The passive hydrophone hears a braod band spectrum of sound waves. Any kind of noise was detectable, of course dependent on own speed as of distance and level of the noise source.
The active hydrophone (ASDIC/Sonar) emitted a 'ping' - a sound bubble with it's perimeter expanding 360°. Any object hit by this perimeter reflected the ping and partially back to the emitter.
The active hydrophone had special (passive) receivers for the own ping frequency. Different models were developed. Common was a mixture of a pretty 'broad' receiving device and a small extra sensitive search device. Both devices were turnable and searched for the ping echoes. If a U-Boot was hit by a ping perimeter it reflected some of the sound back to the emitter/receiving device. If the broad search receiver was turned into the direction of the U-Boot it could receive this echo under given circumstances. This echo was transformed into an electric signal and visualised for the ASDIC/Sonar operator.
The broadband passive only hydrophone would have heared a very loud echo of the ping, therefore it wasn't used while active ping operations.
The extra sensitive search device was able to receive the echo only in a very small cone - therefore ideal to determine the correct course to the target. Even the depth of the object could be detemined with this 'search beam' (it wasn't a beam at all), because the listening cone was adjustable (up to a certain degree) in search height (or better search depth) too.
The distance to the object was measured by the time it took a ping to travel back to the receiver.
The U-Boot crew never knew if they were detected after being hit by a ping until the actions of the search dog were interpretable.
This is indicated by "... is pinging US!" They go for US right now. In the German version they say "We are detected!"
As I understand it, in SH3 the passive hydrophone stays operational all the time and it works perfectly even after a depth charge run destroyed the usable audio conditions completely for several minutes.
There is only a fixed deaf spot in the wake of the vessels due to their own propellor noise and the hydrophone doesn't work as long as the attacker is at high speed (to avoid it's own depth charges).
The ASDIC can't cover the area under the ships hull and a short deep zone forward as the complete 90° to 270° sector aft. Under good conditions the ASDIC will detect you within ~1800 meters if you show your broad side or your upper deck and the dive planes to reflect the ping.
|