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Noise Control Engineering (NCE), LLC is a premier acoustic engineering consulting firm that specializes in noise and vibration measurement and control for marine, industrial and commercial applications. Founded in 1991 by Raymond Fischer, NCE is a small business that readily responds to client needs. The company has the experience, tools and innovative ideas to provide cost-effective solutions to all types of acoustic problems.
A serene working environment is not the first thing that comes to mind when you think of U.S. Navy aircraft carriers. More likely, you’ll picture an F-18 swooping in for a touch-and-go landing amid the shudder of a near-sonic boom.
For the more than 5,000 sailors aboard a typical U.S. Navy aircraft carrier, like the USS Dwight D. Eisenhower (CVN 69), excessive noise is unfortunately part of the job. The U.S. Navy realized that they needed to address this issue. Noise levels on the flight deck easily exceed 150 decibels (dBA) in many areas during takeoff. Constant exposure to high sound levels can affect the aircraft personnel’s health and well-being and can lead to Noise-Induced Hearing Loss (NIHL). The figures reported by the U.S. Department of Veterans Affairs are quite alarming: NIHL and tinnitus are by far the most prevalent disabilities in the U.S. Armed Forces, with more than 1.5 million affected veterans. This figure is three times higher than disabilities from other afflictions such as post-traumatic stress syndrome. Even worse is the fact that most NIHL cases can take up to fifteen years to surface.
Currently, NIHL is already one of the highest U.S. Department of Veterans Affairs expenses, which according to some calculations is over $3 billion in claims annually, in addition to the impact on individual day-to-day life and operational performance.
This is not a new problem. The Office of Naval Research (ONR) is the research arm of the US Navy and is one of Noise Control Engineering (NCE) LLC’s principal clients. Kurt Yankaskas, NIHL Program Manager for ONR’s Warfighter Performance Office, has been battling hearing loss in the US Navy for many years, and has funded numerous NIHL projects, including this one. However, this is the first time that serious action has been taken to reduce noise in aircraft carrier spaces and attack the cause of NIHL head-on. But just addressing the flight deck, where the most effective option is hearing protection, wasn’t enough. Aircraft noise also permeates into the lower gallery decks – just under the flight deck – where many crewmembers live and work. Even in these spaces, the sound level exceeded the U.S. Navy 8-hour exposure limits of 85 dBA. Finding a universal way to reduce noise level throughout the entire aircraft carrier would have the most impact.To help address this challenge, the U.S. Navy contracted with Noise Control Engineering, LLC, an acoustic engineering consulting firm specializing in noise and vibration measurement and control for marine applications. NCE Senior Engineer Jeffrey Komrower decided to follow a very pragmatic and well-proven approach to validate noise reduction techniques on the aircraft carrier USS Dwight D. Eisenhower.
“The noise levels are very high on aircraft carriers,” Komrower explains. “In many of the offices and staterooms below the flight deck, levels exceed 85 dBA and have been measured as high as 105 dBA during launch events. The other issue is that these spaces are typically highly reverberant, making the situation worse. To reduce the noise successfully, it was really important that we understood what the dominant noise sources were and how the acoustic energy flows from sources into the ship compartments.”
“In this project, we combined simulation and measurement techniques to accurately model the physical system and come up with the optimal noise control treatment schemes in terms of effectiveness, cost and weight,” Komrower says. “These treatments were applied to several spaces on the Eisenhower to test out the effectiveness, with the intent of installing them on the rest of the fleet as they go into their maintenance periods.”
The ship structure was modeled using Designer-NOISE®, a software program designed exclusively by Noise Control Engineering. This proprietary software, which was initially developed under the U.S. Navy’s Small Business Innovative Research (SBIR) program, quickly and accurately predicts the noise levels on ships. It has been used successfully on more than a hundred commercial and military programs. Its core solver uses a hybrid Statistical Energy Analysis (SEA) approach to predict the spread of noise and vibration throughout the vessel. All that is required is a relatively simple description of the vessel, based on the physical properties of the decks, bulkheads, insulation material and joiner facings. Solver time is a matter of minutes, which means that Komrower and his team can work quickly.
