I needed to find a solution to an expensive alternative. Sometimes scuba diving involves catching the attention of others underwater. There are solutions on the market for $30. These solutions are much more scientific than my solution, although my solution suits my needs. My diving rattle is simplified. I made it small because it is paramount when diving. In this case, the small size produces just enough sound to be heard (without making noise when not in use). My rattles are 2 or 3 inches long. Use 2 end caps and 3 ball bearings per rattle. If you have a friend who has a cutting tool, adhesive primer and glue (I had), your cost is $3 per rattle. (Both 3-inch and 2-inch rattles made the same sound underwater.)