Why do I need bass traps??

If you plug everything in direct and always listen with headphones you don't.

If you never record or listen to any sound lower than about 200Hz you don't.

OK. What are bass traps and what do they do?

Bass traps are either broadband or tuned and are placed in corners. Low frequencies have high energy levels in corners.

A tuned bass trap operates on the same principle as blowing across the mouth of a bottle in order to create a note. The frequency can be changed by varying the liquid's level in the bottle. The tuned bass trap absorbs energy at the "hoot" frequency. They are generally placed in corners and are used when there is a large single note anomaly. Their official name is Helmholtz Resonator and they're normally fairly large.

Broadband absorbers are also placed in corners and have high density sound absorbing insulation and air layers. A search of the interwebs will yield an amazing amount of designs.

How do I know what kind of bass traps I need?

Donate $10.00 and get REW software. You will need a mic with a flat frequency response and an open mind. You will also have to deal with the realization that your room sucks.

Pretty soon I'm going to run out of pictures