You can actually receive those 3-30kHz signals (which are still being broadcast around the globe today) with a garden variety soundcard, as long as you have a s

You can actually receive those 3-30kHz signals (which are still being broadcast around the globe today) with a garden variety soundcard, as long as you have a suitable antenna (which actually isn't that hard to build, since you don't need a particularly efficient antenna to receive signals). Helps that you can get sound cards with ridiculously high SNRs (compared to say, regular radio receivers) for dirt cheap.