According to the introduction of ISO 8201, there has been growing interest in in the development of an international audible signal which, when heard would unequivocally mean "evacuate the building immediately"
In searching for an appropriate audible signal, it was considered that levels of background noise and frequency patterns are so variable, particularly in the industry, that no signalling device would be able to “penetrate” all background noises and frequency patterns.
A temporal evacuation pattern typically consists of three audible pulses (T3) with a 0.5 second ON phase and a 0.5 second OFF phase. The three pulses are then repeated after a 1.5 second delay OFF, and should repeat indefinitely in this sequence or for a minimum of 180 seconds. In certain environments where the ambient (A-weighted) sound pressure level exceeds 110dB, additional visual and/or tactile signals shall also be provided to complement the audible warning signals within the building.
An additional advantage of using a standardised temporal pattern as the distinguishing characteristic of an audible emergency evacuation signal is that the temporal pattern can be applied to visual and tactile signals to aid those who have impaired hearing.
Australia adopted the Temporal "T3" tone from 2003 and makes the tones described in Australian Standard AS2220 redundant in for new buildings.