Nominal range of a sound signal
The distance at which, in foggy weather, an observer has a 90% probability of hearing a sound signal when he is situated on the wing of a ship's bridge in an ambient noise level (as defined by the I.A.L.A. sub-committee on the rated range of sound signals) which is equal to or greater than that found on 84% of large merchant vessels, propagation between the sound signalling apparatus and the observer being effected in relatively calm weather, with no intervening obstacles.
Please note that this is the term as it stands in the original IALA Dictionary edition (1970-1989)
Level (technician training - WWA)
Latitude (in degrees)
International Association of Marine Aids to Navigation and Lighthouse Authorities - AISM