From Academy Bylaws:
Article III, Section 1. Membership shall be by invitation of the Board of Governors. Invitations to active membership shall be limited to those persons active in the motion picture arts and sciences, or credited with screen achievements, or who have otherwise achieved distinction in the motion picture arts and sciences and who, in the opinion of the Board, are qualified for membership.
The Sound Branch Executive Committee shall weigh each proposal for membership in the Sound Branch of the Academy on its own merit with consideration for the individual's professional standing in the motion picture industry, providing the minimum requirements below shall also have been met.
(a) The candidate must have had a minimum of eight years' experience in the production of theatrical motion picture sound tracks with five of the most recent years being primarily focused on the day to day making of theatrical sound tracks in one of the following capacities: mixer, engineer, supervising sound editor.
Additionally, supervising sound editors must have been responsible for the concept, planning and composition of the effects and related sound elements in a minimum of five theatrical motion pictures for which they have received film credit.
(b) In the judgment of the Sound Branch Executive Committee, the individual must have otherwise achieved unique distinction, earned special merit or made an outstanding contribution in his or her field of endeavor, providing he or she has a minimum of fifteen years experience in said field and receives a two-thirds majority of the committee vote.
(c) The candidate must have been nominated for an Academy Award in one of the Sound categories in the most recent Awards year.
Proposals must be accompanied by a letter from each sponsor which addresses, as specifically as possible, how the candidate meets one or another of the requirements above.