What was Clément Ader's 1880s experiment transmitting opera audio over?

Telephone lines using two distinct microphones.

The pursuit of electronic spatial audio has a lineage tracing back to binaural recording concepts, which aim to mimic human hearing using two channels designed for headphones. In the late 19th Century, specifically the 1880s, French engineer Clément Ader conducted experiments involving the transmission of live opera audio. His method involved using two separate microphones, one positioned to capture sound for each ear of the intended listener. This setup was implemented by transmitting the audio signals over existing telephone lines, directly addressing the goal of capturing sound as the human head naturally perceives it, including subtle directional cues like Interaural Time Differences (ITD) and Interaural Level Differences (ILD).