It is perhaps surprising that forces as mundane as vibration and sound waves can play an important role in the design of ships and in naval operations. But from the environmental impact of cruise liners to the stealth capabilities of state-of-the-art submarines, understanding noise and vibration can make a huge difference.
We spoke to Brüel & Kjær's Northern Europe managing director William Egan about the systems available for monitoring acoustics and vibration, with applications as diverse as structural dynamics, materials testing, acoustic ranging and noise source location. Noise and vibration specialist Brüel & Kjær operates in 55 countries around the world, and in the marine market has provided systems to the likes of the Royal Norwegian Navy and DCNS. The company also provided acoustic and vibration analysis systems to Alstom Marine during the construction of the Queen Mary 2, one of the largest ocean liners ever built.
Chris Lo: What marine issues do noise and vibration measurement cover?
William Egan: In the widest possible sense, we provide acoustic and vibration solutions for customers. That covers just about everything you can hear and touch. A lot of our customers are trying to make sure that things have the right shape, form and usability. We also do things like telecommunications standards. If people are using communications inside a vessel, and it covers both surface and underwater, communications standards, sound quality and speech intelligibility are absolutely key.
We also do things like acoustic and vibration signatures - we measure and monitor those. Our equipment is used for troubleshooting; some of our equipment is used for maintenance, repair and overhaul, for investigating issues. We do things like structural dynamics, so that's the structure of ships, how they move and the materials they use. Also noise source location, both interior and exterior. We do acoustic ranging, and that's on the whole vessel. Then we obviously work with a lot of sub-suppliers into that sort of market, so you're looking at things like component testing, vibration and shock testing, material testing. It's a vast range of products that we can offer.
CL: For military vessels, how important are acoustics and their measurement to operations?
WE: I would say it's absolutely vital. If you're talking about submarines, for argument's sake, you're talking about maintaining stealth. So what they're actually looking at would be checking the radiated acoustic signature. So they'll be detecting, classifying and tracking to identify whether they need to engage or disengage.
They're also looking internally at their own vessel, looking at all the components that are contributing to an acoustic signature - things like running machinery, sensor systems, crew activity, the hull moving through the water (what we call flow noise), and things like the propeller, the blade rate. They have to monitor these all the time to ensure that they are stealth. If anything is changing this acoustic signature onboard the vessel, that can be the difference between being detectable and not. There's airborne noise, structure-borne noise, and there's even changes in the isolation stiffness.
CL: How can noise and vibration properties be tested and analysed?
WE: For things like material testing, we can do things like structural dynamics and modal testing. Modal testing is looking at how an object reacts from a structural point of view when you inject a force. Classic modal is where you actually put a defined force at a particular point and then measure the response of the object at other points. You can then work out how an object will actually flex and move. That can contribute to noise as much as it can to the durability and fatigue of that object.
We've also developed something called operational modal. As you can probably appreciate, something like a ship or a submarine is a pretty big object to try and inject a force into. So we've developed some software called operational modal that allows an object such as a ship or submarine to utilise the existing forces around it. So it creates its own input forces by cutting through the waves or from the wind or the energy in the surrounding area, then measures the outputs from that and recalculates back.
CL: With these kinds of acoustic trials, is it important to make the systems intuitive?
WE: I suppose, by the very nature of acoustics and vibration, they are quite complex, and if you look at our equipment it's state-of-the-art. We've been working on two things very heavily. One is on the hardware side - we are making our systems modular, flexible and scalable. You can actually measure from one channel to a thousand channels on the same software platform. That's absolutely unique - nobody else does this. From a customer's point of view, you've got one type of training, one software interface, and once you've learnt the system it doesn't matter whether you're talking about one or a thousand channels, it's the same process.
On the software side, it really does depend on how complex the operator wants to get. We spend an awful lot of time to make sure our user interfaces are intuitive and easy to use. As an expert user, you can dive right down into the bowels of our software and play with the numbers. But if you just want to pull up a display showing frequency, just do some basic investigation or monitoring work, you can do that easily as well.
CL: What are the applications of vibration and noise measurement systems for non-military vessels like ferries and passenger liners?
WE: Obviously, they're not trying to do stealth! Our customers are quite varied on the civilian side. If you take fast cruisers, for example, they want to ensure that the structural dynamics are right. A lot of these things are designed by computer, using what they call finite element. That's all computer-generated, but you need to physically test it and make sure your finite element model is correct. We close the loop; we have the ability to make the measurements and feed it back into the models to make sure that their model is correct.
We work with engine manufacturers as well, so that's looking at the vibration for troubleshooting and getting more power out of the engine. Acoustic quality is absolutely imperative. You don't want to spend £2m on a fast boat, only for it to sound like a tractor coming around the lane. We have software which allows our customers to identify and design the sound before they've even designed the boat. If you can actually design the sound before you design the product and you've got a benchmark for what you're meant to achieve before you go to prototyping, that's an awful lot of time to market saved. And time is money for our customers. So they get their product to market quicker, they reduce their overall R&D costs and they get revenue in quicker.
If you're looking at bigger ships, sound quality is key, as are structural dynamics. Sometimes, certain vessels can be noisy to the extent of having an environmental impact, so we look at it from that perspective as well.
CL: What are your R&D and expansion plans for the next few years?
WE: The marine market is a key focus market for us. Our R&D is driven towards making sure we maintain and grow our position in that market. We are constantly developing new products, we are looking at systems required for that marketplace, and we are also working very closely with key customers to drive new systems that they might be looking for and develop the marketplace. Some of these marine programmes have a very long gestation period, so what's on the design board today may not actually go into operation for another 10 to 20 years.