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A Sound Education

Sub Sonic

submarine MASTER copy

Captain of the 'Weser': What's it like down there, in a submarine?
Der Leitende: It's... quiet.”
Das Boot, 1981

The Aussies were hoping to quietly update their fleet of submarines, but the French are making a lot of noise about it. As part of a contract from five years ago, Australia was to receive new diesel-electric powered subs from France for their patrols in the Indo-Pacific region. But according to officials, "faster, stronger, and stealthier" subs are now required because of China's growing navy (now the largest in the world) and expanding borders. So earlier this month Australia torpedoed France and made a controversial deal for nuclear-powered submarines from the United States and the United Kingdom. Why change to nuclear-powered subs? One advantage is the extended length of time that can be spent under the ocean without resurfacing (months vs. days). The other is their superior quietness of operation.

Finding a sub
The whole reason for having submarines is to be stealthy, and that technology is always improving. But detection technology is also sophisticated. There are many ways to find a submarine, including radar, satellite imagery, synthetic aperture radar (SAR), and hydroacoustic monitoring. Hydroacoustic monitoring entails both traditional active sonar (the familiar pinging for objects) and passive sonar (listening for movement or mechanical noises). Active sonar isn't used by submarines very often because other subs can easily hide from it, and it would give away your location if you also wanted to remain hidden.

Passive sonar uses very sophisticated listening sensors. You've probably seen a thriller movie where two enemy submarines are trying to hide from each other. One sub stops their engines, drops to the ocean floor, and everyone gets really quiet. When the sonar operator hears the motors of their adversary in his headphones, he excitedly whispers "Enemy sub ahead!".

Hiding a sub
The chugging of engines, the turning of propellers, and any air pumps or other life support systems generate the most noise from a sub. To make a quieter sub, experts start with the engines. Although diesel engines are not as loud as in past decades, they can't match a nuclear sub's low noise. Experts believe that because China's Jin-class subs are very noisy, they remain close to shore and stay out of deep water. A nuclear-powered sub is not completely silent – a nearly silent electric motor turns the props and a low-noise pump is needed to cool the reactor – but one can stay submerged and maneuver for weeks and months, not needing to frequently surface or to fire up diesel motors to recharge batteries like diesel-electric subs.

A sub can use tactics to avoid detection. Sonar works best in constant-temperature water. In the ocean however, varying temperatures cause different densities of water, which in turn create a barrier called a thermocline. If a sub is behind one, they will go undetected because thermoclines bend sound waves away. Another maneuver is to hide a sub under cliffs and in trenches so that active sonar and radar will have difficulty picking up their signature.

A submarine's shape also plays a role in it's stealthiness. The rounded shape gives radar and sonar pings very little surface to bounce off. The long bullet-shaped fuselage allows the sub to slide through water with minimal noise and wake. In addition, special acoustic tiles on the sub's surface reduces echos from sonar and abates any noise that may escape from within the sub.

On the horizon
Emerging stealth technology called fluid cloaking zeros out a sub's wake by accelerating water that enters small jets (similar to a jet ski) inside a mesh shroud, then decelerate upon exit. The two different speeds negate each other and result in no measurable water disturbance. Fluid cloaking also eliminates other signs of movement below water such as a bulge on the ocean surface (Bernoulli hump) and a V-shaped Kelvin wake behind a craft.

This concept of zeroing out is also employed for sound waves that impact a sub. In acoustic cloaking, any sound waves (such as active sonar) hitting varying densities of material surrounding a sub will bend around to the other side. Sub detectors will not perceive any distortion or reflection of sound, making this one of the more interesting cloaking technologies to emerge in recent years. In time, scientists say that nearly full cloaking can be achieved with one material that combines fluid cloaking and acoustic cloaking.

As happens frequently, new technologies are born from war and conflict. I think that someday cities might adapt and erect this sound cloaking technology around busy intersections, along interstates that border neighborhoods, and even near outdoor sports and entertainment venues. During the lockdown for the pandemic in 2020, many people discovered just how quiet their cities could become. Those memories may drive future city planners to require sound cloaking technologies in new buildings and around neighborhoods. What a wonderfully quiet world it will be.