Or do they?
The Felsenreitschule in Salzburg, which hosts a number of Festival operas, is just one of many venues to employ the Vivace acoustic enhancement system.
An article by the suppliers, Salzbrenner Stagetec, describes how the system works:
"Stated simply, the best known purpose of a Vivace system is to simulate or complement a desired acoustic environment. This functionality allows you to change certain acoustic properties of a venue by electroacoustic means. To this end, Vivace uses a number of techniques in order to create convincing virtual acoustics that sound authentic to the audience."
"At the Felsenreitschule, depending on staging, two to four microphones are used to pick up the onstage sound. A NEXUS digital network routes the signals to the Vivace system. The Vivace mainframe performs real time analysis, processes the signals with a convolution algorithm and outputs them to a sound reinforcement system with digital amplifiers and speakers positioned precisely in the hall. Gunter Engel calculated a total of 54 locations at which speakers were hidden in the ceiling, the side walls, rear wall, stage and orchestra pit. These speakers output the supplementary audio computed by the Vivace system to enhance the natural sound. The large number of speaker locations helps prevent acoustic localisation of individual speakers. There are also eight mobile loudspeakers which can be placed to suit the current production."
"Things are completely different in a very recent, temporary Vivace installation. Franz Lehar’s opera, The Merry Widow will be staged at the prestigious Palais Garnier of the Opera National de Paris. For the performances, which will take place between late February and early April, the management hired a Vivace system which improves intelligibility. In addition, it enables acoustically correct localisation of soloists on stage. With Vivace, there are three basic ways of positioning a source: using a computer mouse or — as in Paris — a new touch-sensitive screen, to move the actor manually around within a computer model of the hall, using the innovative direction pointer which is pointed manually in the real room to the desired direction, or automatically in combination with an external tracking system. This makes it possible to achieve a correct acoustic representation of the actor not only across the entire width of the stage but also with regard to the stage depth. "
If you've ever wondered how acoustics can be manipulated without the use of cumbersome microphones or obvious speakers (and in some cases without the singers' knowledge), the whole piece is well worth a read.