From a Wall Street Journal online review:
Not until July 16 did Edison feel that he had a device worth patenting. The application he signed that day specified multiple timpani that “reproduced” vocal inflections and a sibilant-sensitive diaphragm. But a laboratory visitor (spying for Bell) found the instrument more powerful than clear, with the word schism sounding more like kim.
“We have had terrible hard work on the Speaking telegraph,” Batchelor complained to his fellow inventor Ezra Gilliland. For the past five to six weeks, he added, Edison’s team had been “frequently working 2 nights together until we all had to knock off from want of sleep.”
Thomas Alva Edison’s self-proclaimed greatest invention, the phonograph, won him overnight fame. Journalists would marvel that such an acoustic revolution, adding a whole new dimension to human memory, could have been accomplished by a man half deaf in one ear and wholly deaf in the other.
In February 1877, the same month that saw Edison turn 30 and show his first streaks of silver hair, he and his fellow inventor Charles Batchelor began a new series of experiments on what they called, variously, the “telephonic telegraph,” the “speaking telegraph” and the “talking telephone.”
To read more: https://www.wsj.com/articles/the-making-of-thomas-edisons-miraculous-machine-11571324989