What’s the best way to record bass frequencies? With bass mics, of course. They’re the best way to capture low frequency information. They’re specialized tools and you might not be prepared to plunk down a few hundred bucks for a bass mic.
That doesn’t mean you have to go without. This article from Instructables shows you how to make your own bass mic from a small drum and a speaker:
I was able to build this mic for under $20 by “scrounging” various parts out of old junk. Even if you need to buy all the components, you should be able to build this mic for a fraction of the price of the retail version.
This design goes slightly beyond the [Yamaho] SubKick, as far as electronics are concerned, with a dual coil design, and internal direct injection (DI) box.
You should be comfortable using a power drill and a soldering iron, and be able to read a schematic diagram. There is a little sewing, but it isn’t too difficult.
I’m a little dubious about the $20 claim, but I’d love to see one of these things in action. The quality of the microphone is going to hinge on the speaker you choose and any of those choices is probably best augmented with some low-pass filtering and another microphone to capture the top end of a bass cab, for instance.
At the very least, it looks to be a fun project. Happy soldering!