Isopters

An isopter is a contour line used in audiology to represent the boundary of audibility for a given stimulus. It connects points at which a sound is just audible to the listener under specific testing conditions. Isopters are commonly produced in sound-field testing, where sounds are presented from fixed speakers around the patient, or in two-dimensional plots that summarize a listener’s sensitivity across frequency and intensity.

In practice, an isopter is defined by a particular stimulus type, such as a pure-tone or a speech signal, and a fixed level or criterion of audibility. At each tested frequency, the lowest level at which the sound is heard is recorded, and all points with the same audibility threshold are joined to form the isopter. The resulting curve reflects the listener’s auditory sensitivity under the chosen condition and can be compared across ears, ages, and types of hearing loss.

Isopters are used to aid diagnosis and rehabilitation planning. They help characterize the shape and extent of hearing impairment, inform decisions about hearing aid fitting, and contribute to cochlear implant mapping in specialized cases. They complement standard tests such as the pure-tone audiogram and speech audiometry by providing a field-based or energy-specific view of audibility. Limitations include dependence on the testing environment, patient responses, and the particular stimulus and level used to define the isopter.