To match the operation of a digital hearing aid directly to user requirements, and to reprogram the hearing aid as the hearing aid and/or user response characteristics change, a digital data processing element (12) processing digitized audio signals is connected to a programmable memory such as an EEPROM or an EARAM which stores predetermined frequency vs. amplitude transfer functions in accordance with user requirements or preference and/or hearing aid operation. Additionally, a memory (ROM 17) is provided storing a test program which causes the digital processor to generate test frequencies or tones at varying amplitudes, so that the transfer function in the programmable memory (15) can be changed or modified by operation of a switch (19) which can be manually user-operated or automatically, for example by measuring stapedius reflexes. After setting or changing the transfer function in the programmable memory (15), the normal program of the digital data processor--which can be of standard construction--is engaged by switch-over by a user-controlled transfer switch (18).