Well, technically that's what the autocrew does and also what manual classification is about too. Problem is that when you only have a very faint signal its usually not enough to tell what the contact type is... just that theirs something there.
For example, the lowest frequency is usually detected before the other sound frequencies (because low frequencies travel further in water)... in the game its usually the 50 or 60Hz signal. However, just about every ship in the game has either a 50 or 60 Hz signal as its lowest frequency (its related to the electric power frequencies used on the ship...) so this signal in and of itself is not enough to tell the ship type and you'll need to get more frequencies to classify the unique frequency pattern of a specific type.
To see what I mean try Heinrich Liebe's stellar and generous Sonar Profile Utility
http://www.subsim.com/radioroom/showthread.php?t=82476
(note: I think the Oscar II frequencies are wrong in the database.