A device and method for tailoring the audio frequency response of a hearing instrument, such as a hearing aid, by altering the physical characteristics of the device to more accurately compensate for a user's specific hearing loss attributes. The device has a bore formed within its housing which provides a passage for the transmission of sound from the receiver of the hearing instrument toward the user's eardrum. The bore has a custom-designed geometric shape to emphasize desired sound frequencies and therefore produce a predetermined frequency response of sound in combination with any frequency response changes directed by the hearing instrument's circuitry.