Another factor affecting sound pressure for fans in a chassis is the distance of the obstruction from the fan. The top curves shown here are sound pressure measurements taken for varying the distance of a faceplate from the inlet of a fan. The 10 mm distance shows the highest sound pressure, and it decreases as the L dimension increases. Oddly, the distance of a faceplate from a fan on the outlet side does not seem to make any significant effect on the sound pressure level. Therefore, selecting an optimum distance of an obstruction is an important consideration for the fan inlet.