Understanding acoustic signatures

20 October 2017 (Last Updated October 20th, 2017 15:35)

Australia’s Defence Science and Technology Group has invested several years into developing a tool to model acoustic signatures for all classes of naval vessels. Acoustic signatures allow friendly forces to recognise their own vessels, but can also give away position. Claire Apthorp finds out how new modelling is improving the safety of military vessels at sea.

Understanding acoustic signatures
Virginia-class attack submarine firing a torpedo, which acoustically speaking, is a noisy affair. Courtesy US Navy.

With two major naval programmes underway to deliver new, once in a generation frigates and submarines, the Australian government is going all the way with investment into game-changing technologies to equip them.

With the focus of future warfare planning once again addressing a return to peer or near-peer military adversaries, technologies such as stealth, via low acoustic signature, are a primary concern for the Royal Australian Navy. In support of this, the Defence and Science Technology Group (DST) is working with industry to develop a new tool that will make building this capability into programmes including the SEA 1000 Future Submarine programme and the SEA 5000 Future Frigate programme, more effective and efficient than ever before.

Under SEA 1000, Australia is procuring 12 new submarines to enter service within the early 2030s, while under SEA 5000, Australia is seeking nine anti-submarine warfare frigates. Both programmes require vessels to be able to operate with high levels of stealth, part of which relies on a low acoustic signature.

AusRAT

The tool, called the Australian Rapid Assessment Tool (AusRAT), assists in modelling the acoustic signatures for all classes of naval vessels. The acoustic signature – the noise and vibrations a vessel and its on-board equipment and systems make in the water – is a key concern for navies aiming to build low visibility vessels. An adversary looking to find, track and identify vessels must only use equipment to ‘listen’ for acoustic noise in the water in order to track the location and movement of its enemy’s vessels. As a result, acoustic signatures must be kept as low as possible in order to avoid detection.

In order to do this from the outset – before the vessel has even been built – the AusRAT project is developing a tool which will provide the ability to predict the acoustic signature of a vessel while it is still in the design and construction stage.

AusRAT can be applied to any naval platform, including both surface ships and submarines. By predicting acoustic signature at the concept and preliminary design stages, when only limited information is known and fast turnaround of results is needed in order to assess different configurations, the right result can be found before programmes are deep into the build stage and investment has been committed.

The tool can also be used to provide updated predictions during the detailed design and build phases; and to investigate the potential impact of components not meeting their noise budget. It provides approximate analytical models that allow a whole-boat radiated noise estimate to be run in just a few minutes.

The DST and industry team have also developed a flexible and extendable software architecture that allows for seamless integration of new acoustic signature models.

Heggernes testing

The DST believes that the AusRAT tool provides a big advantage compared to other commercially available modelling tools which require detailed information to develop models and can take significantly longer to produce results. However, there certainly remains a place for acoustic measurement solutions that test the vessels once they are built and as they move through their service life, undergoing refit and upgrades, to ensure their noise levels do not deviate from stipulated limits and remain in accordance with requirements.

One of the few facilities in the world able to study acoustic noise is the acoustic range at Heggernes in Norway, which tests NATO military vessels.

The range performs static and dynamic testing in order to characterize the acoustic signature of NATO submarines and surface vessels, supported by the Brüel & Kjær Underwater Acoustic Ranging System (UARS). UARS provides an integrated solution supporting navies with the task of managing and maintaining the acoustic stealth of their submarine and surface fleet.

Static ranging is carried out by mooring the vessel being tested between three buoys, with two underwater acoustic transducers – called hydrophones – located on the sea bed on each side of the vessel, around 50m below the surface and connected to the range house by cables. Each on-board system is then turned on one at a time and the noise they produce is measured. Dynamic ranging is carried out while the vessel is on the move, with the generated noise measured as it passes specific points on the course.

Keeping one step ahead

L3 Oceana also develops solutions to measure and communicate data through water, including its Submarine Acoustic signature measurement Range (SAR). SAR is an ultra-low noise acoustic measurement range that incorporates co-operative tracking and highly automated data analysis and management software. It measures the acoustic signatures of submarines and surface vessels using submerged hydrophone array and electronics.

However, the development of technologies to out-stealth adversaries remains a game of one-upmanship as does all types of electronic warfare. With the US military’s research arm, the Defense Advanced Research Projects Agency (DARPA) delving into new realms of submarine hunting technologies under its Anti-Submarine Warfare (ASW) Continuous Trail Unmanned Vessel (ACTUV) programme, even the quietist diesel electric submarines will no longer be immune to detection.

The aim of the programme is to demonstrate the capability of the ACTUV system to use its unique characteristics to employ non-conventional sensor technologies that achieve robust continuous track of the quietest submarine targets over their entire operating envelope.

As a result, systems such as AusRAT will become ever more important and valuable as a development tool. Knowing what the answers are before a hull has been laid or an engine procured will allow signature technology development to continue to outpace detection technologies.