The reduction of full acceleration truck pass-by noise conforming to Type ECE-51 regulation (Reference 1) was predicted in a hemi-anechoic chassis dynamometer chamber with microphone arrays and compared with actual test track results. This gave a close match to the track data, with both showing a 4 dB reduction in the A-weighted overall noise level after identical acoustic treatments were applied.
Noise control materials were selected to perform as acoustic barriers and absorbers. These were optimized by analyzing the 1/3 octave spectra, determining dominant frequency bands, in critical source locations and engine speeds, and using combinations that dissipate or contain energy well within those bands. With the truck being stationary while tested, important source locations could be quickly identified both subjectively and with localization tools such as Beamforming.
Chamber acoustic, engine (CAN), dynamometer and HVAC data were examined in light of the inherent test track variability, arising firstly from varying starting speeds, and secondly from varying engine performance, due to changing ambient temperature and humidity during track tests.
By comparing with the much more stable chamber data we describe how these factors - fluctuations in specified starting speed, ambient temperature & humidity - cause significant variation in the pass-by test result, which may not only influence a passing or failing result, but may also obscure the influence of treatment iterations for noise reduction.