Development and Validation of an Acoustic Encapsulation to Reduce Diesel Engine Noise

This paper describes a study to demonstrate the feasibility of developing an acoustic encapsulation to reduce airborne noise from a commercial diesel engine. First, the various sources of noise from the engine were identified using Nearfield Acoustical Holography (NAH). Detailed NAH measurements were conducted on the four sides of the engine in an engine test cell. The main sources of noise from the engine were ranked and identified within the frequency ranges of interest. Experimental modal analysis was conducted on the oil pan and front cover plate of the engine to reveal correlations of structural vibration results with the data from the NAH.

The second phase of the study involved the design and fabrication of the acoustical encapsulation (noise covers) for the engine in a test cell to satisfy the requirements of space, cost and performance constraints. The acoustical materials for the enclosure were selected to meet the frequency and temperature ranges of interest. The “noise covers” were designed to reduce the airborne sound power level from the engine by 2-5 dB in the frequency range of 500-2000 Hz. The effectiveness of the treatments was done by measuring the sound pressure and sound power of the engine with and without the encapsulation in accordance with ISO 6798. The results of the study showed that the overall sound power level from the engine was reduced by 3-5 dB depending on the type of materials used. Recommendations are made for the commercial production and implementation of the acoustic treatments on a practical vehicle.