Soprano Noëmi Nadelmann takes advantage of a rehearsal interruption to blow a kiss to the acoustics in the concert hall of Lucerne's culture and convention centre.
It's not an unusual gesture in a contemporary architectural gem that has been upstaging the central Swiss town's famed medieval attractions since it opened a few years ago.
Most people booking a backstage tour of the centre designed by renowned French architect, Jean Nouvel, do not get to sit in on rehearsals – this time by Germany's Baden-Badener Philharmonic Orchestra, with Nadelmann and tenor Zoran Todorovich.
But the acoustics are, along with the much-praised architecture, the star of the tour. The Concert Hall has ideal "shoebox" dimensions complimented by an acoustic canopy that can be raised or lowered at will and an echo chamber that can increase volume by up to a third.
"Just imagine that in the 18th century a Mozart symphony was typically performed in a castle near Vienna for a few hundred people," explains Pirmin Zängerle, head of acoustics and events at the centre.
"And now symphonies are played in halls with seating for 2,000 or more, so you need to adapt the hall for the type of musical repertoire, and the variable acoustics we have here provide musicians with the sound they need."
The spatial volume is "adjusted" by opening or closing any number of concrete doors, each weighing up to eight tons. They open onto a sensual red backdrop.
Entering the echo chamber, guide Denise Fehlmann recounts showing the space to British conductor, Sir Neville Marriner, who told her: "There's no equivalent concert hall like this in London and if I could I'd put it in my pocket and take it with me wherever I go."
The accolade for putting Lucerne on the world map for concert halls belongs to American theatre designer and acoustic engineer Russell Johnson.
Backstage visitors learn how Johnson and Nouvel spent a year waging a friendly war by fax machine before they could agree on the pattern of the structured surface to be used on the doors. Johnson wanted the plaster reliefs to provide a further acoustic advantage by multiplying the reflected sound while Nouvel was worried about appearances.
Nouvel had to make few compromises. Forced to abandon his initial plans to build a concert hall in the shape of ship going directly into Lake Lucerne, he instead chose to channel the water into the building and construct a flat roof projecting over the lake.
The Concert Hall and the centre's two other elements, the Auditorium and Lucerne Hall, represent three ships in a dock, with the Concert Hall exterior resembling a boat hull or – with a little imagination – the smooth, polished wood of a giant contrabass.
Real life postcards
Large rectangular windows in the centre's restaurants, cafés and concert bars are Nouvel's real life postcards, creating a frame for Lucerne's picturesque old town and mountain-fringed lake.
At dusk, the lights of the passing boats reflect off the underside of the aluminium roof.
The backstage tour over, the night begins. Concertgoers take their places at tables in the elegant dining room and trendy lakeside bar, enjoying the panoramic view of Lucerne's illuminated grand hotels lining the opposite shore.
The musical repertoire on this night will include Puccini, Verdi and Bernstein.
"The concert which made a deep impression on me was Rachmaninoff's piano concerto," Fehlmann explains, whetting the appetite of her group.
"I was sitting on the third balcony looking into the reverberation chamber. I could see how it was painted. I could see the lights. I was listening to the music and I felt that the eyes and ears came together and I could see and hear all the things I had learnt about Russell Johnson's acoustics and Jean Nouvel's architecture.
"They came together in a symphony."
swissinfo, Dale Bechtel in Lucerne
Lucerne's culture and convention centre (KKL) offers a range of guided tours to the public.
Standard tours include a look behind the scenes at the technology responsible for the Concert Hall's state-of-the-art acoustics, and provide interpretation's of Jean Nouvel's elegant architecture.
On its website, the KKL offers a limited number of packages including concert tickets, dinner in the centre's RED restaurant and one night accommodation in a four or five star hotel.
Built at a cost of SFr210 million ($173 million) and completed in 2000, the construction of Lucerne's world-class culture and convention centre was a bold achievement for a town with a population of only 60,000.
It was designed by French architect Jean Nouvel. The acoustics of its Concert Hall was engineered by American expert, Russell Johnson.
The key to the excellent acoustics is the hall's echo chamber and acoustic canopy. 50 concrete doors can be opened onto the chamber increasing the spatial volume by 6,000 cubic metres to 25,000 cubic metres.