Fans are sometimes installed onto the inlet side of the chassis, with the inflow of air through a perforated faceplate. This slide shows a variety of inlet obstruction examples that have been measured for sound pressure on a 9S0812M401 model. Shown here, the faceplate with the smallest perforations results in the highest sound pressure level, and this decreases with a combination of larger slit spacing and perforation pattern. The best sound pressure level comes from using a fingerguard, which allows for the most airflow into the fan. Obstructions on the inlet of a fan have a larger effect on sound than on the outlet side.