Sounds that are not able to bend around an obstacle, for example,
those in an enclosed space such as a room are partially ABSORBED
and partially REFLECTED by the barrier. Thus, when a wave of sound
encounters a wall, some of the energy of the wave will be absorbed
by the wall, and the remainder will be reflected as a somewhat weaker
wave traveling back from the wall. The amount of ABSORPTION
and the amount of REFLECTION depend upon the nature of the material
on the wall. Hard surfaces, such as brick or polished stone, absorb
little sound; most of the sound is reflected. Wooden panels absorb
more sound, but nevertheless reflect most of the sound they receive.
Curtains, drapes, and carpets absorb quite a bit of sound, however.
In particular, sounds with short wavelengths higher frequencies tend
to become trapped or scattered by the fibers of carpets and the folds
of curtains. Thus, when a loudspeaker is placed on the floor on a
deep carpet, the bass frequencies become relatively more prominent
as many of the higher frequencies are absorbed by the carpet.
Because of their ability to diffract around obstacles and to avoid
absorption, and because of the greater amount of energy often
required to produce them, lower frequencies carry well over distance.
The sounds heard from a faraway stereo system are those of the bass
instruments, the kick drum and bass guitar (and maybe even a tuba
or bass clarinet). The frequencies of the higher instruments do not
survive the journey nearly as well.
from Barry Truax's Handbook of Acoustic Ecology.