The 18 db and 24 numbers tell the rolloff of the filter, for example an octave is a doubling of frequency. If you set an 18 db/oct ss filter to 40 hz, then at 20 hz it will be 18 db quieter, the 24 db/oct would be 24 db quieter. If you're looking for extreme protection of the sub, then the higher slope (24) is better. If you want a little less rolloff, go with an 18 or 12db.
Sound quality wised, less rolloff usually means better quality, I'de go for the 18 or even 12 if available. Higher freqs. hitting the subs won't damage it, rather make a smoother integration, that is, if the other xover are on the same slope. Enjoy
actually for a subsonic filter, you want as steep a rolloff as possible to prevent the subs from losing their damping in a vented box below it's tuned frequency.
where joao got the idea that a shallower rolloff is always better is beyond me. the rolloff curve is determined by what order the crossover is usually. a 1st order xo is 6dB/octave. 2nd order is 12dB, 3rd order is 18dB etc. this is just a matter of circuit complexity but you generally want to match the Q or slope of the crossover to what it's being used for.
some tweeter crossvoers use a 6dB/octave slope, but 12-18dB is normal for tweeter/midrance slopes as well as midrange to subs. steeper is good sometimes if you're using more notch filtered drivers for things like midbass, and you don't want a lot of overlap. it's a matter of fitting the horse to the course.
I got that experience from my ears, and the way I like music to be played (What seems to me as the nearest of the original), but that, of course, is only my opinion! Enjoy
the slope and overlap will be different for every system. you can't use one statement to cover everything or there wouldn't be any point to having different Q factors for crossovers.
Depends on the speakers you're using, too. Dynaudios are a good example, the drivers are designed to roll off near the cutoff frequency, which makes a shallow slope crossover better for them, because you have a much simpler crossover that doesn't interfere with the music as much.
Yep I agree with Jonathan, though there should be examples of the opposite. Nevertheless, all brands I heard, I usually like the 12 dB better, sounds more neutral to me. Maybe it's just my ears. Enjoy
Glass gave examples of the opposite, subsonic filters for subs that are ported, another a good example is this: suppose you have a component system and the mid is capable of playing to 7 khz (pretty high, but just assume) loudly. If you let it play that high though, you would get a screechy midrange that would detract from the SQ. This is when you want a steeper slope. Also, with tweeters that may be damaged more easily with lower freqs, you need a steeper slope. Assume you are making a high quality system in your car, you naturally want the sub to play the most it possibly can while still blending in with the front drivers, but the sub you have begins to lose SQ and increases distortion above 100 hz. If you have a shallower slope (say 12 db/oct), the sub will be down 12 db at 200 hz, and 6 db at 150, etc, so the sound wouldn't blend as well as you think it would, plus the distortion would be greater and SQ would lack because of this. Put a 24 db oct crossover in this case, and you get better blending of drivers AND the sub will still play all the way to 100 hz, w/o as much distortion. Everything has it's benefits and downfalls. The downfall of steeper slopes is that you have more electronics to go through, the efficiency drops because the speakers are getting less power below the crossover point. The vehicle you use is temperamental and also the design of the components within all contribute. Glass is right, I was just adding to that, it's all in what you're using it for.
don't bother, Jon. I don't know if it's the language barreir or just lack of understanding on Joao's part, but I'm getting a bit tired of correcting the misinformation, and arguing about it.
if he thinks a 12dB per octave slope is the best thing since sliced bread, terrific.