The short answer is: no, adding a deadeningmaterial to the inside of a cabinet will not generally
increase SPL, but you should know by now that I
don’t like to leave it at the short answer. Let’s look
a bit deeper…
I used the word “generally” in the first sentence
because it is possible gain an increase in SPL;
however, there are a lot of different qualifiers. If you
start out with a really poorly built cabinet, you can
gain a greater increase than you could with a wellbuilt
cabinet. However, the gain will likely only take
you to close in SPL to the well-built cabinet, not
over. By building the cabinet correctly in the first
place, you eliminate the need for deadening at all.
And we are talking about “deadening,” which does
nothing to stop a cabinet wall from bowing. For
that you need “stiffening.”
But go ahead and add deadening material to a
near perfectly constructed cabinet and you will still
gain an increment, which is a slick way of saying
“tiny amount.” By tiny, I am referring to fractions of
a decibel. You need to get in the range of three
decibels to hear a perceivable difference, so anything
less than a single decibel is about feeling
good rather than real SPL gains. If you like the
idea of gaining a half-decibel, go ahead, but don’t
expect to hear any difference.
As you can tell, I like many others place great
importance on the quality of construction first.
Using heavy, stiff materials and bracing, build the
strongest box you can with sides that don’t flex
or bow. Then feel free to add sound deadening
material to “enhance” the cabinet. The use of
sound deadening material in a well-built cabinet
will be the icing on the cake by helping with internal
cabinet resonances through adding mass to
the panel.
I remember Dynamat introduced a product
called Dynaboard several years ago. This unique
material was composed of essentially two sheets
of wood with a sandwiched layer of Dynamat
inside. I really liked that idea for its approach to
anti-resonance and recently noticed that the custom
home theater industry is using the same
material.