A door is complicated enough that we have to break it down into components for discussion. As with most things related to vehicles, inner, outer, front, rear, left and right are relative to a seated driver. The outer door skin is the painted exterior of the door. It's often one of the more resonant panels on the vehicle. In most cases you'll only need to install CLD Tiles on the outer door skin. Barrier layers may be appropriate if inner door skin placement won't work, you are concerned about your door mounted speakers being heard from outside of the vehicle or you are pursuing quiet deep into the realm of diminishing returns.
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