Bookshelf speaker satellite speaker series dali zensor system 2 way system principle passive frequency response 53 26500 hz sensitivity 87 dbmw noise level 106 db impedance 6 ohm crossover frequency 2900 hz recommended amplifier power 25 100 power supply passive colour white housing surface veneer vinyl housing material mdf enclosure speaker enclosures bass reflex magnetically shielded removable speaker cover wall mounting possible number