Placing Treatment. Having set the dimensions for your room (taking into account a desire to spread the modes evenly), estimated the room’s Sabin content (absorptiveness) including people and furniture and calculated the room’s RT60, it is generally necessary to add some absorptive treatment in order to get the room’s RT60 down to desired length and compensate for strong modes / flutter echo.
First consider placement of broad band absorbers in the corners of the room whenever this is practical. The corners are locations where all the normal modes of a room will exist and absorbers placed here will have a chance to diminish all of the frequency components of a room’s reverberant field. Also absorbers in corners act on the pressures created by propagating sound waves more efficiently (just as speakers in the corner of room exhibit a boost in bass response). A panel of 703 placed across the diagonal of a corner from floor to ceiling (creating a variable depth cavity behind the panel) creates an excellent low cost broad band absorber. Then consider placing additional broadband absorber panels in locations that are likely to exhibit flutter echo until your calculations indicate you brought the room’s RT60 down to your target.
Flutter echo is one of the most common acoustic problems for bare rectangular rooms. Addressing this problem in a rectangular room will generally address the problem of strong modes. It is an artifact of strong axial modes. Flutter is the most likely artifact of a strong modal coincidence in a rectangular room. It happens when two large parallel surfaces give rise to a very strong axial reverberation. Flutter can be controlled by either simply not leaving the surfaces bare (making at least one of the surfaces absorptive / diffusive), or canting one of the surfaces so that the pair is no longer parallel. Canting one of the two surfaces is very common as to the floor / ceiling pair (vaulted ceilings are a typical construction method), but this is not quite as practical for walls. Non-square walls are not often already present in the case of space conversions, are a bit more difficult to build than squared walls in new construction, and can render spaces that are inefficient and awkward to use (especially when it comes time to install equipment). Also, you won’t be able to calculate the modes of a non-rectangular room through simple analysis; high powered software will be required to do this. For these reason canted walls are considered by many to be too much trouble for the too little benefit. Flutter echo can usually be brought under control by the simple expedient of installing the absorbent material typically required to bring RT60’s in line around the parallel walls of a room in the form of wall mounted panels. Diffusive treatments also defeat flutter.
Finally treat the early reflection points to enhance the stereo image and the clarity of mids and highs. Read more on early reflections, ITG, and the "mirror trick" here: