Acoustic signature—literally the noise something makes—is the primary survivability factor of a modern submarine. Every aspect and component of a submarine's construction takes this reality into account. And it's not just about quieter mechanical systems, pump-jets, or coatings on a submarine's hull, but also about how systems are mounted inside a submarine to isolate vibrations and sound waves as well. Even the shape of a submarine and the water flowing around it can be a noise factor.

SEAFAC is where new technologies and equipment configurations are tested aboard multi-billion dollar American nuclear submarines and where their acoustic signature is measured while underway.

The waterway is the home of the United States Navy's Southeast Alaska Acoustic Measurement Facility, more commonly known as SEAFAC. Built in 1991, it's located roughly 675 miles from the major submarine bases in Washington State. For years it seems its existence was kept relatively hush-hush, although locals clearly knew a major submarine testing facility had been established. But to this very day, the place catches people off-guard. I have received roughly a half-dozen emails over the years from various individuals, from pilots to visitors to the area, who wondered why they saw a huge submarine plowing the waters in a fjord near Ketchikan.

If you venture just north of Ketchikan, Alaska, around the picturesque Behm Canal, you may stumble upon something very unexpected—the sinister black silhouette of a nuclear fast attack or ballistic missile submarine.

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Behm Canal is an ideal setting for an underwater acoustic test range because it is shielded from the ocean and from large amounts of vessel traffic, and thus from ambient noise. Its basin is also ideal, with a smooth tub-like contour that provides as close to lab-like acoustic conditions as possible. It can also be cordoned off relatively easily when tests are underway. The facility allows for static and dynamic acoustic tests, the latter of which can take place throughout the submarine's performance envelope. Two acoustic arrays are attached to the seafloor, which has a depth of roughly 1250 feet, to capture a submarine's audible signature as it travels back and forth across the canal while executing various test profiles. The Navy describes this range as such:

"The Underway Measurement Site acquires the signature of the ship while it is moving through the water. Measurements during movement ensure that propulsion and flow noises can be captured. The measurement process is slow and takes about 30 minutes to conduct 6 minutes of data acquisition for every single measurement. The remainder of the time is spent repositioning the ship for the next measurement. Typically, a submarine will spend two to five days conducting underway testing."

You can read more about the range's acoustic arrays, which were upgraded in 2006 and are called High Gain Measurement Systems (HGMS), as well as how they are installed, here.

Navy document linked above.

There is also a pair of custom barges that provides support for static tests. These tests can include an entire submarine or components of a submarine being submerged while suspended on cables between two hydrophone arrays at various depths. The barges can provide external power to the submarine so that all but the systems being evaluated can be turned off while testing occurs. In some cases, a full submarine isn't needed for these evaluations and just a unique piece of machinery or mounting concept can be tested on its own before being installed aboard a submarine and evaluated in an integrated manner. The Navy explains how the static site works in some detail:

"At the Static Site, suspension barges lower the submarine on cables to hold it in a stable position. The submarine can be suspended at various depths to evaluate how sea pressure affects the acoustic signature. The Static Site primarily measures the signature of rotating equipment onboard the vessel. The vessel does not have to maneuver so the testing can be conducted more rapidly. Also, better communication lines exist between the vessel and the Operations Center, therefore, the testing is more controlled and efficiently conducted. Typically, the submarine will spend one to seven days conducting Static Site testing."

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A data analysis site and general base of operations is located on Back Island which sits on the eastern edge of the restricted test area. From data collected at SEAFAC, the performance of new components and submarines both new and old can be evaluated and signature control practices and methods can be developed, validated, and improved upon. There also is a validation aspect to SEAFAC's mission, wherein submarines that have undergone overhauls or major work can have their acoustic signature thoroughly checked before heading out on patrols. The facility also does some work with surface vessels as well and continues to receive upgrades as America realigns its naval forces for high-end peer-state naval combat in the future. With all this in mind, and considering that U.S. Navy submarines carry the country's second-strike nuclear deterrent and provide critical intelligence-gathering abilities in addition to some of their more traditional tactical purposes, this unassuming test range north of Ketchikan is an incredibly valuable tool for U.S. Navy. The submarine can fight where no other vessel can—as long as it remains undetected. With SEAFAC's help, the future of American underwater stealth technology is better assured. Contact the author: Tyler@thedrive.com