Zahra Moradpour - Department of Occupational Health Engineering, Occupational and Environmental Health Research Center, School of Public Health, Shahroud University of Medical Sciences, Shahroud, Iran.
Sajjad Farhadi - Center of Excellence for Occupational Health, Research Center for Health sciences, School of Public Health, HamadanUniversity of Medical Sciences, Hamadan, Iran.
Mohsen Mohsenabadi - Department of Ergonomics, School of Public Health, Mazandaran University of Medical Sciences, Mazandaran, Iran
Mahdi jalali - Department of Occupational Health Engineering, School of Public Health, Neyshabur University of Medical Sciences,Neyshabur, Iran.

Air conditioning systems are the main source of background low frequency noise in administrative buildings,restaurants, classrooms, and hotels. There are different ways to reduce the low frequency noise. Silencers are generictools which are utilized for controlling noise produced by air conditioning systems. Sound assessment and frequencyanalysis were performed in accordance with standard methods to evaluate the exposure of workers in separate workstations. In the next step, an absorptive- dissipative silencer which its inner side was covered by convoluted acousticalfoam was implemented in the ventilation system. The exposure of employees was re-evaluated. Moreover, PreferredNoise Criterion (PNC) and Speech Interference Level (SIL) were two acoustic indices used for assessing theeffectiveness of the silencer. Before implementing the silencer, the sound pressure level in the rooms was between57.8 and 61.1dBA with a peak frequency at 125Hz. The implemented silencer with dimensions of 1.4 ×1×1m, andinlet and outlet area of 0.45×0.45m was able to reduce the sound pressure level of about 13 to 14.2dB. Further, thePNC and the SIL indices were reduced to 14.33 and 15.31dB, respectively. The implemented absorptive- dissipativesilencer reduced the sound pressure level of about 13.6dBA. Moreover, PNC and SIL indices are two important indiceswhich can be used for assessing the level of comfort in office buildings.

Noise Pollution, Silencer, Air Conditioning System, Acoustic Comfort