I tried posting about subs in beds once and got absolutely burned up, but I'll have another shot at it anyway........if you put subs into a trunk, which is exactly what the AV's bed is when shut on both ends and covered, you have a somewhat sealed enclosure with a sub box in it. When they are playing, they are trying with all their might to move air, hopefully to the listener's area so that he/she can hear it. The problem is that the 'trunk' is sealed away from the cabin, and a nearly sealed enclosure itself, so the air is attempting to escape, and the trapped acoustic energy is manifesting itself as vibrations and flexing panels, making the AV sound like its about to fall apart. Dropping the midgate should allow air movement to the cabin without problems at all, but makes the backseat useless. Why didn't Chevy's design guys think about this?? Should car audio be their top priority when designing a vehicle?
To make matters better, play some music that you listen to and hear a lot of noise with, and play it at your usual volume. Track down vibrating pieces of the somewhat flimsy Av interior pieces (it is built better than most Chevy's seem to be I will say in its defense) and remedy the issues one by one. You have two options on a fix for these parts. You can either mass load them, with Raammat or Dynamat, concrete, lead, you get the picutre. This will increase the amount of energy required to make the objects vibrate, thus reducing or eliminating totally the noise issue. The other method is to increase the rigidity of the objects, for example attaching the flimsy little midgate hinge cover pieces to the Av in a more substantial way with hot blue, silicone or whatever adhesive you choose. Either way can get tedious and time consuming, but is well worth the work. You will be tremendously pleased if you can quiet the interior noises; your subs will sound totally different. Human hearing seems to detect what should not be present moreso than what should be present with music. Trained ears can tell if there's a response drop in the upper midrange or a midbass peak that is muddying the low end punch, but any ears will automatically home in on vibration parts that are buzzing and humming. If you ever hear a really good competition car, the subs will probably be impossible to localize, this is in part due to the fact that when objects vibrate, you hear the sounds they make and, being of higher frequencies, they are more directional and help identify where the energy, which is sub bass, is coming from.
Port noise is just as bad and means you are probably not getting the absolute best performance from you enclosure due to restriction. This is not always the case, some ports are just poorly shaped or installed haphazardly to generate the noise. I have seen many cases where "port noise" was in fact a leaky enclosure, a couple times around the ports, and the whistling was the air being pushed through these crevices.
If your box is nicely sealed, except for the ports of course, consider altering their size to allow for less restriction, thus eliminating the port noise. You seem to have a pretty severe case since you can hear them in the bed from the drivers seat. Stuffing foam into the ports will simply add resistance to the ports and alter the tuning of the enclosure, which defeats the entire design goal of a vented enclosure in the first place. I you were to go that route, you'd be better off just making the box a sealed enclosure.
As for the back seat sound quality, this is a common problem. Ignore it. You drive the vehicle in the front seat, so make it your primary concern. The backseat is in a totally different area of the vehicle and if it were optimized, the front seats would suffer greatly. However, getting rid of the vent noise and rattles will definately make the back seat sound
much better!
What type of subs do you have, as in model number, what is the length and depth of the ports, and how large is the enclosure? If you don't know the internal size, give me the external and I can calculate the internal from that.
Hope I helped,
DS
