Acoustics is the science of sound: its various branches deal with the production, transmission, reproduction and recording of sound, as well as the way it propagates and the effect enclosed spaces have upon the way we experience it.
Being a wave motion of air molecules in the atmosphere, sound obeys the rules of reflection, diffraction and dispersion in a similar fashion to the far shorter wavelength electromagnetic waves which we call light, but obviously from, and through, different materials. The wave length difference is, in fact, crucial. Sound waves are generally long enough to be diffracted quite severely by everyday objects, because their wave lengths are usually similar to the dimensions of the objects.
The wave nature of sound is particularly important in architectural acoustics, where the detailed design of a concert or lecture hall depends crucially on how the designers wish sound to propagate through it. Other branches of acoustics deal with the design of sound recording and reproduction systems (engineering acoustics) and the highly specialized field of musical instruments (musical acoustics). But perhaps the most interesting, and latest field of acoustic research is psychoacoustics which deals with the way we actually perceive sound.
Collaboration between the engineers and the psychoacousticians has led to some very sophisticated hi-fi systems which can fool the listener — via headphones or specially designed loudspeaker arrays — that he or she is hearing sound images which are not really there.
Hi-fi originally stood for high fidelity, and the primary aim of hi-fi systems was to reproduce, as faithfully as possible, the listening experience of a live performance. With the advent of purely electric instruments, such as the electric guitar and the synthesizer, the aims of the hi-fi designers changed slightly. Much modern music is recorded as a collection of separate tracks on a studio master tape — and the aim of the new generation of hi-fi equipment has been to reproduce the recorded information (which has probably never been performed live, with all the instruments playing together) as accurately as possible and as lifelike.
So today the demands put upon a music reproduction system are twofold: it must be able to recreate live music recorded in a real acoustic, conveying something of the sense of space and reverberation which resonates through such performances — but it must be able to handle program sources which have been created only on tape and generally have all sorts of added reverberation and tonal doctoring.