We had SBA at Ternhill when I did basic training on the Piston Provost.
My recollection was that the audio zone to the left of centreline was an "A" zone (._) and to the right was an "N" zone (_.). The centre line was, of course, a constant tone if you were lucky enough to stay on it.
The outer marker was an audio "Onk, Onk, Onk" and the inner marker was an audio "Peep, Peep, Peep".
Once you got the hang of the thing, it was quite accurate. The only other installation in UK at the time was at Stornaway so that wasn't a hell of a lot of use in the event of a diversion!