Each audio signal should be unambiguous and easily distinguishable from every other tone in the control room.
Additional Information:
Current sound generation technology allows the design of alarm signals that make better use of the operator's ability to process audio information. It is possible to design signals that are not only more discriminable from one another than are conventional signals, but also have the potential to carry more information. Signals should be composed of unique combinations of tone pattern and frequency. See also Guideline 4.2.6.3-8.