what about this one:
http://www.liverpoolmuseums.org.uk/m...boa/asdic.aspx
btw, i read in a book from a german ww2 sonar man that
they picked up the asdic sounds first in the hydrophone,
then after some time it was heard in the whole boat.
he also described the sound as a high-pitched trilling
(in the hydrophone).