What is Alternator Noise?

Alternator noise is a high pitched whine caused by the car's electrical system. When the engine spins the alternator around, the alternator induces an AC voltage that is converted to DC and used to charge the car's electrical system. It acts pretty much as an inverted electric motor (motion is put in, and voltage comes out). The problem is that a small amount of AC voltage remains in the system. Frequencies change accordingly to the engine's RPMs. If the engine spins faster, noise frequency is higher. That is why you would hear alternator noise coming from mids and tweeters, but not subwoofers, since subwoofers only play low frequencies.
  

What causes alternator noise?

1. Induced noise through RCA's:

When a wire has current through it, a magnetic field circles around it (i.e electromagnets). Conversely, if there is a magnetic field perpendicular to a wire, current will be induced. If you have your RCA wires going from the radio or equalizer to the amp running in parallel to your power wires, an AC current will be induced and added to the sound signal. The sound signal travelling to the amp is a low voltage signal (in the mV range - thousands of a volt). The induced signal will be amplified along with the music.

Avoiding this problem is very simple: DON'T run power and RCA wires together. If there are points in which they do have to cross, try to place them perpendicular to each other. Run the power wire from the battery to the amp on one side of the car, and the RCA wires along the other side of the car. On most cars it is better to run RCA's on the passenger's side, and power wires on the driver's side. Note that noise may be also be induced by factory harnesses, car computers and other electronic equipment.

2. Ground loops:

Your car's electrical system (and your stereo) use the car metal chassis as a ground (there is always current flowing through your car's metal parts). If your battery and alternator are (typically) under the hood, and you are installing an amplifier all the way back in the trunk, then current flows through that power wire you ran from the battery to the amp, and back through the metal chassis to complete the circuit.

Theoretically the car's metal has no resistance, and it should not matter where you tie grounds for amplifiers, radio, battery and alternator. They all should "look" like the same point, right? Well, the metal in your car does have resistance, and there is a potential difference from the front of the car, where the battery is to the middle of the car, where the radio is, and to the back of the car, where most amplifiers are. The potential difference of the grounds makes the whole system act as an antenna, where they pick up noise. Measure voltages at battery, amplifiers and radio. There should be very little difference between the measured voltages. If there is a difference more than 1/2 volt, then you might have noise problems.

To fix this problem, make sure that the amplifiers have a good ground first. Use at least 10 Gauge wires for the grounds (and power). If you have 2 or more amplifiers, DO NOT go from the ground terminal of one amp to the other and then from there to ground, most likely you will have noise. Ground each amplifier independently. Same thing if you have added stiffening capacitors, go to a separate ground for the cap.
  

Troubleshooting

If you installed everything using the above guidelines and you still have noise, then try to figure out what is causing the noise (a very LONG and tedious process). First, double check grounds at amplifiers, crossovers, radio, etc. Make sure AM/FM antenna has a good ground. Try to figure out what is causing the noise. For example, if you have crossovers, equalizers, etc, bypass them by hooking RCA wires straight from the radio to the amplifier. If noise went away, you know problem is maybe RCA wires or grounds hooked up to crossovers/equalizers. If you have more that one amplifier and have noise only on one amplifier try switching RCA wires around. If noise stays the same, then problem is the amplifier, if it switches, noise is coming from previous components up the line. As said before, it is very hard to find out what is causing alternator noise.

Don't get one of those noise filter boxes unless you have completely figured out that the head unit or equalizer are causing the noise. 99.9% of the time you will be wasting your money in buying noise filters.

If you have tried everything in the world, and still have that annoying noise, contact your nearest car stereo shop. Some of them will be reluctant to fix something not installed by them, or maybe will charge you a lot for something you could not figure out that only took a couple of minutes for them to fix, so shop around first.