Silence Really is Golden

By Douglas Kelly posted 07-25-2012 05:16 PM

  
In a previous life, I worked as a deckhand aboard a group of ferries operating on the Connecticut side of the Long Island Sound. My duties included tying up lines upon docking, safely unloading and loading passengers, and undoing lines for departure. Nowhere in the job description did it say anything about working on top of a small volcano. 

Two of the ferries were powered by engines that were noisy but tolerable. In fact, I recall actually falling asleep during break one time as I lay on the smallest vessel’s gray doghouse, the power plant idling away as we sat at the pier. But the other large ferry sported engines that shook windows all over the harbor. If we had the hatchway to the engine room open when Captain Frank or Captain Joe got underway, it was like a constant barrage of artillery fire. A full day of that and my ears would be ringing the whole way home. 

I was talking with Ray Fischer, one of our (mt) magazine contributors, last week about noise onboard ships, and (no surprise) he confirmed that engines are the dominant source of noise on most vessels. “Whether it’s diesel or gas turbine…that, along with the propeller, accounts for most interior noise on a ship.” Fischer is president at Noise Control Engineering in Massachusetts, and he and his colleagues put together an excellent piece on modeling noise and vibration (“A Sound Approach”) for our July issue of (mt). He says that a key challenge is getting accurate data from vendors on the noise levels of their components, a task made somewhat easier by standards established by outfits such as ASTM. 

The aim is to have everyone measuring noise the same way, which is easier said than done when it comes to powerplants. “I think it becomes more challenging when the requirements get more stringent,” says Fischer. “You really have to know and understand what the engine is contributing. For instance, on the NOAA fisheries research vessels, where there are stringent limits on both interior (crew) noise and on exterior/underwater radiated noise, understanding the exact level that’s produced by the engine, both in terms of noise and vibration, becomes more critical as those requirements get tighter.” 
 
One of the methodologies that Fischer explores in his (mt) feature is that of statistical energy analysis (SEA) modeling, which he feels has shown good promise. “I think the advantage of the SEA approach is that, when you have an accurate model, you can do tradeoff studies, you can optimize the treatment so that you minimize the impact on non-acoustic factors like cost, weight, and space. From an overall perspective, for those trying this for the first time, the advantage is that you get accurate information and you optimize the entire system. You don’t just look at one part…rather than saying “This is a diesel problem, who do I handle it?” You instead can look at options like handling it along the path or handling it at the receiver…you have a lot of options available if you have a good model. So I think it facilitates you coming up with the best answers you can, given all the constraints you have to deal with in designing any type of ship.” 
 
Fischer and I also talked a bit about what’s on the horizon with noise and vibration control. He says that the IMO code is now becoming mandatory, which will change things for ship owners. “That means that, to register an IMO SOLAS-approved vessel, you will have to meet these noise requirements. No longer will it be, yes you have to look at them but you don’t have to achieve them. In the future, when the IMO gets this resolved, it will be an obligation that every owner will have to meet.” 
 
If you haven’t seen it already, check out Ray Fischer’s article on noise and vibration modeling, in the July (mt), at www.sname.org/mt/DigitalIssues

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08-03-2012 09:10 AM

I appreciate the opportunity to work with Douglas and SNAME to spread the word - noise can be controlled. Hearing loss prevention should be on the mind of the entire marine community.