... up to 400sqm, 2)voice clearly and use anti-feedback technology, 3)no string frequency. infrared wireless dual-band audio speaker system(ks668 ... turn of the teachers ,the transmitting distance can be 10m. 4.Away from the electromagnetic radiation and interference, create a healthy ... into infrared speaker system. Which saves money in the upgradation. 2. Has two group audio frequency inputs, a group audio ...