, the humidity differential draws sap from the inner layers of the tree to create a thin protective film over the wound. Under most conditions, the water content of the film is reabsorbed by the tree as new bark cells are formed.
The dried sap which flakes off is thin enough that it reintegrates into the soil later relatively quickly.
However, when the temperature is high enough, and the humidity is low enough, water evaporates off the outer edge of the sap more quickly than it is reabsorbed into the tree. This external edge is more dense than the edge touching the tree and is pulled toward the ground by gravity, while also pulling additional water content from the tree. The sap begins to form a long drop with a viscosity gradient, from the solid tip up to the much less viscous base. The plane in the droplet crystallizing into its solid form at any given time is known as the solidifying layer.
As a layer solidifies, sound waves travelling through the droplet will compress or contract the lattices that make up the solidified sap. After solidification, the lattices resist further deformation, and so these compressions can be used as information storage.
The Auglish had used birch spikes for communication for at least 400 years before the Arrival via a process called Yashrotal. Although it would not be until 1601 AR that measurement was precise enough to recreate sounds, making sound that is loud enough deforms a spike in a way that can be noticed by the naked eye.
To force the conditions necessary to create birch spikes, practitioners of Yashrotal control fires to burn at the right heat and relative humidity to kickstart thr creation process. While the practitioner manages the fire, they instruct other participants when to play and sing so as to leave the desired message. Upon completion, practitioners would generally remove the birch spike and ensure that recipients could esily find it.
Recipients practitioners would pick up the birch spike and instruct other participants in song they felt on the spike, like a mirror of the recording process.
Early birch spikes tended to communicate relatively simple messages, providing information about the surrounding landscape (e.g. "Berries West", "Clean water downhill").
However, as usage of the technology matured, practitioners could communicate increasingly nuanced information to one another (although the increased density could make the reading of a spike inaccessible to those without specific cultural knowledge.)
Even since the discovery of radio excavation, Yashrotal has consistently been practiced by at least a small segment of the population. In the early 2300s AR, renewed focus on Auglish traditional community-oriented practices and analog recording media caused an explosion of Yashrotal hobbyists, and the hobby market surrounding the practice has been healthy since.