Podzharov E., De la Mora Galvez F., Alvarez Sanchez J.

The statistical energy analysis was used in the acoustical design of an electrical emergency plant to reduce the outdoor noise level. In the past when the plant was working a high annoying noise was heard all over the University camp. At a first glance the principal ways of noise propagation were the open door of the plant which was used for the suction of fresh air and a vast hole in the ceiling which was used for gases outlet. Also, a spectral analysis of the noise inside the plant showed that the dominant frequencies of the noise were in the range of 120 -270 Hz. This frequency range is near the critical frequency of the brick walls that is 129 Hz, at which the walls are transparent for noise. A two-block diagram was used for the statistical energy analysis. Two ways of sound transmission was considered through the inlet and outlet holes and through the walls and ceiling. This analysis shows that the exclusion of holes is not sufficient for reducing noise to an acceptable level in the low frequency range but the increase of noise absorption by the wall coating material. The transmission loss is calculated for different wall coatings and hole areas. A layer of fibreglass of two-inch width is selected to increase the wall absorption coefficient. Special silencers are designed and put at the suction of air and at the outlet of engine gases to reduce the noise propagation through the holes. The noise measurement shows that the noise level is considerably reduced after implementation of these measures. The reduction of noise is 7-8 dB(A) in the inside of the plant, 19 dB(A) at 10 m from the plant and 23 dB(A) at 15 m distance from the plant. Now nobody hears when the plant is working.