Discussion
Study of noise propagation around the blast furnace showed some locations in the BF had high noise levels exceeded than the recommended limits. The BF was main source of the noise pollution related to the furnace equipment. The frequency analysis showed that the noise of the BF was continually in the dominant frequency of 4000 Hz.
Results of noise dose before applying the interventions showed the noise overexposure condition. In this regard, the allowed work time was calculated about 3.38 hours. After applying intervention, the allowed work time was increased to 6.18 hours. This allowed work time to be increased by using of personal hearing protection up to 8 hours in order to cover all time of work shift. Golmohammadi et al. showed that noise pollution up to 94 dB (A) and daily mean noise dose about 240% in a petroleum industry. In the mentioned study, the noise enclosure as the best solution was proposed.
The passive control method in target rooms were identified as the best intervention methods in the BF unit. The findings of acoustic conditions of target rooms showed that high noise levels are produced due to some imperfections and improper design. Moreover, noise leakage from all the openings area of the doors and windows which were directly exposed to the sound source. In the control room, noise isolation by replacing steel door, using of UPVC double layer vacuumed glass window and adding an internal wall for rotating on entrance could increase noise transmission loss up to 17.1 dB. In rest room, implementation of armed concrete wall and install a steel door for entrance with rotate of 90° could increase noise transmission loss about 23.5 dB. In the current study, the efficiency of noise control interventions was better than the study of Golmohammadi et al. related to designing noise control refinery room with noise transmission loss about 20 dB. Further, Nassiri et al. showed the efficiency of various types of enclosing rooms is about 20 dB in a petroleum industry.
Nikola et al. investigated noise controls in an industrial room with local noise sources such as pumps and electric discs in Australia. They proposed the barriers with noise absorbent layers to separate the sources from workers leading to 10 dB decrease in noise levels. However, in the current study, the noise reduction was 27.4 dB and 27.7 dB in rest and control room, respectively. There was only a 2-3 dB difference between the predicted and measured noise levels after applying noise interventions in the studied rooms. After implementing the noise interventions, noise levels in control and rest room was about 52.6 dB and 58.4 dB respectively, this was lower than recommended noise exposure limit.