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Standard sound power test methods have existed for numerous years to allow for appropriate noise labeling of products for validation or for monitoring of changes. More recently, advanced methods such as acoustic holography and beamforming have also been successfully used for measurement of sound power and noise source identification. Sound power is a standard requirement for off-highway and agricultural vehicles, construction and power generation equipment, refrigeration and cooling devices, and many other consumer products. In the automotive industry, the engine and a few accessories (AC compressor, power steering pump) are tested for sound power. While sound power testing methods are well known and tests are conducted in most labs by efficient and often automated test procedures, the root-causing strategy in the case of lack of compliance to a specification is still mostly based on trial-and-error.

This paper investigates the ability of Nearfield Acoustic Holography to “focus” on a source from the not-so-nearfield. While the theory of Nearfield Acoustic Holography (NAH) has been around for half a century, practical commercial products using NAH are less than three decades old, with application in just about every field that measures noise from on-highway to industrial/construction to agriculture to marine to aerospace to white goods to electronics and more. With the wide variety of applications, in many cases it is not possible to measure in the nearfield over the entire measurement area. With the advent of new algorithms, one approach is to create a “conformal” array; another might be to perform patch measurements. This paper instead investigates the use of a planar array measuring at a distance from the calculation plane to identify sources. Since evanescent waves are not measured, using this technique to feed into CAE models is probably not applicable.

The high cost and competitive nature of automotive product development necessitates the search for less expensive and faster methods of predicting vehicle performance. Continual improvements in High Performance Computing (HPC) and new computational schemes allow for the digital evaluation of vehicle comfort parameters including wind noise. Recently, the commercially available Computational Fluid Dynamics (CFD) code PowerFlow, was evaluated for its accuracy in predicting wind noise generated by an external automotive tow mirror. This was accomplished by running simulations of several mirror configurations, choosing the quietest mirror based on the predicted performance, prototyping it, and finally, confirming the prediction with noise measurements taken in an aeroacoustic wind tunnel. Two testing methods, beam-forming and direct noise measurements, were employed to correlate the physical data with itself before correlating with simulation.

For many years engineers in the automotive market have struggled to find ways to accurately and efficiently map the noise sources found inside a vehicle. Many techniques, both theoretical and measurement based, have been proposed and used, but there has always been a trade off between accuracy and efficiency. Techniques like sound intensity mapping and Statistical Energy Analysis have proven to be accurate when mapping noise sources in vehicle, but require a large investment in time and money to create a simple, easy to interpret picture showing where dominant noise sources come from. In this paper the authors will introduce and demonstrate a novel technique, spherical beamforming, which can overcome the issue of test time and produce fast, accurate noise maps from the interior of a vehicle.

Farfield beamforming is a powerful tool for identifying spatially distributed noise sources. The technique yields an image of the relative sound levels within the measurement aperture. The latest version of the beamforming software is now able to estimate the total power within its measurement aperture. In this work, the noise sources on three types of construction equipment are imaged with a beamforming array, while simultaneously the radiated sound powers are determined by a six-microphone hemisphere per ISO 6393 or ISO 6395. Of particular interest are: noise induced by turbulent flow at the exit of an exhaust stack, the effect of a noise reduction package in the engine compartment, and crawler track noise during motion. The absolute levels of the mapped source regions are compared with the total radiated sound power.