Features: Perfect for safely troubleshooting and testing audio systems. This phase meter can test all speakers of all car, such as small treble, door speakers, subwoofer, etc. Low battery light on indicates that the battery power is low. Effectively used to test speaker polarity. Professional manufacturing, high reliability. Specification: Condition: 100% Brand New Material: Plastic Color: Black + Silver Size: As Picture Shown Built-in Battery: 1 x 9V 220mAh battery Package Weight: Approx. 139g/4.9oz #How to use the tester? the CD with the package include 4 section test music. section 1: left channel noise signal section 2: right channel noise signal section 3: phase test signal section 4: same as section 3 Put it in car cd player, and play section 1 or 2, check if left/right channel connection is right. Play section 3 or 4, the sound speaker will produce a fixed frequency signal, then press test button. 3 green light flash and 1 red light indicates that the speaker output is in positive phase correct connection); 3 red light flash and 1 green light indicates negative phase (reversed connection). Package List: 1 x Phase Meter 1 x CD .