Passive Amplifier Optimized for a Single Frequency

First off, I know next to nothing about acoustics or sound science of any kind.  My idea is to make some sort of passive amplifier, like a horn, that is optimized for a single frequency, I'm thinking about 4KHz.  I have no clue how to go about designing such a thing and all the info I found on google was too complicated for me to understand.  Does anyone has any starting points for me or just general advice/suggestions?  Or, if you're feeling generous and/or really bored, would anyone like to design such a thing for me?