Appliance Design - August 2008 - (Page 30)

NOISE & VIBRATION CONTROL Silence Simpliﬁed Fig. 1. Principle of active noise control. The unwanted acoustic signal (red) is cancelled by creating a signal with the same amplitude, but 180 Deg out of phase (gray). Active noise control technology delivers low cost solution. B by ron niv Ron Niv is vice president for marketing and business development, Silentium, Rehovot, Israel. 30 applianceDESIGN August 2008 usinesses are developing their own standards and protocols for regulating noise in the workplace and defining the permitted noise levels generated by specific items of equipment. The employer is required to take certain actions, such as providing hearing protection, information, training, and so on, when the daily exposure level is likely to exceed 80 dB(A). A quiet office exhibits about 40 – 45 dB(A) while a noisier environment of 60 – 65 dB(A) requires raising the voice while talking. The culprit behind this office noise is mainly the low humming generated by the cooling fans for personal computers and servers. The fans circulating air from an office HVAC system also contribute noise. Before attempting to control noise, it is necessary to understand it. Sound waves travel through any type of media, including air, water, wood, masonry, or metal. Depending on the medium through which it travels, noise is considered either airborne or structure-borne. While airborne noise radiates from a source directly into the air and travels through it, structure-borne noise travels through solid materials, usually in direct mechanical contact with the sound source or by an impact upon that material. All structure-borne noise must eventually become airborne noise in order for people to hear it, but to achieve effective noise control, both airborne and structure-borne noise must be addressed. The basic idea behind noise control, sometimes called noise reduction or cancellation, is preventing the sound waves from getting to the ears. The traditional method of controlling fan-generated noise is the use of acoustic (sound absorbing) materials, known as passive noise control. It is common to find equipment lined with foam and where the cover can serve as a barrier. The foam is used to absorb acoustic energy, while the barrier prevents the residual noise from escaping. There are two main limitations on using acoustic materials. First, these materials are less efficient on low-frequency noise, which can be the most irritating. The lower the frequency, the thicker and heavier the acoustic materials required. Second, optimal results require sealing of the noise source along www.applianceDESIGN.com http://www.appliancedesign.com

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Appliance Design - August 2008 Contents Editorial Shipments/Forecasts News Watch Plastics Switches Noise & Vibration Control Joining New Products Design Marts Association Report: CEA Advertiser’s Index

Appliance Design - August 2008

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