It’s actually quite simple really. Although don’t throw away your home cinema system away just yet, it isn’t going to be that good.
Essentially a speaker is made of three parts. Firstly, you have a magnet. Around that you have a coil, which is attached to a cone. The coil (known as the voice coil) surrounds the fixed magnet and when an alternating current is passed through it, a magnetic field is induced, causing the cone to rapidly move backwards and forwards – producing sound.
Our speaker design is about as simple as it gets. Our permanent magnet is glued to a cardboard base, as well as half of a plastic cup. The coil is made with 0.29mm enamelled copper wire, wrapped ~80 times around a paper tube, slightly larger than the diameter of the magnet, and covered in glue to secure it. A circle cut out of polystyrene was used for the cone, which is glued to the top end of the tube. The tube is positioned over the magnet. Finally, the ends of the coil are attached with crocodile clips to the ground and the left connection of a 3.5mm jack lead. When assembled the speaker is two components- 1: base (with magnet) and 2: coil and cone. These are not attached to each other and can move freely. The two bent strips of card you see are just for support.
The loudness is in part determined by where the magnet is in relation to the coil so it is best to experiment with the positioning of coil or magnet before gluing everything down. It seems best when the magnet is near either end of the coil.
Some things to note: The wire is enamelled, which means the ends need to have their insulation removed either by burning or scraping. Also, this speaker is powered purely by whatever the aux is plugged into, so it will be noticeably louder when connected to a computer than a phone. Finally, to be clear, in this instance we used a stack of magnets that reaches up half way into the tube, however only one or two magnets are actively contributing to the speaker: the rest are used to make the stack tall enough. The bottom two of three could be replaced with anything the same size (or the coil could be lower on the tube).