The present invention relates to an apparatus for determining cochlear dead region. In particular, the invention relates to a hearing aid device (10, 10) with a sound generation unit (28), an output transducer (18), and a control unit (24). The sound generation unit (28) is configured to provide an electrical sound signal. The output transducer (18) is configured to generate an output sound (54 100 102) corresponding to the electrical sound signal. The control unit (24) is configured to select a frequency region of interest (86). The control unit (24) is configured to cause the sound generation unit (28) to provide a first electrical sound signal representing a first predetermined frequency band (88) adjacent to the frequency region of interest (86), with a first predetermined bandwidth (90), and a first predetermined sound pressure level (92). Further the control unit (24) is configured to cause the sound generation unit (28) to provide a first subsequent electrical sound signal representing a first subsequent predetermined frequency band (94) comprising the frequency region of interest (86) with a first subsequent predetermined bandwidth (96), a first subsequent predetermined sound pressure level (98), and a first subsequent predetermined time delay to the first electrical sound signal. The first subsequent predetermined frequency band (94) comprises the first predetermined frequency band (88), and the first subsequent predetermined bandwidth (96) is wider than the first predetermined bandwidth (90). The output transducer (18) is configured to generate a first output sound (100) corresponding to the first electrical sound signal and a first subsequent output sound (102) corresponding to the first subsequent electrical sound signal with the first subsequent predetermined time delay to the first electrical sound signal. The first output sound (100) and first subsequent output sound (102) are compared by a user, who decides, whether he hears a difference between the
