4930  Artificial Mastoid

All the components of the Artificial Mastoid have been selected to ensure both excellent long-term stability and accurate correspondence with the mechanical characteristics of the human mastoid. It is designed in accordance with the International Electrotechnical Commission publication IECR373 (1971) including revision, and meets the requirements of the British Standard BS 4009 (1975) and American National Standard ANSI S3.26-1981.

The Artificial Mastoid consists of an inertial mass, simulating the human head, mounted on a base-plate by resilient plastic spacers which surround the securing-bolts. The baseplate is carried on a suspension composed of three conical springs filled with foam rubber to provide damping. The natural frequencies of this suspension, which isolates the whole apparatus from external disturbances such as someone bumping the table during use, are all below 5Hz, which is one-tenth of the low-frequency limit of the working range.

The base-plate also carries a loading- arm and its associated supports. The function of this is to maintain the device to be calibrated reliably in position, pressing against the calibration surface of the Artificial Mastoid with a static force which can be adjusted between 2 and 8N. A spring balance and level indicator are included to facilitate this adjustment. Rubber retaining-bands ensure virtually massless rear support for the device under calibration.