1. Intrinsic features
1.1. Acoustic signals shall:
(a) have a sound level which is considerably higher than the level of ambient noise, so that it is audible without being excessive or painful, and
(b) be easily recognisable, particularly in terms of pulse length and the interval between pulses or groups of pulses, and be clearly distinct from any other acoustic signal and ambient noises.
1.2. If a device can emit an acoustic signal at variable and constant frequencies, the variable frequency shall be used to indicate a higher level of danger or a more urgent need for the requested or imposed intervention or action in relation to the stable frequency.
2. Code
The signal for evacuation shall be continuous.