4.2.6.3-16 Pulse Codes

No more than three pulse repetition rates should be used for coding purposes.

Additional Information:
Repetition rates should be between 1 and 8 pulses per second, since faster rates may not be perceived as pulses. Repetition rates should be sufficiently separated (e.g., differ by a factor of 2) to ensure operator discrimination. Sounds with the same temporal pattern, including signals with similar duty cycles (on-off times), may be confused, despite having very different pulse speeds (i.e., periods). Such signals are therefore more appropriate for coding the level of urgency of a condition than for indicating different types of conditions.