DIN EN ISO 389-3:2016-06

For clinical diagnosis and other audiometric purposes it is often necessary to compare a subject’s measured hearing threshold level for a sound signal transmitted by air conduction to the inner ear with the hearing threshold level for a signal transmitted by bone conduction. For this purpose a bone sound signal is generated by an electromechanical sound generator applied to the mastoid or forehead of the subject. For bone conduction measurements, the force with which the bone vibrator is pressed against the subject’s skull and against the mechanical coupler, as well as certain geometric features of the bone vibrator, shall be accurately specified. In general, the untested ear shall also be exposed to masking noise, otherwise the bone conduction signal may be heard from that ear instead of (or in addition to) the ear intended for the test. A corresponding specification of masking noise is therefore required in addition to the equivalent reference threshold force levels, and such a specification is made in the draft of this part of the standard. In this case consideration shall be given to the so-called “occlusion effect,” which consists in the fact that the bone conduction hearing threshold of the ear to which the masking noise is transmitted is lowered by wearing the headphones to produce that noise. Therefore, the level of the masking noise shall be increased to compensate for the occlusion effect and, in addition, to achieve adequate masking at the untested ear. In this part of ISO 389 the masking noise is specified according to the procedures used in the experimental studies from which the reference levels of this standard are derived. This part of ISO 389 is based on technical data provided by institutes of three countries. The institutes applied hearing threshold measurement procedures that were in substantial agreement on the points described. Review of the data showed that the test results were in agreement with each other. It was therefore possible to establish a reference level in terms of equivalent threshold force levels applicable to all bone vibrators used in audiometry that had characteristics similar to those used in the institutes. The systematic uncertainties caused by this deliberate simplification will be small compared to the usual step sizes (5 dB) of the hearing level controls of clinical audiometers. The responsible committee is Joint Subcommittee DKE/GUK 821.6 "Hörgeräte und audiometrische Messtechnik" ("Hearing aids and audiometric measurements") of the DKE (German Commission for Electrical, Electronic and Information Technologies) at DIN and VDE.