Whatever material the wall is made out of, it is better to use two thin walls
spaced apart rather than one thick one of equivalent mass.
Remembering that soundproofing is best achieved by reflection, and that reflection
occurs at the boundary between one material and another, it makes sense to provide
four boundaries rather than two.
At first thought, it may seem that if a partition has a sound transmission
class (STC) of say 35 dB, and two such partitions are provided, then the overall
STC will be 70 dB. This is not the case.
By doubling the mass you get an extra 6 dB, and by spacing apart the two leaves
of the partition you might gain another 3 dB. This may not sound like much,
but it costs hardly anything so it is worth having.
The reason why you don't get twice as many decibels of sound reduction is that
the two leaves remain coupled together.
In fact the more closely they are coupled, the more the object of the exercise
Double-leaf brick walls ('cavity walls') are often constructed using wire or
plastic ties which couple the leaves together for mechanical strength.
For a wall that is designed for good soundproofing, the use of such ties should
Care should be taken not to allow cement to fall onto the ties. In normal building,
this would not matter.
Also, builders are known to have a habit of depositing rubbish between the
leaves of a cavity wall.
You probably didn't want to know this but it ranges from food waste to plastic
bags containing human excrement. This of course must not be allowed to happen
as it couples the leaves of the partition.
The space between the partitions should be filled with absorbent material such
as mineral wool.
This is where absorbent material does have a place in soundproofing.
If the cavity is left empty, sound will bounce back and forth between the leaves
and some of the reflected energy will end up being transmitted. If this can
be absorbed significantly, then the insulation will be better.