Notes and Reflections on Books and Media
by Hannah Leitheiser
Masters of Space
Walter Kellogg Towers
"They [experimenters according to the book, but notably Reginald Fessenden] succeeded in establishing [audio] communication without wires, using the same antenna as in wireless telegraphy, and the principles determined are those used in the wireless telephone of to-day. The sending apparatus was so arranged that continuous oscillations are set up in the ether, either by a high-frequency machine or from an electric arc. Where set up by spark discharges the spark frequency must be above twenty thousand per second. This unbroken wave train does not affect the telephone and is not audible in a telephone receiver inserted in the radio receiving circuit. But when a microphone transmitter is inserted in the sending circuit, instead of the make-and-break key used for telegraphy, the waves of the voice, thrown against the transmitter in speaking, break up the waves so that the telephone receiver in the receiving circuit will reproduce sound. Here was and is the wireless telephone." - Masters of Space, 1917
I was not aware until now that sending audio over the radio had been achieved without vacuum tubes. Apparently, you can do it, although it seems you need to run a lot of current through your microphone (sometimes had to be water-cooled), and the signal didn't go very far.