The next step in OEM 
integration is how to get around the factory “EQs” or “Noise cancellation” systems. Let’s break down these OEM features:
EQs or Equalizers – General Motors incorporated a simple protection system for their OEM speakers in their trucks during the late ‘90s. The volume control in the vehicle went from 0-30 on the factory volume knob. At volume 0-10, the head unit would boost the head unit’s treble and bass. From 10-20, it would remove the boost that was added from 0-10, a flat equalization curve. At 20-30, the head unit would remove treble and bass from the system; this EQ was in place to protect the factory speakers from being overdriven or blowing speakers. But how does this relate to today’s OEM installations? If you added an amplifier and subwoofer to this style of factory head unit when you turned up the volume knob past “21,” and the bass would disappear. Defeating the whole purpose of a subwoofer system. Many other manufacturers use a similar system; I am just using GM’s version as an example.
How do we get around this “lovely General Motor’s innovation”? At the time, you would have to set up the vehicle to reach maximum volume between 10-20 on the volume knob. The customer would have to be advised that his system was only to be used between 0-20 on the volume knob. It wasn’t perfect, but it worked.
Fast forward to today’s aftermarket equalizers; they can adapt to the factory EQ curves. Essentially giving you the same sound at every volume level. I will cover these EQs in Part III of the OEM integration series.
Noise Cancellation Systems – Recently, I called a dealer who was installing an eight-inch Bazooka tube. When the vehicle was off (not running), everything worked great. But when you started the car, a loud hum would come from the speakers. By hum, I mean so loud you could not talk it over. At first, I was thinking bad ground or induced noise from somewhere in the car (seemed logical). After checking the installation over, I placed my ear to the rear speakers from which I was getting the signal for the Bazooka tube. And it was humming in a low tone when the car was running. The Bazooka tube was amplifying the hum of the factory speaker.
Ford, Toyota, and BMW are some of the manufacturers using noise cancellation technology on their vehicles. How does this work? Bose “Noise-canceling” headphones have been using similar technology for years. How is this technology relevant to automotive applications? Road noise is the noise of your tires against the roadway. The vehicle analyzes the road noise from your vehicle and vibrates the speaker out of phase to “road noise.” We have all experienced speakers that are out of phase, and the bass disappears. Since road noise is centered on the same frequencies as subwoofers, the car sending out an out of phase tone through the speakers, thus canceling out the “road noise.” It is a cool technology for road noise, but it makes it impossible to add subwoofers. The more subwoofers you add to the vehicle, the louder the factory system plays tones to cancel it out. This is why the dealer had a hum in the system; the hum was being produced to cancel out the road noise, but in this case, the road noise was the Bazooka tube. Is it possible to get around this system while maintaining OEM integration with the factory head unit?
Yes, the solution is easier than you think. In the headliner of the vehicle, you will see a small-perforated grill. It looks like the head of a small microphone or a speaker grill the size of a nickel. This is the microphone that analyzes the “road noise” inside the passenger compartment. Unplug it.
In part III of OEM integration, I will cover adaptive equalizers that automatically adjust to changes in the volume and factory added equalization curves.
CanadianCarStereo.com
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