I was at an audio shop today and saw several sub boxes with something that looked like foam/cotton type stuff inside of it. What is the purpose of this?

 

it's acoustic foam. It's suppose to increase the volume of a sub box.
For example if your sub require .75 cuft, but your box is only .5 cuft (due to size restriction in the car location) then you can use the acoustic foam to increase it into .75 cuft while maintain the original outter dimension.

 

WTF, how can adding something inside the box increase volume?

 

The box stuffing obviously isn't increasing the physical volume of the enclosure. It's just increasing the *effective* volume of the enclosure with respect to how sound waves bounce around inside the box. It's basically just *simulating* a larger enclosure. I can't really give you the technical details of why it works, but it does.

I also don't know (but have always wondered) whether there is a significant sound quality compromise when stuffing a small box versus using a larger, unstuffed box. I would guess that a smaller box with stuffing isn't as good as a larger box without stuffing. Otherwise, why would you *ever* use a large box? I might have to look this up...

 

Yeah it doesn't physically increase the volume. It's mainly used to eliminating cabinet resonance by absorbing the wave energy.

Think of it like this:
In 3 rooms there is a bomb.
Room 1 is huge and if you were in it you can avoid most of the explosion.

Two rooms are the same size, small.
Room 2 of the small room is empty and you will face the full explosion.

Room 3 is filled with cotton, so most of the explosion is absorb by the cotton.

So Room 3 is similar to Room 1, where damage is minimum. The physical volume of room 3 hasn't changed, but it mimics room 1's volume

So when a sub recommend a CuFt it's to prevent internal resonance/standing waves. But if you can't build a box that size, the foam will absorb most of the resonance to mimic a bigger box.


jreiter
I think it's personal taste when it comes to small stuffed box, compare to larger non-stuff. Some will hear the difference, some won't. Just like some will notice the difference between a $1000 stereo system compare to a $10,000 one.