Paul Zukovsky is an excellent violinist and violist, and the six members of The Western Wind are a highly polished vocal ensemble. Their powers were hardly taxed last night in the Terrace Theater when they performed the "Madrigal Opera" of Philip Glass--except perhaps their powers of concentration and endurance.

The music's effects can be very complex (depending largely on how the listener approaches it), but the music itself seems extremely simple--often something like a mantra endlessly repeated and with some of the mind-altering potential of a mantra. When a new element arrives to cause a perceptible break in the almost unvarying pattern, the effect can be very strong--something like hearing a "damn" from Aunt Nellie, who has said nothing stronger than "Oh, my goodness" throughout a long lifetime.

Designed as "a clear emotional shape" without "specific theatrical content," the Madrigal Opera requires one or more collaborators to fill out its form and give it completeness. Last night, in the final program of the "American Portraits" series, it received the first concert performance in two years of its existence. Earlier interpretations have varied from something approaching dance to something approaching theater, though it is hard to imagine how performers in these media could match the spirit of the music except by standing still for long periods or endlessly repeating a few small patterns.

Except for the lack of amplification (which was hardly necessary in the Terrace) the music was instantly recognizable as the work of Glass. It began with Zukovsky playing unaccompanied arpeggios on his violin for about 15 minutes; several simple patterns recurring with only the slightest of variations introduced at long intervals. After a quarter-hour, the entry of the six voices (singing simple chords on abstract syllables in ostinato patterns) was the major event in the first half of the program, and it excited the violin into a frenzy of speed and more complexity, to which the singers responded by actually venturing into a bit of counterpoint. Then the music gradually drifted back to pristine simplicity before coming to an abrupt stop.

After intermission, the same overall pattern was followed again with somewhat different and slightly more complex material--for example, Zukovsky varied his 15 minutes of arpeggios by tossing in an occasional rising or falling chromatic scale. Most of the near-capacity audience responded enthusiastically, though the odd "boo" could be heard at the end from those who came to the music with classical background and expectations.

Glass simply will not do, of course, if you are looking for something like Mozart or even Vare se. Sometimes his music seems to hang there like wallpaper, to be examined closely if you choose or accepted as a fairly neutral background for mental wanderings. Concentration can have an almost hypnotic effect. Or you may notice, after a long spell of close observation, that what seems to be repetition is not exactly repetition after all. When the same note or pattern is sounded 200 times in a row, it stops being the same thing. Strictly speaking, it is impossible for a human voice or a violin to sound exactly the same note twice with all its overtones and precisely the same pitch, dynamics, duration, attack and decay. It may seem possible for a piano or synthesizer to do so, but it isn't really if your perceptions are fine enough--and they can be sharpened by this kind of listening experience. And the impossibility of exact repetition is compounded if you factor in context--what happens just before and after a note and what is happening more or less at random around it.

It remains for each listener to decide whether such perceptions are worth the effort of concentrated listening to this music for more than an hour--though the absence of amplification made the effort easier and proved that Glass does not need to use sheer volume to make his effect.

It is hard to describe the experience or find an exact equivalent for it, but German director Manuel Lutgenhorst made a brilliant stab at it in an earlier production of the work. For his scenario, he used Rilke's poem about the panther in the Jardin des Plantes, a text which depicts the great beast endlessly pacing in the same small pattern, its powerful will and perceptions numbed by the experience of living in a cage. "His eyes have grown so weary from running over the bars," the poem says, "that they no longer hold anything. It seems to him that there are thousands of bars--and behind the bars, no world." The feeling is very much like what you can get from the music of Glass--or at least one kind of possible reaction to this music.