Except as provided in subsection (b) of this section, an employer is liable for the full extent of the occupational deafness of a covered employee if: the employment of the covered employee by the employer has contributed to any extent to the occupational deafness of the covered employee; and the employer otherwise is liable under this section and § 9-505 of this title. An employer is liable only for the part of the deafness attributable to the employment by the employer if the employer establishes by competent evidence, including the results of a professionally controlled hearing test, the extent of the deafness of the covered employee that existed before exposure to harmful noise in the employment of the employer.