Ask A Scientist© Environmental Earth Science Archive

name K. M. Y.
status student
age 14

Question - I will want to ask how does acoustic material work in
sound insulation; what is the principle behind it? Is there any other
ways to reduce the loudness of the sound?
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Sound waves are reflected when they hit a hard surface.  Providing an absorbent surface can reduce 
some of the reflected sound.  In a "hard" room, soft materials such as absorbent ceiling panels, 
carpeting on the floor, and drapes or special absorbent wall coverings, will reduce noise by 
reducing the reflected sound.  Only reflected sound can be treated as described, while direct sound 
will not be affected.

Porous materials of varying density and composition are generally used as sound absorbers to convert 
sound energy into heat within the open pores of the material.  In order to maintain the best 
absorption values of the chosen materials, the air channels should all be open to the surface so 
that sound waves can propagate into the material.  If pores are sealed, as in closed cell foam 
material, the material is generally a poor sound absorber.  Pores should not be sealed by paint, 
or coverings, and any protective shielding for the absorbing material  should generally be 
perforated.

In evaluating materials for their ability to absorb sound energy, its ability to absorb sound is 
usually provided in the form of an absorption coefficient.  The absorption coefficient is defined 
as the ratio of sound energy absorbed by a given surface, to the sound energy incident upon that 
surface.  The absorption coefficient can vary from 0 to 1.  If, for example the absorption 
coefficient is 0.9, then 90% if the sound energy will be absorbed by that material.  Most porous 
absorbers are more efficient at high frequencies, while improving the materials thickness, or mass, 
can increase low frequency absorption..

The above has been taken from a web handout from SILEX Sound Corporation on the World Wide Web.  I 
hope that this helps.

Sincerely,

Bob Trach
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