Yes the 3:15 rule and the nomographs make speed calculations by hydrophone less necessary, but I still use it often as a second opinion. And it just feels so cool to be counting those chug-chugs with a running timer
But still the sound trainer and documentation by Raduz is about so much more than just speed calculation. It is really very informative, though a bit too nerdy for some perhaps.
If you take time to do a search on this forum there are some brilliant threads about using the hydrophone to calculate range, bearing and speed at the same time, and to actually sink ships without ever seeing them.