“Our software is designed to study large ships, where you don’t have time for complicated or overnight solver work,” says Komrower. “We easily perform rapid tradeoff studies comparing the various potential noise control treatments while keeping a high accuracy level. After we predict the relative contribution of the different surfaces to the overall noise level in the compartment, we evaluate various noise control treatments, including absorptive insulation as well as hightransmission-loss and damping materials. We estimate the efficiency of each treatment in terms of acoustic performance, but also in terms of cost, weight and installation complexity.”
Like every engineer, Komrower knows the value of testing, and the next step in the process involved validating the simulated conclusions by performing measurements on selected areas of the aircraft carrier. The compartments on the gallery deck, just under the catapult No. 2 launch area and extending to the area under jet blast deflector (JBD) No. 2, were ideal, since the noise levels here were the highest on the gallery deck during flight operations. More than 100 channels of instrumentation, including microphones and accelerometers, were distributed throughout the flight and gallery deck to acquire data for more than 100 launches of tactical jet aircraft. To tackle this challenging job, Komrower relied on the high-performance Simcenter SCADAS™ Mobile hardware data acquisition system and Simcenter Testlab™ software.
“Because it is ultra-compact and highly precise in regards to signal conditioning for a large number of acoustics and vibration channels, the Simcenter SCADAS Mobile hardware was definitely ideal for the job,” says Komrower. “We validated the aircraft source levels directly from on-deck microphones and measured the resulting noise levels in the gallery deck spaces. We also had accelerometers on the flight deck underside and ship bulkheads to measure transmitted vibration levels, which could re-radiate as acoustic energy into the spaces. This data was then postprocessed and analyzed using Simcenter Testlab to validate the acoustic models and the source levels.”
With the traditional measurement techniques complete, NCE sought to validate how the energy was actually getting into the spaces below deck. Because of the many possible paths (both airborne and structure-borne) and the high reverberant issue, it is difficult to identify the key culprits and develop an optimized and effective noise control plan. Komrower knew he would have to try something different and innovative. This was a job for a state-of-the-art acoustic array, the Simcenter™ Solid Sphere Array. Its 36 microphones capture noise from all directions and the system uses a spherical beam forming technique to indicate the path of the dominant noise sources.
“We joke that it looks like something out of a science fiction movie, but with jobs like these, it is the only way to accurately and quickly obtain a 360-degree image of a room,” says Komrower. “We can be sure that we locate the right acoustic hot spots after just a few seconds of measuring.”
For the work on the USS Dwight D. Eisenhower, Komrower used measurements taken with the Simcenter Solid Sphere Array to determine that the primary source of noise in the spaces measured was radiation from the vibrating structural bulkheads. The very high acoustic energy from the aircraft impinges on the flight deck and transmits through the ship structure, causing the bulkheads to vibrate. The high radiation efficiency of these structural bulkheads results in the vibrational energy being transmitted to high acoustic energy in the compartments.
“Simply reducing the surface vibration is typically a very effective way to reduce the overall noise, explains Komrower. “We team with a company called Tech21 and decided to use their spray-on damping material on the structural bulkheads since it is lighter, easier and faster to apply than traditional damping tiles. This material was also developed under a Navy SBIR program.“
The project didn’t stop here. Komrower verified the material’s performance by taking measurements again after the spray-on damping had been installed. This set of measurements confirmed the predicted result from the simulation model. There was a five to seven decibel noise level reduction in the compartments that were treated.
“This is an excellent result,” says Komrower. “The spray-on damping treatment was a very efficient way to obtain reductions in noise levels from flight operations in these office and berthing spaces. The Simcenter Solid Sphere Array was an essential tool for this project. It helped us identify the exact surface areas that contributed the most to overall noise levels and made sure the acoustic hot spots were covered sufficiently with surface treatment.”
By combining simulation modeling and innovative experimental techniques using Simcenter testing solutions, Noise Control Engineering was able to successfully develop an optimized noise reduction plan with respect to cost, weight and treatment option effectiveness. The spray-on treatment is currently being installed on the USS Abraham Lincoln (CVN 72) and will be applied to other aircraft carriers in the fleet as they enter their maintenance overhaul periods. Thanks to this seemingly small task completed by Komrower and his team at Noise Control Engineering, there will be a significant decrease in the noise levels, and military personnel on U.S. Navy aircraft carriers will be able to hear loud and clear for years to come.