I just switched out the rear speakers in my Nissan Sentra for some 6x9 inch Kenwood speakers, but now the door vibrates too much when reggaeton or bass-heavy music plays. Do I need to add acoustic material or is the problem that the speakers aren't compatible with the space?
6x9s in a rear door are gonna vibrate a lot if there's nothing absorbing the sound, that's completely normal. The problem isn't that the speakers aren't compatible, it's that the metal panel of the door acts like a resonance box and transmits all that vibration. When you crank up the bass, the door literally starts vibrating like a drum. Acoustic material is gonna help you out a lot, but you don't need to drop a ton of cash either.
The most basic thing is to put foam or acoustic insulation inside the door, between the panel and the speaker. A lot of guys use that self-adhesive foam they sell at audio shops or even on Amazon. Slap it around the speaker and on the areas where the door vibrates the most. If you wanna get serious about it, there are complete isolation kits with more heavy-duty materials, but honestly with something decent you're already gonna notice the difference.
Also check that the speakers are properly tightened and secured to the brackets. Sometimes part of the vibration comes from them not being bolted down tight and the vibration transfers to the whole structure. Tighten up the bolts good, throw some foam on there, and it should get a lot better.
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