ASDIC Question

[Sir Big Jugs]

Hi,
I'm currently reading a Swedish book on U-boat warfare in WWII called "Havets Vargar" (Wolves of the Sea) by Michael Tamelander and Jonas Hård af Segerstad.
Without going into too much detail about the book itself, I found the following (very roughly translated) footnote interesting:

An ASDIC pulse can't be picked up by the human ear, but a small "click" sound might be heard inside when it hits the hull of a submerged submarine. The well known 'PING' from many U-boat films could actually only be heard by the escort's ASDIC operator.
The ASDIC receiver converted the returning ultrasound echo to a sound at ~1000Hz (where the human ear is most sensitive) in order to make it hearable to the ASDIC operator. This converted sound was the charasterictic 'PING'.

Can anyone confirm/deny this? The authors both have a reputation for being quite meticulous in their research, and having read other books by them I hold them quite high in this regard.

On the other hand, I would be surprised if this potential mistake hasn't have been caught earlier, so they're probably wrong.

Hope someone knows the answer!

Regards,
-SBJ