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Troubleshooting: NOISE

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  • Troubleshooting: NOISE

    So I picked up this months Car Audio and Electonics mag (May 2005) and the first trouble shooting article just jumped out at me.

    I'm going to reproduce it becuse I think that the article covers what so many people on here have problems with: NOISE.

    This is not the deffinitive source but rather a place for people to start and a place for people to get a better understanding of what can cause noise.

    Quit Yer Whinin’
    By: Derek Lee

    Q: Hi guys. I’m coming to you now for a little advice on the stereo I have in my 1990 Isuzu Impulse XS. I don’t have all the exact model numbers on any of the gear because I’m typing this on the spur of the moment in a little café in the mall. Anyway, I have a 6-month-old Pioneer Premier series deck (flip-down face), a set of 6 ½” coaxials for my front stage in the factory speaker location in the doors, another set of 6” x 9”s in boxes laying on by back seat because the factory locations for the car’s original speakers in the back of the hatch; my sub box is back there so it was drowning out my rear fill.

    For bass I have two 12” Boston Acoustics Competitor Series subs running of a Pioneer PRS 7200 amp. My other speakers all run off a 3-year-old Clarion amplifier of which I can’t remember any details. I also have an AudioControl Epicenter for added bass when I just can’t get enough. I don’t know the exact box size, but I believe it to be about 1.25 ft^3 per sub.

    My problem is that I have an extremely loud whine when I am driving. From what I’ve read, I think it could be alternator noise by really don’t know how to fix this problem. Also, I recently got this Pioneer deck installed by a certified installer. Since I got this deck put in, it pops when I shut my car off or change CDs. I know I didn’t give much info on the model numbers of my gear, but any help would be appreciated.

    Daryl Damsgard

    A: It’s hard to determine whether the problem is related to the system in general or the installation of the head unit. I also can’t tell if your installer is a professional or not. Obtaining a certification means passing a pretty easy test, so it does not confirm skill. Normally a professional has established skills from hands-on training either at a school or in a shop environment. So let’s drill down into the way noise operates to find the source of the problem.
    The first step in noise suppression is to determine the type of noise and the way it enters a system. A loud whine can be from any number of causes, but if the whine varies with engine speed, it’s almost always “alternator whine.” As the alternator converts engine power to electrical power, it produces a tone based on frequency. The frequency varies directly with the engine speed, in which the faster the RPM, the higher the pitch of the noise. If the noise is present when the engine is not running, it can’t be alternator whine. The alternator internally produces AC (alternating current) but internal diode arrangement called a “bridge” converts it into DC (direct current) to power the vehicles entire electrical system.
    That’s where the noise comes from, but if you try to stop the alternator from producing the noise, it also stops producing a charge for the electrical system. At best this proves the alternator is working correctly. It is now our mission to find how the noise is entering the audio system. Ideally, the energy coming down the alternator output wire is DC, but only in a perfect world. There is a significant amount of AC that “surfs” along with the DC. This AC component can infect your audio system, and how it is entering is the big question.
    There are three ways that alternator whine can infect a system: through the power line (called “power line noise”), radiated through the air (“radiated noise”) or induced as a difference in voltages that reach each component (known as “ground loop noise”).
    To determine which one you have, a bit of experimentation is needed. Start by connecting the power leads for your audio system to a new power supply. The easiest way is to run cables from the battery of another vehicle that is not running. Your audio system will then take it’s power from that vehicle’s battery, which will be delivering only clean DC. If your system no longer whines when the engine is revved up, it means that all the noise was traveling through the power lines from your battery. Start moving wires back to your car one by one starting with the amplifiers. My experience is that the head unit will be the one that adds the whine back into your system.
    The solution for that I usually a simple noise filter installed in series on the unit’s power line. This doesn’t indicate a problem with the head unit – your car is the problem. Virtually any head unit would reproduce the whine since car audio manufacturers can’t install each of the several hundred different noise filters into the head unit itself. It is up to the trained technician to determine which is the right filter for the job.
    The next possible noise is called “radiates” since it doesn’t travel through the power wires, but is transmitted from the noise generating wire into whatever wire or circuit is within its radiation field. This type of noise is often called EMR (electromagnetic radiation) and is pretty difficult to correct since the path it follows is invisible. It is, however, very solvable and is not a reason to live with the noise. It does require a different approach to locating the cause and then correcting the source.
    If, in your previous test, you powered the audio system from the battery in the other car yet the noise continues without much change, you can use a device called a “noise sniffer” or EMR detector to find the hot spots in the vehicle that are radiating the noise. For example, one of the most common noise modes is when a head unit picks up the EMR that is radiated from the factory wiring in the dash. In this case you can use the head unit as a noise sniffer since if you were to unmount the head unit and pull it gently away from the dash while it is playing, you would notice the noise fading out. That means that the noise is being radiated from the headunit from surrounding wiring and moving a few inches away solves the problem. Now it is a matter of either finding the offending wire that is radiating the noise (hint: usually the heaviest gauge wire carries the current levels large enough to create a big noise field) and relocate the wire along another path. As an alternate method, using a magnetic shielding metal between the head unit and the wire harness can also yield good results.
    Almost anything in the signal chain can pick up radiated noise. Power and preamp cables, processors, crossovers, speakers – even a noise filter installed in a noisy area can increase rather than decrease noise. I have seen speaker crossovers mounted under the back seat of a BMW pick up noise that collects around the vehicle battery and speaker wires pick up noise from the factory engine control computer in the kick panel of a Honda. The easiest and most effective solution is to keep your audio components out of range of the factory wiring and modules.
    The final method noise uses to enter the audio signal path is thorough “ground loops.” In your case, you have a variety of brands of different ages combined to make a system that is meant to work well together. Sometimes these devices get along as well as the participants on The Apprentice and saying “you’re fired” doesn’t do a thing.
    If you consider what is happening with the ground loop, it makes understanding easy: Each component has a different length or power wire and draws a different amount of current. That means that the voltage drop on each power wire is not exactly the same. So, if your head unit is receiving 12.2 volts after line losses and the amplifier in the trunk gets 11.8 volts after it’s line loss, the difference is 0.4 volts. This difference can try to balance its self out by taking a shortcut through the preamp cables. The preamp cables should be carrying no more than the audio signal, so when you add alternator whine, it adds to the music, creating a frustrating listening experience.
    To deal with a ground loop, you need to find out where the ground loop occurs. If the voltage is trying to return to the negative terminal of the battery by traveling through the body of the vehicle and the preamp cables, there will be a difference. By installing a ground loop isolator (different from an alternator noise suppressor), you can often add enough circuit isolation to force the DC to travel only along the ground wire and chassis of the vehicle, while the musical signal uses the preamp cables.
    Jan Bennett
    FS: VW MKIV Bezel for 8" Lilliput - 95% Finished

    Please post on the forums! Chances are, someone else has or will have the same questions as you!