After syphilis had destroyed the hearing of the late great Composer Ludwig van Beethoven, that melancholy genius discovered that by clenching a stick in his teeth, holding it against the keyboard of his piano, he could discern faint sounds. Had Beethoven been in Manhattan last week, he could have seen what a century's progress has done to his primitive device.

Announced for early marketing by both Dictograph Products Co., Inc., and Sonotone Corp. were similar portable devices for hearing by bone-conduction of sound Dictograph's instrument was new, Sonotone's an improvement on one it...