I have a pair of Avantgarde Acoustics Duo XD horn speakers, which have a sensitivity of 107 dB/W. . I have them connected with 10 m (30 ft) of unshielded untwisted pro speaker cable 2x 6mm2, running beneath the ground floor, because they are on the opposite side of the room to my equipment. Recently I noticed a high pitched hum through the tweeters when a mobile phone was lying on the couch, probably about 1-1,5 meter (3-5ft) from the cable under the floor. Anyone any suggestions on how to get rid of this noise?