Appliance Design - August 2007 - (Page 21)

NOISE & VIBRATION Fig. 4. Set-up for Beamforming on the same washing machine as used earlier, with results for the rear panel plotted from 1.25 kHz to 10 kHz. Results first published at Internoise 2006. Fig. 3. Set-up for Transient NAH on a clothes washer, with results for the front panel plotted as intensity from 315 Hz to 1 kHz. Results first published at Internoise 2006. disadvantages are that, because the measurements are sequential, it can only be applied to stationary processes, and the measurements can be tedious when mapping larger objects. Finally, with Intensity Mapping, you only have knowledge of the sound field within the plane where the measurements were made. Transient near-field acoustical holography Near-field Acoustical Holography techniques began to be developed at the beginning of the 1980s. They have advanced considerably since then with their extension to transient noise sources and the introduction of the SONAH algorithm (Statistically Optimised Near-field Acoustical Holography). They use a parallel microphone array to take what can be considered as a series of acoustical “snapshots” of the sound field crossing the array with time. These “snapshots” can then be used to calculate the sound field in any plane that is parallel to the original measurement plane as a function of position in space and time. This enables the sound field to be reconstructed closer to the surface of the noise source or further away. Fig. 2 shows a Transient NAH measurement of the switch-on of a refrigerator compressor. In the left-hand display, which is taken just as the compressor is switching on, the noise emissions are seen to come from the relay. In the right-hand display, which is taken a few milliseconds later, it is seen that the whole compressor and its housing is starting to radiate. Fig. 3 shows a typical set-up for Transient NAH on a clothes washer. In Transient NAH, the array is always placed close to the noise source in order to include the near-field effects. This information is needed to enable a complete reconstruction of the sound field. All four sides of the clothes washer were measured during start-up of its spinning cycle, where the drum is accelerating from 0 rpm to approximately 1,600 rpm. The noise emissions at the surface of the machine are shown as a 3D plot, and there is a detail plot for the front panel. They are calculated as intensity in a frequency range from 315 Hz to 1 kHz. They show the main noise emissions to be coming from the soap dispensing system and the rattling of panels. It is also possible to calculate the far-field noise at any distance from the source should this information be required. The advantages of Transient NAH are that it can deal with non-stationary sources and that, from a single set of measurements, the sound field can be reconstructed in any plane parallel to the original measurement plane, and as any acoustical parameter, (that is sound pressure, sound intensity, or particle velocity). The disadvantage is that the upper frequency limit is a function of the spacing of the microphones in the array, and going to higher and higher frequencies requires denser and denser microphone arrays. This gives Transient NAH an upper frequency limit of approximately 5 kHz to 6 kHz. Transient beamforming Another technique for NSI is Transient Beamforming. This also uses a microphone array, but now delays are introduced between the microphone channels to “point” the array in different directions without physically moving it. This procedure can then be used to build up a picture of the sound field at any distance from the array. Like Transient NAH it is a “one-shot” procedure where all of the results can be calculated from a single set Fig. 5. Array for combined Transient NAH and Beamforming. It is used close to the source for Transient NAH and then move further away for Transient Beamforming. www.applianceDESIGN.com applianceDESIGN August 2007 21 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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