Why should I place basstraps mainly in corners?

Sound is reflected by hard surfaces like ceilings, walls and floors. At an edge between wall and ceiling (or floor), it is reflected twice, in a corner it is reflected even three times. This leads to an increase of sound pressure which is absorbed by the basstraps. With this fact in mind it is evident that for all resonances in the room – so-called modes – an increased sound pressure level (SPL) can be found in the corners. Thus, placing the basstraps in the corners absorbs energy of all room modes.