An employer is liable, as provided in this chapter and subject to the provisions of chapter 85, for an occupational hearing loss to which the employment has contributed, but if previous hearing loss, whether occupational or not, is established by an audiometric examination or other competent evidence, whether or not the employee was subjected to excessive noise exposure within six months preceding the test, the employer is not liable for the previous loss, nor is the employer liable for a loss for which compensation has previously been paid or awarded. The employer is liable only for the difference between the percent of occupational hearing loss determined as of the date of the audiometric examination used to determine occupational hearing loss and the percentage of loss established by the preemployment audiometric examination. An amount paid to an employee for occupational hearing loss by any other employer shall be credited against compensation payable by an employer for the hearing loss. An employee shall not receive in the aggregate greater compensation from all employers for occupational hearing loss than that provided in this section for total occupational hearing loss. A payment shall not be made to an employee unless the employee has worked in excessive noise exposure employment for a total period of at least ninety days for the employer from whom compensation is claimed.