One of the most frustrating aspects of car audio is noise in the system. Unlike home audio systems, car audio systems have many sources of noise to deal with. Here's a few of the most common noises found in car stereo and how to fix or avoid them.
Alternator whine is the granddaddy of car stereo noise. The most common and the most annoying. Alternator whine will be heard as a high pitched whine that will rise and fall with the engine speed. Most of the time this is caused by a poorly chosen ground for a piece of equipment. It is usually cured by grounding the equipment directly to bare metal on the chassis rather than an available factory ground bolt as is often used to save time. You'll also need to make sure your charging system is in top condition and that your connections between the battery and components are secure as well as the integrity of the factory ground strap, an often overlooked component. You might also consider switching your RCA cables to a twisted pair model. Twisted pair cables will usually be less prone to noise than their coaxial counterparts.
Accessory pop is associated with one particular electrical event in the vehicle. This can be switching on your turn signal, headlights, brakes, windshield wipers or even the rear window defrost. These high current drawing accessories are causing a voltage spike that is traveling into your car audio equipment with the result being heard as a sharp pop. Adding a small bi-polar capacitor (0.47 uF) between the accessory's power wire and ground will often absorb these surges. You may need to place the capacitor on the load or the power side of the switch (maybe both). See the diagram below to see how this is done. Note that the diagram is slightly different if the accessory has a relay in the circuit.
I'm sure everyone has heard a car booming down the street with the body panels shaking and rattling with every bass note. This is an extreme case of body rattle. Less noticeable examples are interior panels that are worn and may slap together when a bump is hit. Door noise is less common and is usually more of a problem on the less expensive cars. Large and luxury cars are usually built with thicker metal and more sound absorption materials and are less prone to body rattle. Body rattle can be solved as easily as tightening the bolts and screws that hold the panels together or may require a full array of noise dampeners.
Noise dampeners can be any material that is used to reduced body panel noise. The thick padding underneath the factory carpeting is a type of noise dampener. Various clays and putties are also used to act as an absorber between two body panels. The most commonly available product in the automotive aftermarket is the viscoelastic sheets and spray on materials. The most well known brand of sheeting material is Dynamat, a product of Dynamic Control. Other well known manufacturers are Stinger (Road Kill), Scosche (Accumat) , Rockford Fosgate (Dead Skin), Raamat, Second Skin and B-Quiet which makes a product called Brown Bread. Dynamat has the most extensive product line as this is their specialty. B-Quiet also specializes in sound deadening and though their line is not as varied their prices tend to be less expensive. Sheets tend to work well and are relatively easy to apply. If you have a rattling license plate than adding a layer of sound deadener behind the plate will definitely help. Rattling trunk lids may be caused by a loose trunk latch or a worn weather seal. Have this checked out by a qualified body repair person. Liquid spray on deadeners are popular for the interior door panels but are a little messier to apply. These are best left to the professionals or the adventurous DIYer. Keep in mind that sound deadeners tend to be heavy so if vehicle performance is important to you make sure you keep the weight down by using sound deadeners sparingly or by going with a lighter weight deadener sheet or spray on material.
Noise suppressors are not a solution in my opinion. Instead of fixing the cause of the problem a noise suppressor will try to cover it up in a "band aid" approach. This is a poor attempt to fix the problem and doesn't always work. It costs extra money and the noise filter drops the voltage going to the component as well. This is especially important with amplifiers as their power output is usually dependent on the input voltage. Less input voltage, less output power. I feel strongly that a shop owes you a noise free installation without charging you extra for the work. If they can't figure it out then that's their problem. You shouldn't have to pay for their inexperience. Get it in writing that they will give you a noise free guarantee with no extra charges. Have them sign it and you keep the original.
Most non-equipment related problems in car audio are the result of poorly chosen ground points. Always check the integrity of every connection including the battery, head unit, amplifiers and signal processors. Any part of the audio system can bring noise into the system. In general your connections need to be secure (grounds should be to the chassis), the charging system should be in top condition and all factory connections (battery posts, ground strap, alternator connections) should be secure. You may need to increase the size of your factory ground if you install a very large stereo system. It should also be noted that the negative battery post is usually a terrible place to ground car audio equipment. All of the ripple (noise) from the alternator and other items in the vehicle travel to this point. If you choose it as a ground point then you are inviting all of these elements into your sound system.
You may also be interested in How to Install Your Own Car Stereo System . It covers many topics including in depth car audio amplifier installation. Click here.