Speaker polarity

To make sure all your speakers are in phase, unhook the speaker you want to test at the amp (both wires preferably). Using a 1.5 volt battery (any size), touch the positive terminal of the battery to the positive wire going to the speaker, then do the same for the negative wire.  Have a friend look at the speaker.  If the speaker pops out, the polarity is correct.  If the speaker pops in, the speaker is hooked up backwards (out of phase).  To fix this, simply reverse the wires when hooking the speaker back to the amplifier.  A word of caution here: DO NOT hold the battery power to the speaker for more than 1 second, all you want to do is to see if it pops in or out.  You will damage the speaker if you hold constant power to it.   Do not use a higher voltage.  Also, do not try this test on tweeters, you could fry the voice coils.  If there are crossovers with capacitors along the line, this test will not work (capacitors block DC voltage).  Bypass the caps momentarily.

A much more elegant and quicker way to do this is by using a commercially available polarity checker, which uses a test CD.  All you have to do is pop the CD in the head unit and hold the polarity tester in front of each speaker.   The advantage here is that you can test for absolute polarity of the system on all the speakers, including tweeters.  Polarity checkers are available from various companies such as Monster Cable.  Retail for the Monster Cable polarity checker is about $120.

Sometimes, when speakers are not mounted close to each other (i.e., mids on the doors and tweeters up in the dash), reversing the polarity on tweeters or mids makes the system sound better because it makes up for phase differences due to distance.   Try different combinations and see what sounds better.