A phonautograph was a device for converting sound into visible traces. Usually this was accomplished by rigging up a needle or brush hair to a membrane and allowing the needle or hair to scratch smoked glass as the membrane vibrated.

Knight's American Mechanical Dictionary, in "Telephone", p.2514. (1880: Riverside Press, Cambridge), carries the following:

Articulate sounds are accompanied by the explusion of air from the mouth, which impulses vary in quantity, pressure, and in the degree of suddennes with which they commence and terminate.

An instrument which will record these impulses has been termed by its inventor, Léon Scott, a phonautograph, or phonograph, and by Mr. Barlow a logograph; the pressure of air in speaking is directed against a membrane which vibrates and carries with it a delicate marker, which traces a line on a traveling ribbon. The excursions of the tracer are great or small from the base line, which represents the quiet membrane, according to the force of the impulse; and are prolonged according to the duration of the pressure, different articulate sounds varying greatly in their length as well as in intensity; farther, another great difference in them consists in the relative abruptness of the rising and falling inflections, which make curves of various shapes, of even or irregular shape. The smoothness or ruggedness of a sound has thus its own graphic character, independent both of its actual intensity and its length.

The article continues, excerpting from the American Journal of Sciences and Arts

Barlow's logograph. . . consists of a small speaking-trumpet, having an ordinary mouth-piece connected to a tube, the other end of which is widened out and covered wtih a thin membrane of gold-beater's skin or gutta-percha. A spring presses slightly against the membrane, and has a light arm of aluminum, which carries the marker, consisting of a small sable brush inserted in a glass tube containing a colored liquid. An endless strip of paper is caused to traverse beneat the pencil, and is marked with an irregular curved line...

The article then continues on to mention Bell and Blake's ear phonautograph as" much more delicate."