Determine whether the noise is caused by the common ground: 1. Use the headset to connect with the car MP3 and the AUX (3.5mm interface) of the car navigation. If there is no noise heard from the headset at this time, it can be confirmed that the noise of the car equipment is caused by the common ground. Caused. 2. Use the audio cable to connect the battery-powered mobile phone or MP3 (pay attention to the mobile phone, MP3 should not be connected to any power source in the car, such as a telephone) to the AUX of the center console of the car, and play the music of the mobile phone, so that there is no noise, then It can be confirmed that the noise in the past is caused by the common ground. 3. Computer, power amplifier, audio, CD: Use passive headphones to plug in the computer to test whether there is noise, use the mobile phone to plug in the power amplifier, and listen to the audio. 4. After starting the car, the installed power amplifier has a current sound, and some models will also have the situation that the bigger the accelerator, the louder the current sound, this product can solve this problem. 5. There is current sound between the power amplifier and the speaker or the speaker. This product can be installed between the power amplifier and the speaker. 6. If the laptop is connected to the projector and the power amplifier at the same time, there will be noise. You can connect the laptop to the adapter power supply or use the battery. At this time, the noise disappears when the power amplifier is unplugged, or the same noise disappears when the projector is unplugged. Add a common ground isolator between them to eliminate noise.