Joe X

You can easily run kicker 12s which like 2.25-2.5 cubic feet each, You likely don't have 8 cubic feet net, most trunk cars don't have that much. There are some 15s that will run in lesser airspace. like DC Audio brand which will need 3 cubic feet each but very few 15 subs will run great below that. When you have space limitations you alternative is more power and subs that can handle the power to get loud in smaller boxes, there are subs that can handle many thousands of watts of power each.

Here is a box made to be similar to the one you linked but a bit better for the lows and the cutsheet can be completed just with a table saw and a jigsaw, no need for router: Power: to sub(s) rated, amp subsonic filter to 28Hz, minimum port clearance 4", figure is just an assembly guide:

Those are specified to work in 3 to 5 cubic feet net INTERNAL each, say, we pick 4 cubes for each as kicker subs prefer larger boxes and you would need 8 net, you don't have that. You are calculating external volume, net internal volume is the airspace inside the box, the space occupied by the sub, the port and the wood doesn't count. You will only be able to properly fit 1 sub of those. but it would be louder to use 2 12s that would need 4 to 5 cubic feet net internal that you should have.

It could be bad RCA terminals (either end) or the amp itself, very possible if you installed the amp in the sub box. You need also to check your power lines and your turn on signal as well. And also the sub to amp lines at both ends, it could be even the sub. Test everything and see what you find.

1) Yes you have to account for the displacement of 45s but in most cases this displacement is very low. 2) If you add damping to the interior of a ported box it results in a reduction of output around tuning, it also can reduce port noise in a misdesigned enclosure, it is beneficial for flatness but it will reduce output and therefore usually is not used. I will look into the modeling of the P3s however it would be necessary for me to know what amp are you using.

The best way to get flat response is what you did, run an array of high diameter subs in sealed if you don't mind the larger box. To me two standard 12s in a large vehicle cabin is insufficient but if you have to work with them then the box is the way to get them going lower by tuning low with proper port area and going with the maximum allowable internal volume. Once the box has the ability to play low you can use a DSP to try to even things out if needed, and hopefully you will like the outcome.

Depends on what 18 and the maximum dimensions of the trunk. (h,w,d) 89

You can use a 1500W RMS amp no worries, just no clipping, subsonic set to 28Hz, careful with bass boost and other tone controls after setting gains.

So here is a suggestion, power: to sub(s) rated, amp subsonic filter to 28Hz, minimum port clearance 3 1/2", figure is just an assembly guide: NOTE: If building outside you need to build a cardboard dummy box to test fit before building the real box. NOTE: Box can still be used with better subs for a future upgrade.

OP post the model for the speakers and also the model of the amp. Maybe you will need two amps but let's see.

I would definitely look into seat lift kits to fit a box of proper size for those subs, make that enclosure with 3/4" birch wood, dual layer for the baffle, proper bracing inside, specs are 4 cubic feet net 64 square inches of port area tuned to 32Hz, I believe the sub cutouts need to be 7.34". Depending on how much space you find to work with I could fine tune that spec to increase output and low end extension as possible.

So here is a suggestion, power: to subs rated, amp subsonic filter to 28Hz, minimum port clearance 3 3/4", figure is just an assembly guide:

That box is designed for SA-8s as a result the X subs are going to find themselves in a too small box with low port area and tuned too high, surely above the 36Hz specified as driver displacement for X subs is larger, maybe it will get to 38Hz or more and could experience some port noise. Used to a large box tuned very low you will definitely notice a loss in bottom octave bass. Regarding to your question, you can always strengthen the box but if it is made of cheap thin 5/8" wood then it is not impossible. Lastly the pre cut sub holes cannot fit X subs, you will have to work on the holes. Overall that box seem expensive and not adequate for the subs. I would lift up the seats and do a proper custom enclosure for those subs and even then you probably have better bottom octave bass right now than you will ever get with some 8s.

