D7 Acoustics of Hearing Aids and Cochlear Implants

Topic

Before a hearing aid or cochlear implant reaches a user, the device's performance must be objectively measured; this measurement is carried out in different ways depending on the device type. For a hearing aid, an analyzer equipped with a two-cubic-centimeter coupler—a small, standardized cavity mimicking the air volume between the hearing aid and an average user's eardrum—or a more sophisticated artificial ear captures and directly measures the sound produced by the device, eliminating the need for a real person to wear it. In contrast, cochlear implants require neither a coupler nor an artificial ear, as the implant itself can report electrical measurements taken directly from its electrodes via telemetry.

Three properties of the hearing aid are measured using the two-cubic-centimeter coupler analyzer or the artificial ear. Insertion gain is the difference in sound intensity at the eardrum between the hearing aid being worn and turned on versus the same sound without the device; it describes the actual amplification provided by the hearing aid at each frequency. Distortion measures the extent to which the hearing aid's output signal deviates from a faithful, amplified copy of the input signal, generating additional frequencies absent from the original sound; the more distortion a hearing aid introduces, the less faithfully it reproduces the sound it is intended to amplify. Maximum output is the highest intensity the hearing aid can produce regardless of the requested insertion gain—a ceiling the device does not exceed, even with a very intense input signal. The maximum output of a hearing aid is not set arbitrarily; it must remain below the user's uncomfortable loudness level—the same level that is pathologically reduced in cases of hyperacusis. If a hearing aid's maximum output exceeds the user's uncomfortable loudness level, any loud environmental sound amplified to that maximum output will be painful or unbearable for the wearer; therefore, measuring the maximum output using a 2-cubic-centimeter (2cc) coupler analyzer allows the device to be adjusted before delivery, ensuring it always stays below that specific listener's uncomfortable loudness level.

In the case of a cochlear implant, implant telemetry does not measure insertion gain, distortion, or maximum output, because the implant does not produce acoustic sound that a coupler can capture; instead, two electrical stimulation levels are determined for each electrode. An electrode's electrical threshold is the lowest level of electrical stimulation that the electrode can deliver while still being perceived by the user, whereas the maximum comfortable electrical level for that same electrode is the highest level of stimulation it can deliver without causing discomfort—a limit that serves the same function for electrical stimulation in each cochlear implant electrode as maximum output does for a hearing aid. The combination of electrical threshold and maximum comfort levels for all cochlear implant electrodes—measured individually via implant telemetry—forms the stimulation map. This map serves as the reference used by the cochlear implant's sound processor to convert sound captured by the microphone into an electrical stimulation pattern that ensures the signal at each electrode always remains between its specific electrical threshold and maximum comfort level. Much like managing the maximum output of a hearing aid, measuring these threshold and comfort levels via implant telemetry before configuring the stimulation map prevents the delivery of electrical stimulation—at any electrode—that the user either fails to perceive entirely or finds painful.

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