What is a sound reflection?
This happens when sound hits a hard flat surface and bounces off the surface. Wiki defines a sound reflection as "When a longitudinal sound wave strikes a flat surface, sound is reflected in a coherent manner provided that the dimension of the reflective surface is large compared to the wavelength of the sound."
Why are sound reflections a problem?
Reflections can be a problem, when the sound is reflected directly or indirectly back at the source. This actually is the problem, the problem occurs when the sound hits another hard flat surface and, and bounces back again. The problem, compounds when this cycle is repeated hundreds of times. This is perceived as reverb or echo.
What does a room sound like with many sound reflections?
Here is a demonstration of one sound reflection, a hand clap in a squash court. The reflections go on for over 10 seconds. Watch and listen to this video.
What are some solutions?
Where to place panels or diffusers?
Place them on perpendicular surface, there are three perpendicular surfaces in a room. Read more about placement.