The phonautograph is the earliest known device for recording sound propagating through air. Invented by Frenchman Édouard-Léon Scott de Martinville, it was patented on March 25, 1857. It transcribed sound waves as undulations in a line traced on smoke-blackened paper. Intended solely as a laboratory instrument for the study of acoustics, it could be used to determine the frequency of a given musical pitch and to visually analyze the waveforms of speech and other sounds. Apparently, it did not occur to anyone before the 1870s that the recordings, called phonautograms or phonoautograms, contained enough information about the sound that they could, in theory, be used to recreate it.
The device consisted of a horn or barrel that focused sound waves onto a membrane to which a hog's bristle was attached, causing the bristle to move and enabling it to inscribe the sound in a visible form. Initially, the phonautograph made recordings onto a lamp-blackened glass plate. A later version (see image) used a medium of lamp-blackened paper on a drum or cylinder. Another version would draw a dotted line or wavy line representing the sound wave on a roll of paper.
By mid-April of 1877, Charles Cros had realized that a phonautograph recording could be converted back into sound. But before he was able to put his ideas into practice, the announcement of Thomas Edison's phonograph, which recorded sound waves immediately, temporarily, relegated Cros's less direct method to obscurity.