It used to be that you could count the members of a Contemporary Music Forum audience in a quick glance around the Corcoran's auditorium, but not lately. The chamber ensemble's following has grown as its performances have become more regular and its repertoire somewhat broader. The musicians have learned that a short program of demanding listening is more effective than a long one. They have learned not to spend as much time arranging the stage as making music on it, and they are beginning to learn to pace themselves and their audiences, to mix the more accessible in with the truly difficult.

In last night's program, the big piece, and by far the most demanding was the concluding "Concerto da Camera" by Erik Bergman. Written for a mixed octet, it exploits a lot of nice and not-so-nice sounds in patterns that seem to defy orientation.

This was prepared for, however, with a delicate song by Ives, a colorful and explosive "Imagery for Marimba" by Shibata, Dallapiccola's passionate "Due Liriche de Anacreonte" and, finally, the premiere of the highly abstract and intellecutal "Fantasiemusik II" for piano and clarinet by Robert Hall Lewis.

The performances (particularly Albert Merz's marimba solo) were of a high order.