this is the other thread where this question was discussed:
http://www.subsim.com/radioroom/showthread.php?t=111277
There is no right or wrong answer to this question within the limit of the very simple sound modeling implemented in SH4. I have left it as is, but its up to the personal preference of each player.
You should also note that even though you cannot hear contacts at PD, your sonar man will still report contacts, so you are not totally deaf.