When the diesels are being run is called low discretion. Ways to improve discretion include choosing nuclear propulsion, using better batteries and/or using Air Independent Propulsion (AIP).
Australia's future submarine debate appears to involve unquestioned assumptions of rejecting AIP in favour of a new fairly untested battery type, Lithium-ion Batteries (LIBs), that have never been used operationally.
Choosing Japan will involve choosing LIBs and rejecting AIP because Japan is specifically rejecting AIP.
Germany (TKMS) and France (DCNS) may adopt or retain AIP and use the old lead-acid battery type or, if encouraged, adopt LIBs.
To rejects AIP while adopting LIBs is a $Multi Billion dollar gamble for Australia.
This makes it all the more important to revisit (now and then) the relatively arcane subject of AIP - below .
AIP Section Loudness