Directivity factor (of a sound source)
The ratio of the square of the radiated free field sound pressure at a fixed point on the principal axis to the mean square pressure averaged over the surface of a sphere passing through the fixed point and concentric with the source.
Reference: I.E.C. (modified)
Please note that this is the term as it stands in the original IALA Dictionary edition (1970-1989)
Celsius (previously Centigrade)
International Association of Marine Aids to Navigation and Lighthouse Authorities - AISM