there is an audio tone present on the cable, between 500 and 3000 hz, when i use my inductive amplifier (tone tester) i think im getting alot of interferance from <500hz, but i haven't got any test equipment so its hard to tell but it sound a low tone if you know what i mean.
my off the cuff research & testing so far consists of this...
i found this video on youtube
YouTube - Hearing Test
it has tones on it, when i play these through my headphones, i use this inductive amplifier **broken link removed**
i can clearly hear the tones when i turn up the volume on the pc but in reality the tones are not that loud, i think about 20db, i need to amplify them some how and filter out the tones below 500hz
hope this explains abit more