For a free progressive wave in air
(e.g., a plane wave travelling down a tube or a spherical wave travelling outward from a
source), sound pressure level and sound intensity level are nearly equal () This is not true in general, however, because sound waves
from many directions contribute to sound pressure at a point. When we speak of simply
sound level, we nearly always mean sound pressure level, Lp,
since that is what is indicated by our sound-measuring instruments.