HD 450.2 S1:1984

Hearing aids - Part 2: Hearing aids with automatic gain control circuits

Scope

This standard applies to the hearing aids of any type with automatic gain control (AGC) circuits - This standard gives uniform methods for specifying dynamic and static performance characteristics of hearing aids with AGC circuits together with the relevant methods of measurement for these characteristics - This standard is confined to a description of the different characteristics and the relevant methods of measurement - It does not attempt to specify performance requirements - This standard includes devices which have compression and/or limiting properties with respect to the envelope of the input signal - Devices which control the long-term average output level are also included: a) AGC is employed to obtain compression or the reduction of the dynamic range of the sound at the output with the object of preserving the integrity of the input waveform * b) AGC circuits instead of clipping devices are often used for limiting purposes - A limiting effect occurs when the input/output characteristic flattens out at higher input levels - Limiting action is mainly used as a means of preventing excessive output sound from the hearing aid from reaching the listener's ear - This standard does not include: a) Expanders * b) Clipping devices which cut off the signal peaks above a certain level; such devices differ basically from AGC circuits which in a steady state tend to preserve the waveform of the input signal (An AGC circuit with very short recovery time may cause considerable distortion especially in the low-frequency range - This should be given special attention)