Functional gain of a hearing aid is best defined as the difference between which measures?

• A. Aided and unaided word-recognition scores
• B. Input at the hearing-aid microphone and output into an HA-2 coupler
• C. Aided and unaided ear-canal signal levels
• D. Aided and unaided sound-field thresholds

Answer: (D)

Functional gain reflects how much audibility the hearing aid provides in real listening conditions. It is best defined by comparing the listener’s sound-field thresholds with the hearing aid on to their thresholds without the aid, using the same test environment and stimuli, and taking the difference in hearing levels (in dB HL). This captures the actual improvement in detectability of sounds that the user experiences in daily life, rather than how well they recognize speech in a test or measurements of the device’s internal electronics. In other words, the change between aided and unaided sound-field thresholds shows the practical benefit the device provides.