There's specialzed hydrophones to locate the pingers.
http://www.phnx-international.com/sp..._SpecSheet.pdf
Deptrai, I hope they're using the "specialized hydrophones" to locate the pinger. The French nuclear sub, looking for AF447 didn't! That's why they went back to review the sonar tapes of that sub. Don't know if the dynamic range of the tapes would have recorded 40khz anyway.