A ``busy'' signal, when used by a public coast station in accordance with the provisions of Sec. 80.111(d), must consist of the transmission of a single audio frequency regularly interrupted, as follows:
(a) Audio frequency. Not less than 100 nor more than 1100 Hertz, provided the frequency used for this purpose will not cause auto alarms or selective-ringing devices to be operated.
(b) Rate of interruption. 60 times per minute 10%.
(c) Duration of each interruption. 0.5 second 10%.