5.54 in the Radio Frequency Table Allocation of Ireland.

Content Type
ITU FOOTNOTE
Title
5.54
Description
Administrations conducting scientific research using frequencies below 9 kHz are urged to advise other administrations that may be concerned in order that such research may be afforded all practicable protection from harmful interference.
Source
https://search.itu.int/history/HistoryDigitalCollectionDocLibrary/1.43.48.en.101.pdf