The building's council chamber, or Rådssalen, has been the seat of Stockholm local government since 1923, with seats for 108 councillors and a balcony that can accommodate 200 members of the public.
Although the council chamber had an induction loop, it was proving very troublesome with less than adequate coverage, so Stockholm-based audio specialists Sennberg AB were asked to design and install a replacement.
"I was asked to do an analysis of the existing loop system because there had been a lot of complaints about it," says Sennberg AB's Johnny Karlsson.
"It had been installed as two 13 x 13m arrays, as the loop amplifiers used by the installation company couldn't supply the impedance required for the necessary coverage of 13 x 26m. The result was a three metre wide corridor down the middle of the room with insufficient field strength, which meant the council president - who uses a hearing aid and who sits at the rostrum right in the middle of that strip - wasn't receiving any signal.
"To solve that problem they had installed a small perimeter loop just to cover the chairman, but this then created problems for other delegates close to the rostrum and didn't solve the problem further down the room. The same problem was found for delegates close to one of the long sides, because of too big a gap between the loop and the wall. All-in-all a very unsatisfactory solution."
To solve the problems, Karlsson recommended completely removing the existing induction loops and installing a new Ampetronic low-loss system, using two ILD1000G loop amplifiers and an SP5 phase shifter / metal loss corrector.
"We designed and installed a low-loss array because there are other rooms in the building which use, or plan to use, loop systems and the spill from a perimeter loop could interfere with them," he says.
The system also had to be carefully designed because the building is K marked - the Swedish equivalent of the UK's listed building status - but installation was straightforward, as it was carefully arranged to coincide with the installation of a new carpet.
"All the furniture was moved to one half of the room, the old carpet was removed, we installed the loop in that half and the new carpet laid. Then all the furniture was moved to the other half of the room and the process repeated," adds Karlsson.
"The loop is linked to a line output from a Televic EQ unit, which acts as a mixer for all of the councillor microphones. The Ampetronic system has already made a huge difference, with many councillors commenting on the far superior performance over the old system."
(Jim Evans)
