These BoomBags come in two styles, Walkman and boombox. All you have to do it connect the 3.5mm jack to your device and drop it in the pocket. There are no batteries or anything required.  BoomBag measures 11.8×5.9×3.9″ and is $39.79, the Walkman is 5.9×4.3×2″ and costs $19.84. Both are available at ShinyShack.com.
