Appliance Design - August 2007 - (Page 20)

NOISE & VIBRATION Seeking Sources Fig. 1. Intensity Map of a household vacuum cleaner. The brightest colors indicate the areas of the highest sound intensity. They indicate the main noise source to be the motor/fan unit at the base of the vacuum cleaner. Fig. 2. Transient NAH of the switch-on of a refrigerator compressor. At the start of the switch-on, (left display), the radiation is from the relay. A few milliseconds later, the radiation is from the compressor and housing. There are different methods for tracking down the origins of noise. n recent years, noise emissions have become an important parameter in selecting one particular appliance over another. It is not uncommon for appliance manufacturers with particularly low noise emissions to quote them in their advertising material or to use terms like “low noise.” However, to reduce noise, it is first necessary to identify where it is coming from. This can be a tricky problem, but fortunately a number of different methods exist for doing it. They are grouped under the term Noise Source Identification (NSI). The following overview surveys some NSI methods and illustrates them with some practical measurement results. The material also looks at some of the latest developments in the field of NSI. I Intensity mapping technique by roger upton Roger Upton, is product noise specialist, Brüel & Kjær Sound and Vibration Measurement A/S, Nærum, Denmark. 20 applianceDESIGN August 2007 Sound intensity is a vector quantity that measures the flow of acoustic energy from A to B. The most popular way of measuring it is to use two closely spaced microphones separated by a solid spacer. By measuring the sound pressures at the two microphones and knowing the spacing between them, it becomes possible to calculate the sound intensity. Intensity Mapping involves setting up a measurement grid around the source, making measurements of sound intensity at each point on the grid and perpendicular to the grid, and then putting the measurements into some mapping software. The software then allows frequency-selective mapping of the results. An example is shown in Fig. 1, which shows an Intensity Map of a domestic vacuum cleaner. Fig. 1 shows both a three-dimensional map of the sound field and a map of a single plane. The measurement points are shown in white, and then there has been some extrapolation between them to improve the quality of the map. The selected frequency range is from the 315 Hz to the 500 Hz 1/3 octave. Narrow band Intensity Mapping is also a possibility. The advantages of Intensity Mapping are that it is relatively simple to carry out, requiring only a two-channel analyzer, an intensity probe, and some mapping software. The www.applianceDESIGN.com http://www.appliancedesign.com

Table of Contents Feed for the Digital Edition of Appliance Design - August 2007

Contents Editorial Shipments/Forecasts News Watch A Reed Switch is Simople, yet Versatile, and Designers are Finding New Uses for this Time-Tested Technology A Novel Actuation and Sensing Technology can Register a Light Touch, but Resist Blows from a Hammer Tracing the Source of Noise is Made Easier with Noise Source Identification Methods Appliance Noise can be Annoying, but Designers have Many Material Choices to Help Rein in the Racket Nanocomposites are No Longer a Technology for the Future. These Plastic Materials are Already on the Job A New High-Strength, Low-Density Glass-Microsphere Additive can Improve Plastic Part and Molding Performance A New Electromagnetic Weld/Bonding Process Helps Ensure Long-Term Integrity of Joints in Plastic Parts Snap-Based Technology Eases Installation and Makes Products More Serviceable Two-Part Silicone Adhesives Provide a Rapid-Cure Alternative to Traditional One-Part Silicones Classified DesignMart Advertiser’s Index Association Report: GAMA

Appliance Design - August 2007

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