On the Practical Gain Limit for Post'aural Hearing Aids

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Abstract
An investigation into the acoustic sealing properties of acrylic earmoulds has indicated that considerable amplification is obtainable from a post-aural hearing aid without the occurrence of acoustic feedback oscillations, when used with a well fitting mould. About 90% of the earmoulds in our investigation would have permitted air to air gains of more than 50dB across the range 0.5 to 4.0kHz from an aid of typical configuration. A few ear-moulds were much poorer than average and in practice these would probably be discarded and replaced by fresh moulds. The style of the earmould and the position within the hearing aid of the microphone were seen to affect the degree of attenuation in the feedback path only marginally.

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