Smartphone speakers are rarely loud enough, especially in public places. Instructables user urant has created a pocket-sized megaphone that amplifies your music. The best part? It fits in your wallet.
We've tested similar DIY solutions before with mixed results, but this one has a few things going for it. First, it's made from heavy card stock that should go a long way to amplify the sound. Second, its structure and tight fit over the phone itself will help ensure the megaphone picks up all the sound. And it conveniently folds down and fits in your wallet.
If the picture above makes it look a little complicated, the instructions include a printable PDF that simplifies the process. All you have to do is print it, cut it out and follow the folding directions. It's built for the iPhone, but it can be modified to work with any smartphone.