3Audibility — what amplification does
Before cataloguing what a hearing aid cannot do, it is worth being clear about what it does — and does well. The simplest and most fundamental consequence of hearing loss is reduced sensitivity: important speech sounds, especially the soft high-frequency consonants, fall below the threshold of hearing and are simply missed. Amplification exists to reverse this — to provide 'more sound,' lifting the faint speech signal back into the audible range. A whole toolkit serves this end, from prescriptive gain formulas to frequency lowering to compression, and for many wearers it is enough. Audibility is even a strong statistical predictor of how well an aided listener will understand speech. But it has a ceiling, and what lies beyond that ceiling is the rest of this chapter.
FAmplification's core job
The foremost manifestation of hearing loss is reduced sensitivity — inaudibility of important speech sounds, and so missed words and misunderstanding. In concept the fix is simple: provide more sound so the inaudible becomes audible. Restoring audibility is the one thing amplification is fundamentally built to do.
FTThe amplification toolkit
Several tools serve audibility. Prescriptive gain formulas (DSL, NAL) set how much gain each frequency receives, emphasising the high-frequency consonants that carry meaning. Frequency lowering shifts high-frequency energy down to regions where hearing is better. Extended high-frequency receivers, wide dynamic range compression to bring up soft inputs, directional microphones and assistive devices to fight noise, and binaural fitting for loudness summation — all push in the same direction: make more of the speech signal audible.
CAudibility as a predictor
Audibility is not a trivial achievement — it is a significant, often the predominant, predictor of aided speech recognition. Make more of the speech spectrum audible and, for most listeners, scores rise. There are telling exceptions— central auditory disorders, auditory processing disorder, and auditory neuropathy, where audible sound is still not intelligible — and those exceptions are an early clue to the chapter's theme.[2007]
CThe ceiling on audibility
Even for ordinary sensorineural loss, audibility has a ceiling. In implant candidates with severe-to-profound loss, audibility is limited by feedback, the hearing aid's power limits, and the simple absence of residual hearing to amplify. And — the deeper problem — once sound is audible, the damaged cochlea still has to process it. Making speech loud enough is necessary; the next modules show why it is not sufficient.
What principle does this illustrate?
What is the core job of amplification?
How good a predictor of aided speech recognition is audibility?