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Noise Control: Fincantieri first with Active Noise Control on cruise ship

The latest whisper in noise control technology is the use of fast electronics, speakers and elaborate software that generate phase-shifted noise which intercepts and suppresses medium and low frequency noise from fans, pumps, motors, engines, etc. This technology is called ANC, short for Active Noise Control.

In a joint development program between Fincantieri, Trieste, and ABB Flakt Marine, Gothenburg, an active noise control system was recently installed in a segment of the air-conditioning system (HVAC) onboard Holland America Lines' new cruise liner M/S Veendam, presently under construction at Fincantieri Marghera shipyard. This is believed to be the first ever application of ANC to a ship.

(The objective of the ongoing development program is to exploit the latest advances in active silencing technology also in shipbuilding.) A leading developer of ANC technology is the Wisconsinbased acoustics and vibration specialist company, Digisonix Inc. The company has successfully applied ANC to a multitude of industrial and automotive noise sources and to many HVAC systems in buildings. The company is the specialist subcontractor to ABB Flakt Marine for the development program at Fincantieri.

On board M/S Veendam, the team installed a single channel ANC system in a circular duct supplying air to a ship boutique. Low frequency noise radiated from the supply register into the otherwise very quiet room. Although the base line noise near the register measured only 45 dB(A), the noise was quite disturbing due to the very low background noise of only 38 dB(A) in the boutique. Most of the sound energy was concentrated to frequencies between 100-500 Hz, and originated from rotational noise from the supply fan in the airhandler and from flow-induced noise in the duct.

"When the ANC system was switched on, the noise from the supply register in the ship boutique simply disappeared said Fincantieri acoustics specialist Ing. De Lorenzo, who sees several candidate noise areas onboard ships where this technology can be applied. "Right now, we are evaluating this particular installation, but we also intend investigating several other areas where we think ANC can be applied. Over the years, we have developed a good understanding of this impressive technology in order to stay at the forefront of technical advancements and to remain an educated customer, able to exploit new technology in the best interest of our own customers." The principle of ANC is simple. Noise propagating down a duct is canceled by "anti- noise" from one or several speakers attached to the duct. However, it was not until recently that enough computation- May, 1996 al capacity could be packed into a small and relatively inexpensive controller so that the principle could be exploited commercially. To suppress or cancel a repetitive tone is simple, but to cancel random noise such as fan noise requires a significant computational capacity, clever control algorithms and very robust hardware. Noise in a duct, from a fan for instance, is picked up by the first microphone that sends a signal to the controller, which interprets signal, and phase shifts the signal and sends an amplified signal to the loudspeaker.

The speaker injects the "antinoise" precisely at the correct moment, the anti-noise intercepts the fan noise the two pressure waves "collide" and cancel each other.

Residual noise is picked up by the second microphone, and this signal is used by the controller to finetune the system, particularly in respect to changes in the air flow, temperature and characteristics of the noise.

As a major supplier to the maritime industry, the ABB group of companies considers ANC a strategic technology and has therefore invested in various development programs since 1991. As a result of these efforts, ANC systems from Digisonix are now being applied both as retrofit measures and as active components of originally manufactured equipment such as air handling units (AHU) and, in the near future, large baffle silencers for heavy duty f




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