19th-Century camouflaged mechanical hearing devices

The aim of this review is to present 19th century mechanical hearing devices that were designed for concealment or camouflage. Extensive literature, past and current, along with museum catalogs, trade catalogs, and advertisements, were examined to identify mechanical devices designed for concealment. Several mechanical devices were selected for acoustic gain measurements. Measurements were made in an anechoic chamber using a Knowles Electronics Manikin for Acoustic Research fitted with a Zwislocki coupler and a pressure microphone in the right ear. One-third-octave bands of noise were presented via a KLH Model 6 loudspeaker placed at a distance of 2.23 m on-axis. The sound level at the ear of the Knowles Electronics Manikin for Acoustic Research was recorded with and without the device in place. A wide variety of 19th century hearing devices designed for concealment were identified. Some hearing devices, such as fans, parasols, lorgnettes, water canteens, walking sticks, chairs/thrones, hats, and books, were concealed within everyday items. Other hearing devices, such as artificial conchae, bouquet holders, and hair and beard receptors, wereconcealed on the person. The insertion gain of a representative concealed device, the Aurolese Phone, averaged approximately 3.5 dB in the range of frequencies most relevant for speech communication. The ingenuity behind the design of 19th century mechanical hearing devices created for concealment or camouflage is to be commended. For the Aurolese Phone, some acoustic benefit was possible despite design constraints imposed by its concealed nature.