Outdoor speaker on wall speaker series polk audio atrium system 2 way system principle passive frequency response 60 25000 hz noise level 90 db impedance 8 ohm recommended amplifier power 10 100 power supply passive speaker enclosures closed wall mounting possible number of tweeters 1 treble chassis 19 mm tweeter material tweeter aluminium number of midbass drivers 1 bassmidtone chassis 127 mm midbass driver material woofer midrange polymer