Noise source mapping for trucks, part 1: development and design

Abstract

traffic noise modeling and mitigation. The purpose of this study is to develop a practical truck noise source localization technique using acoustic beam-forming. An experimental 70+ microphone elliptical array was designed and fabricated for truck testing. Beam-forming software was developed and implemented using a computerized data acquisition system. Proof-of-concept tests were performed at low-speed and high-speed truck testing facilities for a representative sample of trucks with widely different characteristics to validate the measurement system performance. The measurement system design parameters were verified experimentally, and certain improvements to the system were recommended for future implementation based on the field experience. The developed beam-forming measurement system provided adequate noise mapping and localization for various noise sources on trucks, stationary and moving with the speed up to 50 mph. The results of the proof-of-concept testing presented in an accompanying paper (Part 2) confirm that the developed microphone array, data acquisition system and beam-forming software performed generally as expected and required no major adjustments.

Noise source mapping for trucks, Part 2: Experimental results

“…The 0 dB level of the colorbar is referenced to the maximum in each map. At 600 Hz, 922 Hz, and 1938 Hz the images are clear except for the ground reflections; this is consistent with the source-alone as discussed in [1]. At 2999 Hz, however the expected side lobes contaminate the source map as expected from the calculated beam response.…”

“…The measurements exemplified in this paper and the calibrations discussed in [1] demonstrate that the array and its post processing can provide absolute level comparisons of truck noise sources with adequate localization to pinpoint the source location with the sound-generating truck component. For this large-aperture elliptical array, with the 6 m passby range geometry, the spatial discrimination of sources on trucks was as expected for frequencies between 231 Hz and 2000 Hz with aperture limiting at low frequencies and grating lobes limiting above 2000 Hz.…”

“…Measurements were made on various truck models (all by International ®) in various operating states with and without trailers. The extensive spherical-source calibrations [1] confirmed the array's basic beam forming at both on-normal and steering angles (to 45 o ); they also disclosed the magnitudes of ground reflection paths for localized sources at different elevations. In all cases, the array output was computationally inverted to produce two-dimensional spatial maps of source levels in the side profile of the truck.…”

“…The initial phase of measurement of the array's calibration in which a single spherical source was used [1] was followed by a second measurement with the same source now mounted on a truck bed. This measurement provided a second calibration check combined with verification of its ability to discriminate among sources.…”

“…In the previous paper [1] the elliptical array development and its basic beam forming calibration was shown to permit vertical discrimination beam width (-6dB) of less than 3.6m for frequencies above 250 Hz. Earlier developmental work [2] defined the measurement technique that is required to develop a spatio-temporal definition of truck sound at passby speeds up to 80 km/h (50mph).…”