Noise reduction of a portable generator set driven by small natural gas engine using an acoustic enclosure

Abstract

A detailed experimental study has been conducted to evaluate the effect of the two developed enclosures (simple and modified) on the noise emission of a small generator fuelled by natural gas. The sound of generator without enclosure and covered by developed enclosure was measured in front of the generator exhaust at five electric loading conditions (0%, 25%, 50%, 75% and 100% load). The sound was measured on a test site was managed according to SAE J1074 test procedure. The sound signals were measured using a precision sound level meter. The time domain sound signals were recorded in a lap-top computer hard disk. The digital sound was converted to the frequency domain using a developed Fast Fourier Transform (FFT) program to achieve narrow band signals. The narrow band signals were further processed to obtain 1/3^rd octave spectra and overall sound levels in A-weighting scale. The results showed that the loading conditions and type of enclosure parameters had significant (P<0.01) effects on the overall sound. The overall sound of generator without enclosure reached to 93.2 dB(A). The enclosed generator overall sound levels at simple enclosure, semi covered modified enclosure and full covered modified enclosure were reduced to 88.4, 87.2, and 86.1 dB(A), respectively. The generator sound increased significantly (P<0.01) with increasing the electric load so that the maximum sound level was obtained for full load conditions. The measured insertion losses for simple and modified enclosures were 8.7 and 5.7 dB, respectively. The results of 1/3^rd octave frequency spectra revealed that the simple and modified enclosures were effective to attenuate the generator noise in frequencies greater than 800 Hz and 630 Hz, respectively. The simple enclosure was not effective in the range of 315-630 Hz for the all loading conditions. The modified enclosures could not attenuate the generator sound better than the simple enclosure for frequencies greater than 1250 Hz. In general, covering the generator in the enclosures would be protected the human beings against the noise threats because the human exposure time to noise increased from less than 2 hours/day to more than 4 hours/day.