Acoustic tags that transmit pings from 30 khz to 50 khz are used to track various species of large marine animals (i.e., sharks, whales, sea turtles, tuna, seals, etc). The ping interval is typically 1 second.
There's an interesting
opinion piece in the Malaysian Insider by William Meacham, in he which claims the pings heard were not from the ULBs, but more likely from a marine acoustic tracking pinger.
The article is footnoted with his sources from reputable authorities. For example:
"Dr David Gallo, a senior scientist at Woods Hole and co-director of the successful search for the wreckage of AF447, wrote in an email to me: 'I don't know any underwater acoustic people that think the pings have anything to do with the plane.'”
Definitely worth a read.