This is a simple, but important topic.
All speakers as most of you know have a positive and a negative terminal. If these terminals are hooked up backwards the speaker will still work, and if only one speaker is used you will probably not even notice the speaker is out of phase.
The problem comes into play when using more than one speaker cabinet, or multiple speakers within one cabinet.
If one of the two speakers is hooked up backwards then the two speakers will be out of phase and when played together they will sound very thin as they are essentially cancelling each other out. This is because one cone is moving out while at the same instant the other is moving in.
In cabinets with multiple speakers it is also important if you have to replace a speaker that you match it up as closely as possible to the original, or replace both at the same time. The reason is if the two speakers have different efficiencies and cone materials then the cones will move at different rates which creates another type of phasing issue.
When hooking speakers up you cannot always rely on the markings on the speakers. I have seen many speakers over the years that the positive & negative leads were labeled backwards.
The good news is that it is extremely easy to check for phasing issues and proper speaker polarity, all it takes is a 9 volt battery.
With nothing else hooked up to the speaker touch the positive lead of the battery to the positive lead of the speaker, and touch the negative lead of the battery to the negative lead of speaker. As you are looking straight into the front of the speaker it should move towards you.
The motion when the speaker pops when you hook up the battery is what you are looking for. . Do not leave the battery hooked up for more than a few seconds at a time.
When doing this with a cabinet you can simply use the input terminals of the cabinet to perform this test.
Randy Morgan ….. Owner & head technician at All Service Musical Electronics Repair