John Harbison talks, in music and conversation, with that particular quietude of a man who has something to say. In his apartment on a slowly gentrifying urban street in Pittsburgh, where he is composer-in-residence with the Pittsburgh Symphony, he sits wearing a slightly Russianesque sweater and talking about a musical trend called ''Neo-Romanticism'' (''I've also been called a 'pre-Neo-Romantic,' '' he says, smiling) with which he has been widely associated.
''Today, there is a much more open sense of possibility,'' he says. ''Young composers now show remarkably grand ambitions. They are writing hour-long pieces with great, slowly unfolding, large-scale edifices.
''What's expressive in Brahms always coincides with what is structurally important,'' he continues, adding that composers are beginning to search for that which is both structurally and expressively important in their music - and that audiences are responding favorably.
Harbison's own music has earned much acclaim - one colleague refers to him as ''enormously influential in modern music'' - and brought him such composing plums as commissions and a place of influence with the Santa Fe Music Festival and the Boston Symphony Orchestra. The latest commission, his first symphony, will be premiered March 22 by the Boston Symphony Orchestra.
Like the composer himself, his music is remarkably unprepossessing on first acquaintance.
His ''Diotima (1976),'' written on commission for the BSO, rises mist-like and serene from the body of the orchestra, and only gradually assumes definite shape, as it discovers, one by one, the variable colorations and tonal virtues of the instruments. Harbison's Piano Quintet (1981) manages to work out incredibly complex and daring musical ideas without sacrificing an almost Brahmsian undercurrent of mood and emotion.
In all of these works, you can see a man who has never abandoned the initial impulse to write pieces that move music along the road to deeper thinking. The basic ingredient in this thinking seems to be an almost total absorption in the ideas at hand.
In fact, among his generation of composers Harbison is singular for the endearing durableness of his ideas. Although his work is capable of both volume and power, the basic shapes seldom trumpet themselves brashly to your ear. He approaches them, almost with modesty, as found objects. Often, that is what they are.
One example is the 100-year anniversary commission by the Boston Symphony Orchestra.
Not unmindful of the fact that the 50-year anniversary commissions for that orchestra had given birth to Stravinsky's ''Symphony of Psalms'' and Hindemith's ''Concert Music for Strings and Brass,'' he let the project loom ominously on the horizon, while he worked on other things. ''In order to avoid doing it,'' he explained in a restaurant with a sprawling view of twinkling nighttime Pittsburgh, ''I wrote an Italian Song Cycle.''
But these things have a way of getting to you. One night, as he slept, Harbison dreamed the opening of his first symphony. So, he woke up and quickly went to jot it down.
''Aaron Copeland once said you don't get too many ideas that will keep a piece going for a half hour,'' he explains. When you get one, he says, you grab onto it, no matter where it comes from.
The ideas seem to keep coming; and the reviewing press seems to keep finding much to praise in them:
''His heightening eloquence stamps him as a leader in music's humanistic revival - and has made him one of the hottest composers around,'' Michael Walsh wrote in Time magazine. ''John Harbison . . . is one of the more important American composers of his generation,'' says New Yorker critic Andrew Porter, writing in the Financial Times of London, adding that ''he is above all a composer of works that enshrine poetic visions in precise, lyrical, and beautifully wrought music.''
Harbison and his contemporaries have been criticized in various quarters for a nostalgic hankering after the Romantic era - and for abandoning the structural thinking spawned by Schonberg and his pupils Berg and Webern. But Harbison disagrees. ''I absolutely do not want to turn the clock back. I feel just as interested in structural issues as I ever have.''
Judging from his reputation as a serious musical thinker, he has rights to that claim. ''I would say that, when a composer gets so much attention from his contemporaries, it is for a reason,'' points out Elizabeth Larsen, composer-in-residence with the Minnesota Orchestra. ''People are studying his music, because it is worth studying.'' Larsen adds that the way Harbison reportedly got his assignment as composer-in-residence in Pittsburgh was that the orchestra's music director, Andre Previn heard his music on the radio and said, ''That's the composer I want to work with.''
A slender, quiet man with very clear, thoughtful eyes, Harbison slowly weighs and evaluates every question during an interview. His answers reveal a careful, wide-ranging, highly detailed mind. And he seems to approach a musical assignment, now matter how large, with the same modest care.
''I didn't set out to write ten symphonies at once,'' he says of his first symphony, alluding to the composer's problem of confronting a form that the titans have put their ten-league-boot stamps on.
And he says of his first string quartet: ''I wanted the piece to have its own character, not just to make a Grand Statement. I wanted it to have, not the sound of, but the direction of the Haydn quartets, particularly in the (1780s), just after Mozart.'' He adds that he was interested in exploring the character of a true string quartet.
''I wanted to respect the idea.''