By referencing ULB, I'm assuming you mean underwater locator beacon. I think the US Navy decided many years ago that any frequency below about 500kHz belonged to them for LORAN and I don't think they've given them up (military brass are extremely reluctant to revisit the reasons for their rules - it could be embarrassing to the officer corps). Very low frequency acoustic signals travel long distances in seawater (which is why the USN snaffled them) but changing protocols for the non-revenue generating costs of FDR's and CVR's would not contribute to shareholder value for the airlines, to which their boards are legally required to adhere. The financial advisors that manage your retirement plan would be horrified to recommend that airlines spend money on safety efforts which are non-revenue generating investments which would decrease their fees (the ones they never told you about). It's unlikely that ultra low frequency beacons will appear in my lifetime.