Rasmus Lyngdal-Christensen - Noise, Acoustics and Vibration Specialist, LR Consulting
The shaking marine floor mounted with steel rods and colourful balls (or moveable weights), is a good way to illustrate some important fundamentals of vibration and vibration control. The idea behind the shaking floor is to illustrate what happens when a natural frequency of one of the rods is excited. A standing wave is formed within the rod which means that the vibration amplitudes of the rod increases dramatically at some positions and at other positions there is no movement. By simply moving the ball on the rod, the natural frequency of the rod shifts away from the excitation frequency thereby reducing the dramatic vibration of the rod.
An intriguing example we have been faced with was a distinct low-frequency noise heard in the Wheelhouse and in Owner’s Stateroom on a yacht. The noise appeared from time to time and sounded like a diesel generator exhaust. However, the Wheelhouse and Owner’s Stateroom was located far from any of the diesel engines or exhausts. After some scrutiny we were able to verify that the noise only appeared during certain wind speeds. After thorough testing, different measurements and analysis the noise turned out to be caused by wind-induced excitation of the terrace handrails in-front of the Owner’s Stateroom located above the Wheelhouse. The wind caused vortex-shedding at the handrails that at certain wind speeds excited the first natural frequency of the handrails. By simply filling the handrails with sand we were able to sufficiently decrease the natural frequency (moving the ball) without visually impairing the handrails, hence elegantly solving the noise issue.
The above example is extraordinary case and displays a fraction of the experience and knowledge we have gained over the years, since 1980, working with noise and vibration on superyachts. This includes noise and vibration consultancy services on more than 50 yachts and on at least 14 yachts of more than 100m length. We have worked both as the owner’s representative, the yards representative and in some cases as third-party.
Naturally our consultancy services to the superyacht industry does not only include special type trouble-shooting assignments but also includes vibration analysis, noise analysis, writing technical specifications, measurements during acceptance tests etc. Throughout the years we have developed software for advanced analysis and measurements and have our own laboratory facilities with the most state-of-the-art measurement equipment available.