This chapter looks at aspects of sound fields that are particularly relevant to sound insulation; the reader will also find that it has general applications to room acoustics. Airborne sound insulation tends to be lowest in the low-frequency range and highest in the high-frequency range. Hence significant transmission of airborne sound above 5000 Hz is not usually an issue. Sound fields in rooms are of primary importance in the study of sound insulation. The chapter is concerned with the basic principles needed to discuss the more detailed aspects of sound fields that are relevant to measurement and prediction. To gain an insight into the sound field in rooms we often assume that it is comprised of plane waves; so called, because in any plane that is perpendicular to the propagation direction, the sound pressure and the particle velocity are uniform with constant phase.