When information is coded by the pitch of narrow-band signals (i.e., tones), no more than three frequencies should be used.
Additional Information:
The frequencies should not be in a ratio of 2:1 with one another, since it can be difficult to identify pitches an octave apart. Although some sources recommend that no more than five separate frequencies should be used, operators may not reliably distinguish among more than three pitch codes. For critical alarms with differing response requirements, the more conservative guidance should be followed. If more than three critical alarms are to be coded, it is preferable to combine pitch with another dimension to create more distinctive signals. See Guideline 4.2.6.3-13.