I just put a 12-inch Hertz subwoofer with its own amp in my Seat León's trunk. The problem is the bass sound seems to bounce a lot and sounds hollow, doesn't hit the way I expected. I'm guessing it's either the space where I installed it or the wiring, but I'm not really sure what to check.
First thing is to check that the sub is properly sealed in the box; if there are air leaks or it's not secured right, the sound disperses and loses pressure, that's why it sounds hollow to you. Also check the phase on the amplifier (the switch that says 0/180), because if it's inverted compared to your car's sound, the bass frequencies cancel each other out. The power wiring should be well isolated away from the audio cables to avoid noise, but that won't cause that hollow effect you're describing. Once you've confirmed everything is mechanically solid and the phase is correct, try adjusting the amplifier's filters (crossover and gain) so it works properly with your original equipment.
Have you tried adjusting the subwoofer amp's gain or the crossover frequency settings on your head unit? The sealed box thing is fine as a starting point, but before you go opening things up I'd recommend checking that first - sometimes the equipment comes from the factory with the controls set way too low and sounds weak or muddled precisely because of that. Plus, if the signal cable is connected wrong or it's low quality, you also lose definition and the punch dies out. Once you get that dialed in, then yeah, check if the box is properly secured to the trunk without any movement.
Something that nobody really mentions is where the sub is mounted in the trunk. In a León, if you put it directly on the floor or against the side, the sound can bounce all over the place because there's no dampening. Try putting some rubber pads or acoustic foam underneath and behind the subwoofer, especially if it's touching metal directly. That absorbs parasitic vibrations that create that hollow effect you're describing.
Another thing: check how the subwoofer is oriented inside the box. Some Hertz subs have the port facing backwards or to the side, and if it's pointing toward the wheel or toward a corner where air bounces around, you lose a lot of punch. Point it toward the inside of the trunk or better yet, toward the back seats if the box allows it. Sometimes just changing that makes it sound completely different without touching anything else.
Don't start swapping cables or opening the box yet, that's just what people rush into without thinking! What nobody mentions is that in a trunk as enclosed as a León's, the bass needs acoustic damping because it bounces like crazy off the walls and the rear window, and that gives you that hollow, punchy effect you're describing. Try putting some acoustic foam or sound-absorbing material around the subwoofer (in the trunk, not inside the box) and you'll see how drastically the sound changes. Sometimes it just needs you to control the room reflections, not perform surgery on the equipment!
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