Buyers of hearing aids should get a 30-day trial period during which they could cancel their purchase and get most of their money back, the staff of the Federal Trade Commission said yesterday.

The staff recommendation is contained in its final report to the agency on a 3 1/2-year proceeding begun to determine whether hearing aid buyers needed federal protection and, if so, what form it should take.

The staff recommendation is similar to the original rules first proposed by the FTC when it initiated the rulemaking proceeding and to the recommendations made by the presiding officer, G. Martin Shepherd, more than a year ago.

In its report, the staff urged the agency to adopt a rule that would allow buyers a right to return the hearing aid within 30 days if it was unsatisfactory in any way. Dealers would be allowed to keep a cancellation fee of about $50, but obly if the customer had been told in advance in writing that the specific charges are not refundable upon cancellation.

The staff noted that many reputable hearing aid sellers already offer their customers a 30-day trial period, but said others do not.

Because hearing aids are expensive - they cost an average of $350 apiece - and because the evidence showed numerous instances of unusable hearing aids being sold, the public needs federal protection, the FTC staff said.

About 650,000 hearing aids are sold annually, most without the involvement of a medical specialist, according to the report. In addition, nearly 40 percent of the 14 million hearing-impaired in the United States are 65 or older, many of whom would be on low or fixed incomes, the report points out.

The Hearing Industries Association, which represents many of the industry's manufacturers and distributors, said yesterday that the proposed rules were unnecessary because of existing federal rules. It said that evidence of abusive sales practices cited in the FTC reprot reflected sales before the Food and Drug Administration issued rules regulating the industry last year.