how to see if your subs are in phase?

[MACnonsence6]

is there a track or something you can play to check if you subs are in fase for w.e reason it seams like in my car shit vibrates after the bass hits not during the note kinda like a wave effect after the notes over. if that makes sence

[iceman1575]

If they're outta phase, you'll easily be able to tell most of the time. I've heard take a 9V battery & hook it up to the wires going to your speakers (with the amp off) and see if all the speakers go the same way.