After less than 1/100th of a second, the room has developed a
complex wave pattern with energy varying with room position. The
listener is now awash in a series of wave-fronts which will soon
reflect off the wall behind.
A mere 5 milliseconds later, the initial direct wave-front has now
reflected off the rear wall and has made its way back to the listener.
The listener will perceive this reflection as additional bass energy,
though a standing wave has not had time to develop.
It is a common misconception that such low frequency excess energy
is merely the result of inevitable resonances within the room, when a
large portion can be attributed to initial reflections.