Correct me if I'm wrong, but I'm nearly certain the OP was referring to the multiple scenes in The Hunt for Red October where the Russian sonar officer calls out torpedo status from "Torpedo In The Water...."
It's entirely based on sonar and further the sonar op's skills, in the Russian's case the Sonar operator is an officer, whereas on US nukes (and on skimmers as well, basically fleet-wide) the sonar technicians are highly-trained, highly-skilled, enlisted professionals. Anyway- in the case of TH4RO, in the torpedo scene, it's the Sonar Officer listening to what's going on and calling it out, based on his (aural) observations:
"Captain, Sonar, we have just been overflown by a low-altitude multi-engine turboprop!"
(Captain Raimus orders it "put on audio" so the entire control room, and incidentally, the audience, can hear the Sonar Officer's earphones -- this is totally legit and every famous ship captain from Bligh to Tom Chandler would totally do the same, let's pipe the sonarman's ears for all to hear. 'Hey Crew, we're getting torpedoed, and *THIS* is precisely what it sounds like. Remember it if you don't die during the explosion!!")
"Port transients, close aboard" -- ie, the Sonar Officer is detecting loud, obvious sounds, aka acoustic "transients" 'close aboard' aka motherfreaking near the submarine, port being, left side. This is a little odd, as I'm nearly certain the torp comes in on the starboard side later on. Anyway. He's continuing to listen through his sonar arrays, and detailing the action through the hydrophones.
"Water entry of small objects" -- this coincides with the sound of acoustic tones, presumably, active sonar pulses. We know this only means.....
Executive Officer Borodin: -- "Sonobuoys" -- yeah no $h!t XO. (Now as we know, the sonobuoys will begin acquiring detailed, effective acoustic data and transmitting it via radio datalink to the Bear-F ASW aircraft.)
this is where it gets interesting, particularly re: OP's post.
Captain Benjamin Sisko -oops I mean- Marko Raimius- /nods "Battlestations"
<there is some insolence involving the order and some slothly-calculated nav info... the conn officer is a real insolent Cossack and the 'gator is scared borschtless>
<we ain't bottoming this damn boat and you better launch the countermeasures you rookie Comrade Jr. Lieutenant (aka Ensign), who is scared more Borschtless than anyone> - btw of course it's a "full spread" of countermeasures. They're like Photon Torpedoes, seemingly ineffective unless you fire a "Full Spread." Anything less, simply uncivilized.
<acoustic tones heard in the background, AKA, the sonobuoys are sonobuoying and then we see the Russian Bear ..... this can't be good....>
<the bomb bay is opening. It's totally not good at all. Never good when they open that bomb bay and some English actor uses a terrible, terrible Russian accent to ask his even-worst-accented-English-Russian for permission to launch the 'weapon', which soon is 'away' . If only that Catepillar was working, this mess would not have happened..... The Co-pilot reinforces this with his cold, calculating stare at the pilot ..." You authorized him to kill a friend... you Imperialist... you authorized him to kill Raimius..." >
NOW THE MOMENTS WE'VE ALL BEEN WAITING FOR!!!
Sonar Officer starts calling out the action, again, as he hears it in his earphones which are giving him the sonar arrays' acoustic data via the hydrophones
"HiGH SPEED SCREWS! TORPEDO IN THE WATER" --- he knows this via their sound
"TORPEDO IS ACTIVE" -- This is important -- perhaps the crux of the original poster's question. As I realize many know, but the finer points may be a bit blurry, there used to be two types of torpedoes... there still are, but now most torpedoes can use both technologies independently ... In this case, due to the fact that the Russian Bear-F had a (seemingly rock-solid) fix on the 'enemy' sub, they dropped their torpedo with a set of target data, and it went active immediately- ie, it immediately started swimming and using it's active sonar homing technology. *it did not* just swim while listening for a target, aka "passive" , because the Russian Bear-F had positive target data via both the sonobuoys and presumably onboard sensors. So the torpedo went active as soon as it hit the water, and certainly, the Sonar Officer can hear the torpedo's pulses. Now remember- a torpedo has nowhere near the amount of "sonar power" that another submarine has, but that is a key point- it's one thing to transmit sound and receive it back and identify a target... it's QUITE ANOTHER for someone to "hear" that YOU are transmitting sound [with the intention of it hitting you and giving them your location]. The absolute best example I have ever heard is the "basketball arena" example. Let's say you are put in a basketball arena with 5 other "bad guys". And you have to kill the 5 guys to survive, and they all want to kill you. You are given a small, standard battery-operated flashlight, and a gun. *As SOON* as you turn on your flashlight, you're basically killing yourself, because: the arena holds on average 20,000 people. Your flashlight can illuminated a little bit in front of you and certainly would light up someone if you pointed at them right in front of you. But it's a big place, and you're probably not going to be aiming at one of them. BUT AS SOON AS YOU TURN IT ON, *they can all see you*. Easy as pie. They might be clear across the arena, far away from you. But they'll see you and have a fix on your position. And then they'll close on you and kill you.
"Torpedo has aquired!!!" <Captain Raimius: "Launch Countermeasures!!">
---this is based on the Sonar Officer realizing the torpedo's acoustic transmissions aka "pings" have ceased being random 'homing patterns' and pointed straight at you... The pings are coming from the same bearing, at the same interval, and the sound of the screws has altered to point at you. *THAT* is how the Sonar Officer can issue the " Torpedo has Acquired" informative. Based on the acoustic transmissions becoming uniform, and the screw sounds simultaneously altering their course you your bearing, you know the torpedo has "acquired" you. <as we know, an Active Countermeasure emits an array of bubbles spraying like an airstone of an aquarium, which are used in an attempt to force the active sonar of the torpedo to lock onto the bubbles' acoustic signature. Red October emits two Active Countermeasures, correctly attempting to 'jam' the "active" torpedo>
<we continue to hear the torpedoes>
"TORPEDO HAS REACHED COUNTERMEASURES....... TORPEDO HAS LOST CONTACT" --- this is due to the fact that the torpedoes are equipped with microprocessors and <fairly advanced, for the time> logic circuitry to give them a set of things to compare.... In other words.... , from the Torpedo's perspective, "I am now getting a very loud signature.... I know my programming tells me I always want to lock onto the loud signature and go kill it... BUT WAIT.... I have logic .... And a reference... what is this sound I am homing in on ?? " And then the torpedo compares the acoustic signature it is hearing <the countermeasure> with a library of sounds, and either decides <this sounds good enough, I am going to finalize my path to this and kill it> or <hmm... this sounds suspiciously like the things they told me to avoid.... I think I'll keep searching.....>
"TORPEDO HAS REACQUIRED .................. TORPEDO HAS REACQUIRED, AND IS HOMING..." ..... looks like them Russian torps ain't so bad.....
Now we have good old Marco Raimius doing his "give me the count! Full ahead flanK! You're relieved!!!" thing.
The torpedo of course barely misses, but misses so closely it detonates in the Neptune Massive. (well , hey, they were "OUT OF THE LANE!!!!").
RIGHT FULL RUDDER, ALL BACK STARBOARD SHAFT
DOWN ON THE BOWPLANES!
<sound collision!!!>