Appliance Design - August 2007 - (Page 22)

NOISE & VIBRATION of measurements. The results are presented as broadband, 1/n octave, or narrow band sound pressure, and if a few assumptions are made about the sound field, they can be scaled as sound intensity. The measurements are made at a distance from the source, and Transient Beamforming operates best in the mid-frequency to high-frequency range. That is because the resolution is a function of wavelength, and it degrades at lower frequencies. Fig. 4 shows a set-up for Transient Beamforming on the same clothes washer as was used before. Note how the microphone array, which is in the form of a semicircle, is now placed upon the wall of the semianechoic chamber being used for the test. The measurement is again of the start-up of the spin cycle, but is now from 1.25 kHz to 10 kHz. Noise produced by the different components in the machine, the drive belt for instance, will be radiated out through those points in the casing where the insulation is low. The results show the main noise emissions to be coming from a mounting hole at the back of the machine. The main advantages of Beamforming are that it is fast, can be used on larger objects, and has good high-frequency performance. The disadvantages are poor low-frequency resolution and that careful design of the array is required to minimize “ghost images” due to the array having sidelobes in its response. opment of conformal mapping. All of the techniques described so far produce planar maps. In conformal mapping, the map produced follows the surface of the source, making it much easier to visualize the individual noise sources. This subject is very new, and no doubt will be described in more detail in the future. Conclusion Noise Source Identification has come a very long way since it first began to be introduced in the 1980s. By far the most popular method is Intensity Mapping, because of its relatively simple nature and because it can yield relatively good results. The array methods, however, are far more powerful in that they can be used with non-stationary sources and allow reconstruction of the sound field at places other than the measurement plane. As a result, the array methods are rapidly gaining acceptance. < For more information, enter number . . . . . . 306 Or email: RUpton@bksv.com changing sound field during the copying cycle. The transient NAH result, shown on the top of the figure, was taken with the array placed 10 cm from the copier, and is in a 1/3 octave frequency range of 500 Hz to 630 Hz. The Beamforming result, shown on the bottom of the figure, was taken with the array 1.2 m from the copier, and is for the 2.5 kHz 1/3 octave. Both show the main noise emissions to be coming from the paper-handling system. References • R.Upton, M.Quickert “Application of noise source identification techniques to consumer products with illustration on a dishwasher, a washing machine, and a copying machine” Proceedings of Internoise 2006. • J.Hald, “Non Stationary STSF”, B&K Technical Review No. 1, 2000. • J.Hald, “Patch near-field acoustical holography using a new statistically optimal method”, Proceedings of Internoise,2003. • J,.Christensen, J.Hald, ”Beamforming”, B&K Technical Review No. 1, 2004. • J.Hald, “Combined NAH and Beamforming Using the Same Array”, B&K Technical Review No. 1, 2005. Fig. 6. Transient NAH (top) and Beamforming results (bottom) obtained for a copier, using the same array. Result published at Internoise 2006. Combined transient NAH and beamforming A benefit of the SONAH algorithm is that it allows the use of a non-regular array for Transient NAH. Transient Beamforming requires the use of a non-regular array if the effects of sidelobes are to be minimized. Furthermore, Transient NAH is good in the low-frequency to mid-frequency range while Transient Beamforming is good at mid-frequencies to high-frequencies. Therefore, why not combine the two and use the same array to produce results across the full frequency range? Fig. 5 shows such an array, called a “sector wheel array.” Each sector has 12 microphones arranged in a randomized pattern, and an array can have any number of sectors as long as it is odd and equal to or greater then three. The array shown has seven sectors. Fig. 6 shows some results obtained using a five-sector array for a photocopying machine. These plots are selected from a series showing the 22 applianceDESIGN August 2007 Latest developments A further benefit of the SONAH algorithm is that it allows the use of small handheld arrays, as shown in Fig. 7. These arrays contain systems that allow their position in space to be detected. This in turn allows so-called “patch holography,” where the array is used at multiple positions around a source, and the results at the individual positions are then “patched” together to form a continuous map of the sound field. A further development of these techniques is the devel- Fig. 7. Handheld array with array position detecting system for “patch holography” and conformal mapping. 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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