Sonar tech here, this was true 20+ years ago but new torpedo and sub blade configurations are not only highly classified but any nuclear powered subs no longer even use blades to counter this exact problem. Only old russian and N. Korean subs can be identified this way anymore. Even when I was active duty 10 years ago china and us subs were basically silent and torpedoes surround themselves with air bubbles so the acustics dont really escape. You know something is coming at you fast but that it