If I recall correctly the maximum Noise Reduction Rating (NRR) for earplugs and earmuffs is around 30db. You can combine the two for a slight increase in hearing protection but you still hit a limit because of bone vibration.
Is there PPE out there to go even further beyond this? Where would it be commonly used?
Yea, that “low frequencies” is rather ambiguous.
I could see over-the-ear headphones being better at “lower” frequencies than in-ear, both from material absorption and speaker size. Every ANC pair I’ve had, of any style, was pretty good at nearly eliminating the higher frequency noise while flying (engines, airflow noise, etc), and almost eliminated voices.