1.Intrinsic featuresU.K.
1.1.Acoustic signals must:U.K.
(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;
(b)
be easily recognizable, 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.