The technical term is "sound enhancement". In opera performance, it means the use of concealed microphones - on the stage itself, or hidden within the singers' costumes, or both. What reaches the ears of you and me in the audience is, therefore, an amplified version of what some or all of the singers are singing - and, sometimes, also of what the orchestra is playing. Everyone in the opera business knows that it happens, and that it has gone on for years. And yet, to the best of my own knowledge, no major performer in the world of opera has been prepared to talk about it in public.
Enter John Tomlinson, who is probably the finest operatic bass these islands have ever produced. He's a master of some of the great roles in the repertory, notably Wotan in Wagner's Ring Cycle and Hans Sachs in Die Meistersinger von NÃ¼rnberg. He sings these and others on the stages of the world's major opera houses, from Bayreuth and Covent Garden to New York's Metropolitan Opera House. Why should a highly successful and much-respected artist, at the height of his career, suddenly choose to go public on a subject that everyone else in his profession seems to regard as taboo?
"I think it's time that those of us involved in opera began to be concerned about the future of the art form as we know it," says Tomlinson. "The art of singing opera is the art of singing without a microphone. It's about how you project your voice from the stage, over an orchestra, in a natural acoustic, to those listening in the auditorium. It's a form of drama, and this is one of the ways you make it happen."
Many would say, I suggested, that this is a purist view in a world now geared to greater access to the arts, and to the commercial imperatives that go with it. Tomlinson, on the contrary, insists on a balanced view of the situation.
"I'm not in any way against the use of microphones in the right context. Look what Sinatra could do with one. They're accepted practice in musicals, too. And why not, if audiences are happy with this? I went to see Chicago recently, and I had a great time." Even with the amplification that contributes to the show's famously pulverising decibel level? "Oh yes," says Tomlinson. "Besides, it's a good way of cleaning the wax out of your ears."
What about the growing public appetite for staging opera in large, round arenas such as Earl's Court or the Albert Hall?
"Using microphones makes sense here as well. You need them so that everyone can hear what's happening. But I don't think it's a purist approach to insist that opera, performed in an opera house, is about something different. At the moment, most opera performances in most opera houses have no amplification at all. But my concern is that there is an ever-greater temptation to use it, because it's always going to be there as an option.
"The process is very seductive. You sing quite gently into a microphone, and this immense sound comes out all around you. But performing opera in the theatre shouldn't be about this kind of security. It's about relying on your technique and training, and putting yourself totally on the line. There is risk and vulnerability. Even fear. Overcoming these things, and turning them into something that grips your audience - it's what you're there to do. And it's what the public has a right to expect. As time goes on, with microphones being relied on more, there may be fewer singers who are able to do this. You can't put a timescale on it, but the problem is increasingly there."
The catalyst for these thoughts was a speaking engagement at a private dinner given for Tomlinson earlier this week by the Royal Philharmonic Society. A couple of weeks beforehand he had been singing the role of Mephistopheles in Gounod's Faust at the Bavarian State Opera. "I was recovering from flu, and was trying to rest properly between performances. There I was, holed up in a hotel room in Munich, and wondering what to put in this speech I had to write. The whole issue of voice enhancement is something I'd been thinking about for a while. I decided it was time to try to start an honest debate about it."
His own vocal equipment means that Tomlinson is in prime position to do this. As he puts it: "I've got a strong voice". Tomlinson's bass is a phenomenon - huge, rich, sonorous, and at the same time superbly focused. It is difficult to think of any voice that's less in need of any kind of electronic aid. And yet he is used to finding himself in a situation where this happens anyway.
"As far as I know, it isn't something that's ever mentioned in singers' contracts. We're hired hands, we turn up and do what we're employed to do. If I were to ask anyone at the theatre whether or not the performance is being miked, I'd be up against a wall of silence. Apart from the New York City Opera, which is open about it, no one else has had the courage to own up, as far as I'm aware."
The script of Tomlinson's speech to the Royal Philharmonic Society comes up with the intriguing idea that the extent of amplification in opera performances could be graded. Category 1 would be where it is used only as a way of realising special effects, such as offstage singing. As for arena opera, under Category 5, everything would be honestly and blatantly miked. Between these, Tomlinson suggests, come the areas ranging from Category 2, where subtle amplification is used to help overcome a dead acoustic, through Category 3, where spoken dialogue is amplified so that it can be easily heard, to Category 4. This is the one which Tomlinson views as "slightly more sinister".
"I've heard performances where, on the same stage at the same time, some singers have been miked and others not. And others where I thought the orchestra had been amplified, and the stage not. I'm not saying that this is necessarily wrong. Some American opera houses seat an audience of 4,000. Their administrations need to fill those seats, and if people sitting at the back can't hear, they won't come again.
"But what I'd like to see is more honesty about what audiences everywhere are actually hearing. For instance you could list something along the lines of my five categories in the programme, and say which particular one was being used in that performance. That, in itself, would be an incentive to keep the figure as low as possible.
"It's the element of confusion that's the problem. In the mid-Seventies I was singing Leporello in a performance of Don Giovanni with the English National Opera at the Coliseum. My agent was there, and he overheard somebody saying, 'But why is that Leporello using a microphone?' So much for the advantages of being gifted with a magisterial voice, I suggested.
"Yes," says Tomlinson, with a rueful grin. "But this was 25 years ago. And it still sums up the situation today. The audience doesn't have a sure way of knowing what it's getting."
John Tomlinson sings the role of Hans Sachs in Wagner's 'Die Meistersinger von NÃ¼rnberg' at the Royal Opera on 16, 19, 25, 27 May (information: 020-7304 4000), and in a concert performance at the Albert Hall on 31 May (020-7589 8212)