ISO BOX FOR GUITAR AMP

Hi, I intend to build ISO BOX FOR GUITAR AMP. I am playing Gibson Les Paul, my amp is Blackstar JJN 20, 20-Watt Valve Combo. I want to turn on the volume on the amp, so that the sound starts to build... My question is, how big decrease in volume can I expect with iso box? I want to reduce the outside volume as much as I can... Any ideas which is the best way to build it? Thank you in advance, Marko

a box-within-a-box - 20mm plywood with a 100mm gap between boxes - fill with sand. the lid will need venting or better keep the amp outside the box and only the speakers inside. if venting - build it like you would a room-to-room duct silencer and construct the lid like the box with the shell-sand approach to mass and damping. you should be able to get extremely loud inside the box with little exterior sound or vibration.

As Glenn said if you build it with decent mass and sealed, very little sound will get out. But note that a part of the sound of electric guitar is often the acoustic feedback from speaker to string. I recorded an album here with the BlackStar 1 Watt amp!. I would love to hear the 5W one.