The Three Options of a Sound Wave

Any sound wave, upon striking an object in its path has the option to do one or all of the following three things:  Be reflected, diffused or absorbed by the object.

The type of object in question and the angle that the sound pressure wave strikes the object will determine what the sound wave does.  All of the sound energy will be captured by the sum of these three actions.  For instance, a signal emanating in a particular direction through air will be applying 100% of its pressure energy in that direction.  Once it hits an object, the sound will be reflected, absorbed, and/or diffused.  If you were to measure the amount of sound pressure energy in all three of these directions, it would add up to 100% of the initial sound energy.

Reflection

A sound wave that strikes an object perpendicular to a flat surface and bounces back toward the direction it came from has been reflected.  Glass, walls, water, and other hard objects are generally good at reflecting sound.  If the sound strikes the object at an angle, it will be reflected in the equal and opposite direction as light reflects off a mirror.  This suggests that glass and smooth surfaces can be difficult to see using echolocation unless you are perpendicular to a flat surface (See calibration surfaces).  When you are at an angle to the surface the sound reflects off in another direction and does not return directly to your ears.  This makes it much harder to see.

Diffusion

A sound wave that is reflected in many different directions at once has been diffused.  Sound is being scattered in all directions and therefore there is much less sound energy to any of the resultant sound waves.  This can make an object more difficult to see since the signal returned is not quite as "clear".  It sometimes sounds "muddied".  Tree bark or very rough walls are a good example of diffusive surfaces.

Absorption

A good percentage of sound waves hitting softer objects will be absorbed by it.  This means that the sound goes inside the pores of the object and expend their sound pressure energy before coming back out of the object.  A soft pillow, or clothing absorbs quite a bit of sound.  This, similar to diffusion makes objects more difficult to see and makes them less "present", or less "clear".