I'm not sure, but I'll take a shot at it.

It's a general nono. It doesn't make much sense.

Some might think it'd be cool to have an 18" tuned to 20 Hz, a 15" tuned to 30 Hz, a 12" to 40 Hz, and a 10" to 50 Hz, but even if they were all tuned accurately and had the crossovers set properly, each subwoofer would still be moving at any frequency from 20 Hz to 100 Hz, and they'd all cancel eachother out from sheer differences of sound-waves. The ONLY way you could do this is with crossovers that *completely* cut off a signal when it goes out of the intended tune range, not just at -24dB per octave, the subs can't be moving all at once. If you could completely control the 18" to only play from 20-30 Hz ETC than yeah it'd work, but even then it would be smarter to just use the same sized subs. It's not like an 18" hits the lows better than a 10", how low a sub can play has to do with specific parameters, not size at all. The raw sound of the different sized drivers would get in the way of each singular sound, it would be so muddy without very-custom crossovers. I've never heard of crossovers like that so I wouldn't know if it could be done. Even if you had the same sized subs tuned at every octave from 20 Hz to 50 Hz it would be harder to get a flat response, and it's not like a sub tuned to 30 Hz can't give a flat response on up, you're only missing < 30 Hz potential and that just isn't a realistic goal for even rap music.

If you're worried about having a flat response from 20 Hz to 100ish Hz than use a sealed or 4th-order.

Here's an example of what most people avoid, although this guy thinks his setup is the , I really doubt it. Some Canadian-pro insight would be nice because I called him on it and was chewed the **** out by 40 year old Americanos, they really are meaner!

YouTube - Frankenstein 2 15's 2 18's 9,000+ Watts! Part 6

YouTube - Can You Mix Sub Sizes? Frankenstein Sez Yes As He Beats My Camera To Death!

 

Your questions have been pretty well answered here but I'll lend my .02. The reason you shouldn't use two different enclosure types is simply that they are not the same. The main difference between the two are phase and transient response. Phase is what makes two boxes sum in volume properly and transient response is what tends to "sound crisp". Phase differences will lead to some degree of cancellation. A ported box can have good transient response but a sealed will have better transient response near the tuning frequency (all else the same).

 

and I didnt catch the answer to going larger for a sealed box...

so here you go... by going to a larger box you will not be able to put as much power into the woofer as your excursion will not be as limited as it would be in a smaller box... but in return your lower frequency will have more output and your higher bass a little sloppier. it all depends what you are after, if you want to damage your kidneys by resonating them with 20Hz in your body or if you enjoy kidney jabs from Mike Tyson? that will determine what to build for

 

That makes sense. I have decided to go with 2 RE Audio SRX10D2 subs in a sealed box. They recommend .5CF/sub.

I am going to be pushing the subs just under their RMS rating of 600 watts for the pair. My amp will put out ~580 watts RMS @ 2ohms.

Does anyone think it would be a good idea to go a LITTLE bit bigger? Maybe around .7CF/sub?

I listen to mostly rock and metal, so how should that effect my box volume choice?

 

leave it at 1 cu ft total + displacement ... rock metal need to be tight and hard hitting bass or it just has no effect having subs in there... make sure you have good midbass upfront too or your bass will not sound good

 

^second that opinion. Amplify good front component set and deaden front doors (if thats where speakers are installed anyway)

 

If you have the Thiel/Small parameters of the SRX10D2 someone here could model it for you. That would give you the difference in acoustic performance as well as the true power handling of both enclosures. This would give you the proper data to make a decision.

 

^ what he said(kevmurray)

I am pretty sure going from .5 to .7 cuft will be essentially no difference (I assume you have already compensated for the volume the speaker will be taking up)

 

I think I will just build the 1CF sealed enclosure and throw a little polyfill in there. My front stage is taken care of. I have 8" midbass drivers paired with a 6.5" component set. The 8" midbass definitely add some punch up front!

 

Sounds like it should sound good. How are the8's mounted. Are they band pass x'd over? All boxes we build are partitioned for 2 individual boxes. Are you doing common airspace?