Should be fairly easy to determine what's at fault just swap parts, you have another amp, test with that, use another sub and test, check your gains, do it fast so that you can send the product back for warranty if necessary.

RFP are HE2 not HX2, driver displacement are not the same.

Most important is to check the model of your alternator to make sure is actually 120A, if not what I am saying is not valid, secondly you should get an AGM battery of decent capacity, probably that should be enough to reduce the dimming, a big 3 upgrade is recommended, take a look at the link below to read some more on the topic: https://learn.sonicelectronix.com/5-steps-to-eliminate-headlight-dimming/ The other way to to help your electrical system is by using higher diameter subs which are much louder for the watt, a single 10 sub in a big vehicle will make you up the volume level a lot to get decent output, a 12 sub can gain you 2-3 dB just by merit of size, A 3dB difference means you can be as loud with a 12" on half the power that a 10" sub need.

Your vehicle is supposed to have a 120A alternator, if that's the case your electrical should be able to hold at 13.2V so as long as you only play with your car on you are good to go, to reduce the dimming substantially you could upgrade your battery and add a capacitor at your amp terminals.

13.2 V is nothing to worry about but if you are stressing a lot your electrical things may get worse over time so you may want to list what subs and amp models you have and what is the vehicle. That should be enough info to tell you if you are likely to have problems going forward or not.

Don't actually know what are you saying but if the lowest voltage you ever get is 13.2V in your car you might as well do nothing unless you plan to upgrade your audio.

You can play a 5 or so second 40Hz tone and up the volume level until the issue presents itself. Use a multimeter to measure (while the tone is playing) voltage between the negative terminal of the battery to the stud to see how good is your chassis ground for example. It should measure close to zero volts Then to the ground terminals of your amps and then to the positive terminals of your amps, you can do the same to any point of interest including the drop at the battery terminals themselves. If when the tone is playing the voltage drops significantly at your battery terminals then your electrical is not working right. If your wiring is right you should see most of the battery voltage drop at the amps terminals and very little at any other point, if that is the case your wiring is good. Hopefully you get the idea.

Many mainstream brands feature that "signal sensing" turn on you want, a notable brand would be JL Audio, many amps from them are capable of that. But that feature is not available in most high power amps (if not all) so usually you ad a low cost device between the amp and the head unit that will both convert to "RCA level" the speaker outputs of your head unit and create a "turn on" signal for you that you can use with one or more amplifiers that you may have, check out the following link: https://www.audiocontrol.com/knowledge-base/how-to-power-and-turn-on-the-lc2i/ If you happen to find the high power amp with that "auto turn on signal" please let us know which it is, so far it has eluded me.

Someone needs to test if the alternator is charging properly, where the voltage drops in your lines to determine what is at fault, you need to have that done by someone or do it yourself, hopefully you will find what is wrong.

Due to the considerable voltage drop it looks like your electrical has a problem, ideally you should run a 1/0 line to each amp and have two 1/0 lines to ground and you should make sure your grounding location has good contact, with some vehicles is better to run ground lines directly to the battery. So have that electrical system throughly checked, battery, alt , big 3 and power lines. Once that is done set your gains with a scope or a DD-1 both head unit and amps to makes sure there is no distortion in the signal path. See how that goes.

If you have D2 subs wired down to 1 ohm and the amps are the MXD-2000-2 version instead of the MXD-2000-1 then that's your problem. Full bridge amps can't be run below their minimum stable impedance. In any case that setup of yours certainly is odd, high end electrical + walmart amps + low power subs, you sure don't mind to post pics of that system right? Would be awesome to see.

Voltage drops mean your electrical system can't handle the power the amps want, electrical upgrades (agm battery or batteries, alternator, big 3, power wiring) are available to fix those voltage drops and when that is fixed you need to setup your gains so that no clipping occurs. your electrical system upgrades need to be calculated to match the needs of your system but not significantly exceed what is needed as you may end up overspending badly.