It's been quite a while since I built that little box. It works reasonably well, but could definitely be better.
The box now belongs to a buddy of mine who lives in a small flat. I don't know how much use it's been put to, but it couldn't come with me to the new property.
While reserching methods for building a studio, I realised that I could have done the iso-cab so mch better.
The box was made with 9mm ply, glued to a pine frame and some earth wool was placed inside.
Performance was ok, but it could still be heard and was only any good for a small amp.
I think maybe even the 15 watt Fender Princeton was too hot for it.
Something like a 5 watt Valve Jr, Black Heart, Tiny Terror is more suitable.
This is due to two factors, the size of the cabinet and the density of the walls.
The cabinet I built was basically a 600mm cube. This was due to size restrictions with the flat I was living in.
What I've read is that the small volume of air can lead to speaker and output transformer damage, due to back voltages being generated in the voice coil.
Indeed on one test of a 10" driver, I was alarmed to find that the magnet was quite warm to the touch.
This is not something I have ever noticed before. I stopped that test fairly quickly. Every other test I did was at a lower volume and on a speaker with a larger magnet (better heat dissipation, among other properties).
Despite the small sound however, the sound was pretty good, no "boxiness" was present, but I think that was likely due to the mic being pressed right up against the speaker grille (no room for ambient sound).
A better design would be to double the length of the box, leave out the rock wool (it really did nothing in terms of blocking the sound path), use 18mm ply and a couple of layers of plaster board (with an air gap between the inner and outer layer). Essentially building a tiny version of a room within a room. Plaster boards properties lend itself to be quite the good little insulator. Not what I expected when I started my research.
Making a sound proof box it seems, is entirely dependent on just a few factors. Density, Mechanical Isolation and a Perfect Seal.
That means Thick Walls, Rubber & Air isolation between inner & outer boxes, and Blocking the movement of air through any gaps that may appear during assembly.
Anywhere air goes, sound goes.