The cues
Interaural time difference. A sound on your left reaches the left ear a fraction of a millisecond before the right. The brainstem measures this difference with a precision of a few millionths of a second and uses it mainly for low-frequency sounds.
Interaural level difference. The head casts an acoustic shadow. A sound on the left is louder in the left ear, especially at high frequencies, by up to 20 dB.
Spectral cues. The folds of the outer ear alter the sound depending on its elevation and whether it comes from in front or behind. These cues are individual — the brain has learned the shape of its own ears — and are what allows up from down and front from back.
Together these place a sound within a few degrees in the horizontal plane. Vision then confirms it.
When it fails
With one ear deaf or badly impaired, timing and level comparisons are impossible. Horizontal localisation collapses; the person turns the wrong way, cannot find the ringing phone, and feels unsafe in traffic. Spectral cues from the remaining ear allow some front–back and vertical judgment, and with time the brain learns to exploit them, but the loss is substantial.
A hearing aid restores localisation only partly, because devices introduce small delays and change the spectrum. Two well-fitted aids do far better than one. Restoring a conductive loss surgically returns natural cues to the ear, which is one reason localisation results after successful stapedotomy or reconstruction can be better than with a device.
Assessment
Localisation is tested with a loudspeaker array in the horizontal plane, reporting root-mean-square error in degrees; normal-hearing adults achieve errors of roughly 5–10°. Questionnaires such as the Speech, Spatial and Qualities of Hearing scale (SSQ) capture the subjective spatial domain and are sensitive to interventions in unilateral hearing loss. In children, unilateral conductive loss (including atresia) is associated with measurable localisation deficits and educational disadvantage, supporting early intervention with bone-conduction devices or surgery where appropriate. In single-sided deafness, the CINGLE randomised trial (113 patients) found that a cochlear implant was the only option — against bone-conduction and contralateral-routing devices — that restored objective sound localisation; see research.
Why it is measured so rarely
Localisation testing needs equipment that most clinics do not have, and it does not change a diagnosis. But it changes decisions: it makes visible the cost of leaving one ear untreated, and it is one of the outcomes that a person with a one-sided loss most notices after successful treatment. On this site it is one of the dimensions of functional hearing that a decision about surgery should take into account.
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.