ul·tra·low frequency 
(ŭl′trə-lō)
(ŭl′trə-lō)n. Abbr. ULF
An electromagnetic wave whose frequency is less than 3,000 hertz.
- work for hire
- work function
- work hardening
- workhorse
- workhouse
- work in
- working
- working capital
- working class
- working-class
- working control
- working day
- working dog
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- workingwoman
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- work like a charm
- workload
- workman