Dead cabinet - to reduce wood vibrations that color the sound.

Echo - Have you ever been in an empty room and talked? Echo.
Make a small box and stick your head partway inside, make a loud
burst of sound, you hear echo. This echo needs taming with the fiber
material that you install inside the box to reduce that echo that
smears your sound.

Audio is simple. Hammer to test dead cabinets, your head inside
the box to 'test for echo'.

http://www.amazon.com/Test-Echo-Rush/dp/B000002JAI

.. LOL ..