Speaker Phase Checker Check Speaker Phase, Stereo Polarity (Phase) Sound Test
Speakers are out of phase if the wires for one of your speakers are connected the wrong way around. When this happens the sound from one speaker cancels out the sound from the other so you get reduced bass and poor stereo imagine.
If you had to connect your speakers with wire with bare ends pushed into a connector or post it’s possible to inadvertently connect one speaker” the wrong way around”. If this happens the speakers will be out of phase and unless you are an experienced audio engineer you might not notice.
So it need PHASE CHECKER to make sure the speakers work well.