ACOUSTICAL TREATMENTS
Even when the mounting structure is built solid, any areas around and behind the speaker need to be treated so that they do not resonate or vibrate and detract from the sound we want to hear. A number of sound deadening and damping materials from any of several manufacturers can handle this. Metal panel vibrations, as can occur with door outer and inner panels, can be controlled with adhesive-backed mass damping materials. Adhesive-backed absorptive foam and/or reflection-controlling silicone rubber materials can be used to attenuate and control the back wave from the speaker cone, preventing it from reflecting off hard surfaces behind the speaker and then coming through the cone, causing distortion.
MIDRANGE/TWEETER MOUNTING PROXIMITY
The number one question regarding high-frequency speaker placement is, "Where does the tweeter go, relative to the midrange?"
It's a good idea, if possible, to keep the midrange and tweeter close together. This will minimize problems caused by separating them by large distances. These problems include comb filtering and other frequency response problems, instruments and vocalists that seem to come from multiple locations, arrival time issues, etc. In general, separating the midrange and tweeter make it much more difficult to tune the system's overall sound quality response.
In fact, quite often, a good speaker choice is a set of properly designed coaxial speakers. They can sound very good, with very minimal effort. With coax speaker designs, the speaker manufacturers know exactly where the tweeter will be relative to the midrange, and the system can be designed more critically. With even the best quality separates, the manufacturer must make assumptions about where the tweeter will be mounted, and if they are not installed accordingly, the speaker system's response may not be optimized.