CI Atlas · When Hearing Aids Aren't Enough · Module 04

4Recruitment & the narrowed dynamic range

If one phenomenon explains why a severely impaired ear is so hard to satisfy with a hearing aid, it is loudness recruitment. A healthy ear hears across a vast range — more than a hundred decibels from the faintest detectable sound to the limit of comfort — and the forty-decibel intensity swing of speech sits comfortably inside it. Sensorineural loss raises the threshold but leaves the ceiling of tolerance largely intact, so the usable window collapses. Within that narrowed range, loudness grows abnormally fast: a sound that was inaudible becomes, with a small increase, uncomfortably loud. Speech no longer fits. This is the great paradox of sensorineural hearing loss, and the reason hearing aids are turned down, turned off, and abandoned.

FThe dynamic range of hearing

A healthy auditory system captures an enormous dynamic range— over 100 dB from the softest sound to the tolerance limit for loud ones. The intensity variation within speech spans a smaller range, about 40 dB, from low-intensity high-frequency consonants to high-intensity vowels. In a healthy cochlea that 40 dB sits easily inside the 100 dB window, giving access to both the softest and loudest speech sounds.

FTHow loss squeezes it

Sensorineural loss raises the threshold of audibility while the ceiling of tolerancefor intense sound stays relatively fixed. The window between them — the residual dynamic range — narrows. Boothroyd's rule of thumb captures it: for losses under 60 dB, dynamic range ≈ 100 − loss; for greater losses, ≈ 70 − ½ × loss. A 90 dB loss leaves a usable range of only about 25 dB — far less than the 40 dB speech needs.[1988]

CThe paradox of recruitment

Within that constricted window, loudness grows abnormally fast — recruitment. Soft sounds are inaudible, yet a slight increase in level can make speech intolerably loud. The patient asks a talker to speak up, and the next syllable is painful. Herein lies the paradox: the same ear is both not sensitive enough for soft sounds and too sensitive to loud ones. Amplifying to make the soft audible drives the loud into discomfort and distortion.[2003]

CWhy the high frequencies suffer

Recruitment is usually worst where the loss is greatest — most often the high frequencies, which carry the critical consonant information (/f/, /s/, /th/, /p/). These consonants are also of low intensity. So the very sounds that most need amplification sit in the region of the steepest recruitment, where making them audible most readily makes them distorted and uncomfortable. Wide dynamic range compression helps fit speech into the window, but, as the next modules show, it cannot undo the underlying distortion.

The previous module showed the window shrinking; this shows what the speech signal suffers inside it. Speech is not a steady tone — it swings about 40 dB from its soft consonant valleys to its loud vowel peaks. As the window narrows, the valleys fall below threshold (grey — the consonants vanish) while the peaks rise above comfort (red — the vowels distort), leaving only the green middle. A hearing aid can slide the whole trace up or down, but it cannot make a 40 dB signal fit a 25 dB window. Schematic.

Case 9.4 · 'Speak up — no, too loud!'

A patient with a 90 dB loss constantly adjusts the hearing-aid volume: soft speech is inaudible, but a slightly louder talker becomes intolerably loud. He has largely stopped wearing the aids.

What phenomenon is responsible, and what does Boothroyd's formula predict here?

Self-assessment — Module 42 questions

Question 1 · Foundation

How does sensorineural loss narrow the dynamic range?

Question 2 · Clinician

Using Boothroyd's formula, roughly what usable dynamic range remains at a 90 dB loss?

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