Modern sonar technology and possible consequences

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The history of sonar began as the searched for a technological solution for the submarine detection problem during the First World War.
The principle was simply; create a sound and listen technically to the echo caused by the submarine. That was a basic sonar and proved to be adequate till late in WW2.


The Germans (and to some degree the Japanese) had developed several countermeasures to active sonar, the most spectacular being a high underwater speed that rendered classic anti-submarine warfare (ASW) methods of escorts ineffective. Acoustic absorbing coating (made of synthetic rubber) was also developed and a spectacular success in tests, but wasn't durable enough and defects of the coating created problematic noises.

The immediate post-WW2 period saw the modernization of old submarines to emulate the German and Japanese high-speed submarines and the build-up of a huge Russian submarine fleet.

The response was the switch to passive sonars that detected and tracked submarines by mere listening; active sonar was primarily used for range finding.

This approach was also effective against the noisy nuclear-driven submarines. No matter what many Americans claim, even late Los Angeles SSN are apparently louder than contemporary good non-nuclear submarines (SSK).
The used of variable depth sonar arrays (towed sonars that can drop below problematic ocean layers) was also pioneered during the Cold War.
Really modern SSK were very dangerous and not really under control of ASW technology, but they were a side show next to the nuclear-driven doomsday machines (SSBN) and their escorts and hunters (SSN).

The elimination of noises on board of submarines made them so silent that passive detection became unlikely before the submarine opened fire with its torpedoes. That applies especially to SSK and the relatively noisy coastal waters that were quite much ignored by the high seas-focused SSN/SSBN community during the Cold War.
Modern SSK further reduce their vulnerability and enhance their ability to hide by becoming air-independent with Stirling engine or fuel cell-based air-independent propulsion (AIP).


Active sonar had to make a comeback, but not in its old form.

Acoustic absorbing materials now reliably reduce the echo on submarines similar to radar-absorbing materials reducing the radar echo of stealth planes. The shaping of submarines was also altered, to reduce the direct echo by reflecting the acoustic energy into another direction than its origin (this was limited to relatively minor shape changes, though).

The acoustic absorbing coating isn't very effective at frequencies lower than 2.5 kHz, though - and thus such low frequency active sonars were installed - on the German Typ 123 frigates (LFTAS), for example.

This principle still has drawbacks; it exploits only a tiny fraction of the reflected acoustic energy and it gives away the position of the active sonar.
An ASW ship with such a sonar likely doesn't need to be silent; it's giving away its bearing anyway.
Low frequencies also place large requirements on emitter and receiver size as far as I know. The tiny helicopter-based sensors (dipping sonar - see photo, and even smaller sonobuoys) may become less relevant in the future (and therefore the whole concept of helicopters as hunter-killers in ASW).


Another approach seems to be smarter: Bi-static / multi-static sonars.
This is a combination of one or more emitters and one or more receivers - there's not just a single emitter and receiver, though.
This requires a lot of processing power and good algorithms - no insurmountable problem any more.


Multi-static sonars are auspicious because the high-value ships don't need to emit acoustic energy and because there's even no 180° reflection necessary.

Let's think about this a bit. Multi-static sonars place an emphasis on quantity and benefit by radio communication (triangulation).
That sounds to me as if the age of the large ASW ships (FFG/DDG) is probably over. It's just another hint that a cooperating swarm of small units may be more advisable.

The full exploitation of this technology requires medium bandwidth communication, a feature that's not available for submarines because water blocks all radio frequencies that can be used with such bandwidths. Submarines are relatively lone fighters, they're disadvantaged by this technology - not only because it's directed at them, but because they cannot exploit it well against their enemies.

The establishment of a multi-static sonar network by boats, ships and aviation (sonobuoys mostly) is on the other hand threatened by enemy ECM and air power.

It looks to me as if this technology enshrines the old principle that dominant sea powers prefer surface fleets while underdogs are limited to submarines because they cannot use surface fleets and air power at acceptable losses.
Submarines are still valuable for the dominant (= air superiority) power as well, though - just not so much in the focus.


Multi-static sonars may be just another nail in the coffin of the importance of large surface combattants.

Meanwhile, the cards have been mixed anew between submarines and sub hunters, and Cold War wisdom about sub hunting is in great parts already outdated - like most Cold War legacy ASW sensors.

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P.S.: I admit that I'm not fully informed about modern sonar tech. I may have missed essential info, but I wrote this text to show how far ASW sensors have moved away from the perception of most interested observers who tend to look at in-service systems, not so much at avant garde in-service systems and new projects.
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