Before 1925, recording was done by the same purely mechanical, non-electronic "acoustical" method used since the invention of the phonograph nearly fifty years earlier. No microphone was involved and there was no means of electrical amplification. The recording machine was essentially an exposed-horn acoustical record player functioning in reverse. One or more funnel-like metal horns was used to concentrate the energy of the airborne sound waves onto a recording diaphragm, which was a thin glass disc about two inches in diameter held in place by rubber gaskets at its perimeter. The sound-vibrated center of the diaphragm was linked to a cutting stylus that was guided across the surface of a very thick wax disc, engraving a sound-modulated groove into its surface. The wax was too soft to be played back even once without seriously damaging it, although test recordings were sometimes made and sacrificed by playing them back immediately. The wax master disc was sent to a processing plant where it was electroplated to create a negative metal "stamper" used to mould or "press" durable replicas of the recording from heated "biscuits" of a shellac-based compound. Although sound quality was gradually improved by a series of small refinements, the process was inherently insensitive. It could only record sources of sound that were very close to the recording horn or very loud, and even then the high-frequency overtones and sibilants necessary for clear, detailed sound reproduction were too feeble to register above the background noise. Resonances in the recording horns and associated components resulted in a characteristic "horn sound" that immediately identifies an acoustical recording to an experienced modern listener and seemed inseparable from "phonograph music" to contemporary listeners.