The young scientist or engineer looking for a relatively simple but interesting project for the science fair could construct the homemade permanent-magnet speaker shown here. It appeared fifty years ago this month in the October-November 1966 issue of Radio-TV Experimenter.
The speaker is constructed inside a cardboard box, which also serves as the sounding board (which the article incorrectly calls a “cone,” which it would be in a normal speaker).
The only other components, aside from a few pieces of cardboard and glue, consist of a permanent magnet and wire. Just as in a commercially manufactured speaker, a coil of is mounted on a form surrounding the magnet. When an audio signal is applied to the coil, it and the top of the box are made to vibrate.
The 1965 plans call for a magnet from a burnt out speaker, and this would still be a possibility. These days, a more prolific source of powerful magnets would be from the drives of defunct computers, as shown in this video:
Suitable magnets are also available from Amazon or many other sources. These plans call for a coil of 75 turns of 30-gauge enamel wire, although the exact wire size is not critical. The original plans call for using the speaker with a radio or television. The simplest way to make the connection to a modern radio or MP3 player would be through the headphone jack. Another option would be to use an inexpensive audio amplifier such as the one shown below:
The use of an audio amplifier would also allow the use of a homemade microphone, such as one of those shown in an earlier post. And for another somewhat more complex homemade speaker (or microphone), see this post.