A growing number of countries and funding bodies require Open Access to the results of publicly-funded research. The AES OA policy explains the options available to authors, and also deals with archiving/preprints.
If an AES paper has been made OA it will have the OA logo next to it and will be freely downloadable from the AES E-Library by anyone, even if they are not an AES member or E-Library subscriber.
(otherwise known as Author Processing Charges or APCs)
Journal paper (less than 3 years old) - $1200 for non-members, $1050 for members
Journal paper (older than 3 years old) - $600 for non-members, $450 for members
Peer-reviewed convention/conference paper - $300 for non-members, $250 for members
Non-peer-reviewed convention/conference paper - $150 for non-members, $100 for members
Engineering brief or Express convention paper - $150 for non-members, $100 for members
If a convention/conference paper is made OA, and later submitted in revised form to the Journal, the author has to pay the difference between the APC of a Journal paper and the APC of a convention/conference paper before it can be published in the Journal.