From the article there is not much info about what the subs were doing together. If they really were on separate missions and submerged when they hit each other then somebody will get fired over that. A submarine is a very stealthy boat. They can be very quite and can actually hide in water. Sound is bent and deflected in water. It creates blind/deaf spots in the water. There are layers in the water based on temperature and salinity of the water. Sound travels differently in each layer. One sub can be inside a layer masking its sound and another can be just outside the layer and cannot hear the other.
Deep water has lower background water than shallow water. Water moving through a straight is very loud compared to water in the middle of the ocean. Nuclear submarines are very quiet and unless they were running at speeds above say 15 knots they would be very stealthy. Other than the screw making noise the loudest part on a nuclear sub are the circulation pumps on the reactor. I could go into some technical stuff, but I won’t. The pumps will make noise if they are used, so natural circulation is used up to a certain power level.
Think of both subs as two people with blindfolds on. One has their I-pod playing music in their ears. The other has the regular sounds around them with maybe some loud music played from time to time. They start walking around in an empty gym for an hour. They may or may not run into one another, but it is possible. So, are you confused yet? It is possible for two subs to run into one another. The odds are rather high it can happen, but it can.
