"QUIET PLEASE" demands a large sign on the stage of Lisner Auditorium. But there is hardly any need. Virtually all that could be heard in the hall yesterday afternoon was a Steinway piano playing Bach preludes and fugues, and occasionally the voice of Raissa Tselentis Chadwell announcing the next performer in the Johann Sebastian Bach International Competitions: "This will be contestant number 13. She will play--or he, I don't know--Book II, Preludes and Fugues 4, 5 and 8."

Silence and anonymity: both motifs are embodied in a strip of carpet that stretches from backstage to the piano stool so that the judges will hear no footsteps, which might tell them whether the unseen contestant is male or female. Some music competitions publish programs with pictures and biographies of all the contestants. Not this one.

The Bach Competition, which attracts pianists aged 20 to 40 from all over the world, is one of Washington's unique musical activities. It is unique mostly because it is conducted in an atmosphere of deep secrecy--a shield against human frailties and the possibility of corruption.

This year there are three judges, two men and a woman. They are as anonymous as the 23 contestants in this competition, which is marking its 25th anniversary, though an alert music-lover might be able to recognize some of their faces. Their identities will be announced after the winners are chosen tomorrow. The prize fund totals $9,000, of which the winner will take $4,000 plus various concert engagements, including a two-month tour of Germany.

The judges sit at stage left, separated from the contestants by large plywood screens. Each has his or her own table with a water pitcher, pencils, copies of "The Well-Tempered Clavier" (which all the contestants are playing) and a score sheet on which, from time to time, marks are entered. The judges' three tables are spaced far apart; they are not allowed to communicate with one another, with the audience or--naturally--with the contestants.

When an anonymous young pianist has completed the prescribed three preludes and fugues, he or she leaves the stage silently. There is no applause--an attendant at the door explains politely that it "is not encouraged." This competition does not want audiences (which could be packed with friends and relatives) trying to influence the judges.

The judges write intensively for a few minutes after each performance. When they finish, one of them rings a little bell, and an attendant comes around the screen to pick up the three marked-up score sheets and leave three fresh ones. The judges are given no opportunity for second thoughts.

Yesterday was the first elimination round of the competition, which will continue today and reach its finals tomorrow afternoon. A high percentage of the audience was made up of other contestants, nervously studying their music and perhaps picking up last-minute ideas on interpretation. Other contestants paced up and down in the lobby, trying to ignore the piano sounds that came faintly through the doors--which were closed and blocked while performances were in progress. "I'd go crazy with nerves if I sat in there and watched the others," said one anonymous young man.

In past years, all the contestants have been expected to play exactly the same music--the Goldberg Variations, for example. This eliminated one of the wild-card factors often found in music competitions--the fact that choice of repertoire rather than sheer performing skill might influence the judges. Statistically, piano competition prizes seem to go more often to people who play Liszt or Prokofiev--difficult, flashy music--than to those who play the quiet, subtle music of Mozart.

In this competition, everybody plays Bach. This year, the repertoire requirement was loosened slightly. Contestants were allowed a choice; they could prepare either Book I or Book II of the "Well-Tempered Clavier." In the first elimination round, those who chose Book I played Preludes and Fugues 3, 4 and 5, while those who chose Book II played Nos. 4, 5 and 8. This saves the judges from having to compare apples and oranges. It also demonstrates, for those who sit in for an afternoon of the competition, how many different ways the same music can be played.

With its built-in safeguards--undoubtedly the most stringent of any music competition in the world--the Bach Competition has eliminated the kind of favoritism, politicking and backstage deals that rumor sometimes associates with other music competitions. There are still elements of chance; a pianist may be having an especially good or bad day when he goes on stage; a lapse of memory can spoil a technically fine performance, or the music chosen may not be the best to show off a particular talent.

But when the winners of this competition are announced tomorrow evening, the participants and audience can be reasonably sure that the judges' decision was based exclusively on the sounds they heard coming past that big, plywood screen.