What’s the frequency, Karlheinz?
The occasion was part of last year's Salzburg Festival, and a performance by the Stadler Quartet of the pioneering German composer's Helicopter String Quartet. Each of the four string players was to perform from within an airborne Black Hawk helicopter (courtesy of the Austrian Army), while the audience sat in the all glass Hangar 7, the home of an Austrian aerobatics team. "We tested seven or eight microphones for the violins with Peter Heckler, the sound director, before we found one that would work in these conditions," explains Wolfram Kolb, project leader for Neumann&Mueller of Munich who provided the sound reinforcement system tor the event. "In the end a relatively inexpensive piezo pick-up, a Big Twin, proved best. Although you only get about 60dB off a violin we had no problem."
Kolb was fortunate that background noise from the helicopters was a planned part of the performance - in fact, Stockhausen's score demands that the pitches played by each of the performers matches the pitches they perceive the rotors to be generating. "We even put an MKE 2 on the chopper's nose to give a more interesting sound." The two devices plus a vocal mic on each aircraft transmitted signal back to the hangar via a powerful transmitter system. But it was the hangar venue that presented the real challenges. "Not only is the curved roof all glass, but the floor is a very hard concrete; it was all reflection and reverberation, around five seconds in the low/mids, 2.5 seconds for the higher frequencies. Therefore you need loudspeakers with a very well defined directivity both vertically and horizontally." Plus Stockhausen's composition required a spatial awareness of the four instruments, so four separate PA systems were called for.
"We chose the new d&b Q Series line array system for several reasons," says Kolb. "As long time users of d&b equipment we knew that pattern control would be consistent, especially down in the critical lower frequencies. But Q offered us the enhanced benefits of line array propagation, but with a unique pattern. For aesthetic reasons a line array was really the only option; with an audience space 40m wide and 55m deep a point source system would have required delays every 15m, and the roof couldn't support that much equipment." The Q1, the principal component of d&b's three box Line system, has a 75° horizontal by 15° vertical pattern. "I know of no other cabinet of this type with such a wide vertical spread, this meant we could use far fewer cabinets to cover the 2,000 members of the audience. Plus the box only weighs 22kg; weight is a critical factor when you're hanging off a glass roof."
Despite the three flight rehearsals being conducted under less than favourable conditions - the builders were still putting the final touches to Hangar 7 -the concert was deemed a great success. "There was some small problems with a party group at the back of the hangar who chatted throughout, and of course there were still scheduled flights taking off from Salzburg airport. But the PA performed exactly as predicted by the Q-Calc software, and the acoustic impression matched exactly the projected images of the tour violinists up on the video screens. For a new system with which we had comparatively little experience, this was a very satisfying experiment, in every sense."
Text: Daniel Diver