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When using Behringer Ultradrive , which one is preferred 24db or 48db slope and why?

It's not as simple as that.

The unit has capability for 6/12/18/24/48 per octave slopes for a variety of reasons. The real money in PA lies in knowing when and where to apply what type.

With a basic DJ rig there's four filters that comprise the system and they are of two types:
1. The decks, EQ, DSP, and amps - these are a lump sum electronic filter
2. Speakers - the first acoustic filter
3. Room - the second acoustic filter
4. Ears - the final acoustic filter

It's cut and dry to see what an electronic filter will do on paper. Not so much with an acoustic filter.

At a minimum for the speakers and room, this requires RTA. That's how the factory does it and it's how the big sound companies do it.

Most folks here are using an active filter network in the form of a DSP (the Behringer or DBX) to tie together their mains and subs.

The tuning process involves getting to know your gear acoustically by first setting up the rig in an equivalent performance space and running unfiltered sweeps through the sub, then through the mains and compare the resultant responses.

The really hard part is taking into account all the factors that acoustically affect the response of each channel (room boundaries, number and type of cabs, placement), the known behavior of the electronic filters, and the power limitations of the drivers to choose an appropriate slope and cutoff frequency for each filter that maximizes the performance of all the equipment involved (most importantly the quality of the sound) while at the same time keeping all the connected gear within operating limits.

Too much (bad transient response, decreased headroom) can be just as detrimental as too little (poor dispersion, blown drivers). A good rule of thumb is to use the steeper slopes (18,24,48) to tightly control the power in a high pass application, and use the mild slopes (6,12) to contour the frequency response in a low pass application.

It's a complex balancing act.

z-audio wrote:When using Behringer Ultradrive , which one is preferred 24db or 48db slope and why?

It depends on where you want to apply the slopes.

If the application is, for example, high passing your subs, in the past, Bill has said something like: "use as steep a slope as possible".
If you have a DBX driverack, IIRC the steepest slope is 24dB.
So the minimum acceptable slope is 24dB.

Since your question relates to the DCX2496 however, as a high pass for subs, you should use a 48dB slope.
Two reasons:
1. Because you can.
2. It affords the maximum amount of protection for your driver(s).

You could apply the same thinking to low passing tops/mains.

After that, you enter the grey area, that as Radian has suggested, requires experimentation.
That relates to the crossover region.

You could use 24 or 48dB at a set frequency, eg 90Hz low pass on subs output and 90Hz high pass on tops/mains output.
What that does in terms of affecting frequency response is dependant upon which type of slope you use, it varies with Butterworth or LR slopes, as examples.

There are other ways of setting crossover points though.
For example, if you have T39's, you'll notice in the SPL charts that T39's have a rising response from around 70Hz. You might want to utilise that, so you might set a subs low pass at say 60Hz with a 12dB slope, and leave your tops/mains high pass as suggested earlier.

If this hasn't helped, give us more information about what you want to know...

Thank you very much for the responses. It was very helpful.