I think its real hard to have sonar contacts on the surface the allied destroyers didnt hear uboats when they made surface attacks, why not ?

cheers
NoLine

The US fleet boat type sonars have the following 2 parts:

1. T-Bar
Location: Mounted on top of the fore deck
Freqency: Low freqency (human hearing)
Range: Long
Sonar Abilities: Listening only

2. Balls
Location: Mounted below the hull at the fore torpedo room
Freqency: High freqency (ultrasonic)
Range: Short
Sonar Abilities: Detecting ultrasonic enemy transmissions, pinging

The 3rd hydrophone sensor is most likely the ST radar which is basically a range finding radar mounted to a periscope to check range on a sighted target. Or it might be somehow related to the ping ability of the sonar in a "virtual sensor" way.