  Room acoustics is commonly regarded and studied as an applied subdiscipline
of engineering acoustics. It is likely for this reason that many textbooks and
lecture notes on the topic provide a too brief, unsystematic, and incomplete
physical basics of the theory, while focusing on the practical aspects of
calculation, measurement, assessment, and optimization of sound fields in
rooms, such as lecture halls, control rooms, concert halls, and opera houses.
While such approach may be satisfactory for a broad community of acoustic
engineers, it leaves many important physical concepts and mechanisms obscure
and their understanding superficial and unsolid, especially among inexperienced
practitioners and students. The main purpose of this text is to provide a brief
yet self-contained introduction to the physics of room acoustics or, more
generally, acoustics of closed spaces, starting with the governing equations of
fluid dynamics. While covering all essential topics and concepts in the field,
such as modal analysis, geometrical and statistical theory, room impulse
response, subjective criteria and descriptors of sound fields in rooms,
acoustic design and simulations, and acoustic elements (reflectors, absorbers,
and diffusers), it does so in a systematic and, arguably, didactically more
appropriate way. The rigorous derivations of many important theoretical results
rely on well-established methods of modern mathematical physics, but require
only basic mathematical skills of the reader, since all mathematical tools are
introduced and explained in sufficient detail. On the other hand, the thorough
derivations expose the underlying assumptions and limitations of the obtained
results, the proper understanding of which is crucial for the correct use of
the results in practice.
