Room acoustics describes the actions of sound waves in closed rooms and their mutual interferences, which influence the time and frequency responses of the total sound field. The total sound field is created by the sum of the original sound waves (created by a sound source like a loudspeaker) and all those sound waves reflected and partially absorbed at the room walls. The original sound waves define the direct sound field, whereas all the reflected sound waves together create the diffuse sound field, where a certain radiation direction can no longer be detected.

Listening in large or small rooms is influenced by room acoustics in very different ways. Large rooms (churches, theatres, or conference centers) are characterized by a large reverberation time, which may considerably deteriorate speech intelligibility and syllable articulation.

By contrast small rooms suffer from sound coloration due to the frequency dependence of the interference of direct and reflected sound waves. This may result in cancellation or increase (multiplication) of the volume level at certain frequencies. These sound colorations depend not only on frequency, but also on the actual position within the room. For special frequencies or small frequency ranges, a listener will notice cancellations of volume level in some positions, and multiplications in others. In closed rooms, standing sound waves form, which are characterized by nodes (sound cancellation) and antinodes (sound multiplication).