Room acoustics is the broad term that describes how sound waves interact with a room. Each room, and all the objects in it, will react differently to different frequencies of sound. Every speaker will sound different in different rooms.
For example, imagine an empty room with hardwood floors and bare drywall. Lots of echos, right? Now imagine the same size room with lush carpet, lots of bookcases, a big plush sofa, and thick draperies. Quiet and intimate, right? These are the fundamental extremes of room acoustics, and the ideal-sounding room is somewhere in-between.
How to get there can be done with physical adjustments to a room, or to a large extend, electronically. There are a few terms important to understand first.
First Order Reflection
First order reflections are the first locations where sound reflects between your ears and the speakers.. This is often a wall, the floor, and the ceiling at a point roughly mid-way between the seating position and the front of the speaker.
When tuning a room, the first reflections are commonly treated with either diffusion or absorption. Absorption can be simple, everyday items like drapes, fabric walls or all the way up to purpose-built acoustical treatments.
First order reflections can be found only a few feet in front of your main front speakers directly to the left and right of the speaker locations. Another location that sees first order reflections is the ceiling. Installation of treatments on these locations can be dramatically beneficial to an audiophile or home theater installation.
For companies that make acoustic treatments that can be used to fix first order reflection problems, check out Acoustic Innovations, 3-D Squared, Acoustic Art Panels, Acoustic Sciences, Acoustic Smart Home Theater Interiors, Acoustics First, and Core Audio Designs.
Diffusion is an aspect of room tuning with the idea of breaking up standing audio waves in an audiophile or home theater room. At a basic level, smooth surfaces are more reflective than rough. The rough ones surfaces disperse audio a little better, while the smooth surfaces (think glass, smooth walls, stone floors) create more echoes and reflections.
A Brick wall is an example of a surface that provides diffusion in a real-world audio environment. RPG's BAD panels combine a mix of absorption and diffusion, normally covered by a fabric wall.
Note: A room with all-absorptive surfaces is not a going to be a good-sounding room. You need a balance of diffusion and absorption to achieve a good audio environment. Use of a professional acoustics company and/or an acoustical designer is recommended.
Also check out subwoofers, AV Reveivers, and AV Preamps that include digital EQs that electronically adjust the sound to do a lot of what diffusion is doing in the physical domain. These are from companies like Audyssey, Trinnov, and Anthem.
Room Correction and Room Tuning
Room tuning is a way to describe the art and science of adjusting room acoustics using physical and/or electronic means for audiophile and home theater rooms.
Room tuning is normally done by a professional acoustician who has audio measurement systems.
There are many ways to tune a room, but the most common are with physical pieces of room acoustic treatment (like diffusion, mentioned above), and digitally, with some sort of digital EQ.