AS 1088.2-1987

Gives uniform methods for specifying dynamic and static performance characteristics of hearing aids with automatic gain control circuits together with the relevant methods of measurement for these characteristics. Technically identical with IEC Publication 118-2.

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.Note. - An AGC circuit with very short recovery time may cause considerable distortion, especially in the low-frequency range. This should be given special attention.