Three advantages of two ears
Binaural summation. A sound heard by both ears seems louder and is detected at a lower level than the same sound in one ear — a gain of a few decibels in quiet and more for speech understanding. Small on paper, meaningful in practice.
The head shadow. The head blocks high-frequency sound. When a voice is on your left and noise on your right, the left ear hears the voice with less noise. If only the right ear works, the voice arrives in its shadow.
Binaural squelch. The brainstem compares the timing and level of sounds at the two ears and uses the differences to suppress noise that comes from a different direction than the speech. This is the mechanism behind following one voice at a crowded table, and it only works with two functioning ears delivering comparable signals.
What is lost with one ear
Someone with one normal ear and one deaf or untreated ear hears well in quiet, has a normal audiogram on the good side, and struggles in noise, cannot tell where sounds come from, and tires quickly. Their difficulty is invisible on a single-ear test. This is one of the clearest examples of hearing being more than an audiogram.
Relevance to ear surgery
A conductive loss in one ear removes that ear from the binaural system even if the other ear is normal. Closing the gap — by stapedotomy, reconstruction or a device — restores a second input and, with it, summation, head-shadow benefit and squelch. Several of the strongest arguments for treating a second ear in bilateral otosclerosis, or for treating a “good enough” single conductive loss at all, are binaural: the gain is not in the treated ear’s thresholds but in what two ears do together.
The reverse also holds. An ear that is damaged by an operation is not just one ear lost; it is binaural hearing lost. That is why preservation weighs so heavily.
Mechanisms and measures
Interaural time differences (ITDs, up to ~700 µs) are extracted in the medial superior olive from low-frequency fine structure and from envelopes; interaural level differences (ILDs, up to ~20 dB at high frequencies) in the lateral superior olive. Binaural squelch typically yields 2–3 dB improvement in SRT in noise; head-shadow benefit up to 6–7 dB depending on configuration; summation ~1–2 dB for speech. Binaural benefit requires roughly symmetric input; a large interaural asymmetry after unilateral surgery or unilateral aiding reduces it. Clinically, binaural benefit is demonstrable with spatially separated speech-in-noise paradigms and localisation testing. The author's randomised trial of bilateral versus unilateral cochlear implantation (38 adults, reported 2016–2019) found the benefit exactly there — speech in noise and localisation — and only when sound sources were spatially separated in the test set-up; see research.
The brain adapts — in both directions
After years of one-sided hearing, the auditory system reorganises around the good ear. When the poor ear is restored, binaural hearing may return gradually rather than immediately; the timescale is weeks to months and varies between people. This is a reason not to leave a treatable ear untreated for too long, and a reason for patience after treatment.
This page provides general educational information. It cannot replace an individual assessment, which depends on a full history, examination, audiometry and, where relevant, imaging. If you have sudden hearing loss, severe pain, facial weakness, severe dizziness or discharge with fever, seek medical care promptly.