There are two different types of altitudes. MSL and AGL. Throughout the website I refer to the altitude as MSL by default. MSL Altitude is Mean Sea Level Altitude. AGL is the altitude Above Ground Level. This differentiation is really important for communities in the mountains.
Communities in the mountains, far away from the airport,experience much higher amounts of jet noise than most would expect. When planes fly at 8000' MSL, and you are at 3000' of elevation the distance is only 5000' above ground level.
I have enclosed on the left a chart that shows the AGL and MSL for SERFR and BSR.
Image Source: FAA