Box design for Eminence Omega Pro 18C

[ted01]

I'm looking for box designs for the Omega Pro 18C. I'd like a design that would give me a really low F3 (25 - 30 Hz) while maximizing the power handling capabilities of the speaker. I've been looking around on the 'Net and I can find ported designs that will give me a low F3, but with only a couple hundred watts of power. Higher power leads to higher F3's. These have been mostly ported designs (like the ones on the Eminence web site). I have a pair of JBL JRX125's that I would be mating these subs with and they go down pretty far, so I would probably be crossing the subs at 100Hz or maybe even less. They would be used for outdoor rock events. The keyboard player uses some really, really low notes and I'd like to be able to reproduce the gut wrenching lows he plays. In a perfect world, I'm hoping for a set of plans that are free, however, I'd be willing to pay a modest sum for plans that were designed specifically for these drivers. The ultimate game plan is to have four of these subs, two per side, matched with four of the JRX125's. The whole mess will be powered with four QSC GX5's (700 watts/channel into 4 ohms) and actively crossed using a dbx two way crossover. One other concern is the HPF that Eminence calls for. My xover has a HPF, but it rolls at 40 Hz and I'm hoping to go lower than that. How can I provide the protection needed by this HPF to avoid over-excursion?

Thanks!
Ted

[MJL21193]

Originally posted by ted01
I'm looking for box designs for the Omega Pro 18C. I'd like a design that would give me a really low F3 (25 - 30 Hz) while maximizing the power handling capabilities of the speaker.

Hi Ted,
The limitation on all bass drivers is Xmax. In the case of the Omega, it's 4.8mm. This is not enough to get the kind of frequency response and power handling you want.

Here's a look at the modeled excursion of this driver in a sealed 97 litre box. F3 is 61Hz. At 70Hz the driver is starting to exceed Xmax at 200 watts.
Think about looking for another driver if you want low bass at high spl.

[MJL21193]

Here's a 170 litre vented box. F3 is 39Hz.

This is excursion at 200 watts. See how it shoots up below 40Hz?

[technofreak]

Omega 18 is not a good driver for the money - there are better ones. If you really need those mid-30 frq.-s I would go for Precision Devices PD 186 in 250 liters box tuned to 33 Hz. But I quess that actually you don't need anything below 40 Hz with even the most craziest rock band ever. Those gut-wrenching deep freq.-s are actually centered around 50 Hz.

Atually I personally would put that PD186 into 150 L box tuned to 35Hz - IMO those 6 Hz you win with almost twice a big box are not worth it for rock bands (dubstep or DnB is another story), just makes a bigger and heavier box you have to carry around.

[ted01]

Hey John,

I think I'm beginning to understand. More box volume = lower freq. but with less power. So if I want to get down low, I need a big box, but can't shoot lots of power to it. I've been looking at the designs on the Eminence website and it looks like the loss in acoustical output for a larger box vs. a smaller box, and less power (for the larger box) vs. more power (for the smaller box) isn't linear. The larger box with 200 watts at 50Hz produces about 121 dB SPL, whereas the smaller box with 400 watts at 50Hz produces about 120 dB SPL (but goes up to 127 db SPL at around 80-100 Hz, i.e. even more output but at a higher freq.)

So, if I've got this right, I probably want to go with the larger box, get the lower freqs., not worry too much about anything above, say, 80 Hz 'cause I'm gonna be sending most of that to the JBL JRX125's. Oh, and I get the advantage of being able to use smaller amps to drive the subs with. Am I at least a little right here? If so, then the next thing I'm going to guess at is that if I want/need even more low end I would simply add more amp/sub combinations to the sound system.

Now, from a practical perspective, if I have four of the JBL JRX125's (two per side) and I drive them with 700 watts per channel each, and I have four of the subs, each with a single Omega Pro 18C and I drive each with about 200 watts (or perhaps a bit more for some headroom), do you think this would be sufficient to handle a crowd of about five hundred people outdoors?

Thanks for helping me understand this stuff,
Ted


Oh! I had one other question. On the first graph you (John) posted the excursion of the cone went up in more or less a straight line as the freq. decreased. On the second graph there is a bump in the excusion (around 55Hz). What causes this?

[ted01]

Hey Techofreak,

Thanks for your suggestion. However, there are two issues for me. One, I can't seem to find anyone who sells PD drivers in the US. Two, I already own the Eminence drivers.

Cheers!
Ted

[MJL21193]

Originally posted by ted01
I think I'm beginning to understand. More box volume = lower freq. but with less power. So if I want to get down low, I need a big box, but can't shoot lots of power to it.

This is true enough, but...

Originally posted by ted01
So, if I've got this right, I probably want to go with the larger box, get the lower freqs., not worry too much about anything above, say, 80 Hz

That's not it. Your driver will not give you (without distorting) what you want. The linear excursion of the cone (Xmax) will be exceeded. All of these lower bass notes will be distorted be the driver "clipping" or "bottoming out".
Xmax is what determines the maximum power input in a bass speaker.

Originally posted by ted01

Oh! I had one other question. On the first graph you (John) posted the excursion of the cone went up in more or less a straight line as the freq. decreased. On the second graph there is a bump in the excusion (around 55Hz). What causes this?

The first graph is for a 97 litre sealed cabinet. This shows at what frequency the drivers Xmax will be exceeded. In this case, at 70Hz with 200 watts input power.

The second graph is for the same Omega driver in a vented 170 litre box. Here you see the driver is still exceeding Xmax at 70Hz, but it is only by 1mm. This may be a better approach if you really want low bass, as the chances of over-excursion are limited. Still, you will need to dramatically cut below 35Hz or your driver may (will) fail.

I'll do another simulation and see what the best scenario is, as you say you already have these drivers therefore you need to get a usable plan.

I'll be back...

[MJL21193]

Here a closed box. 80 litres, fully stuffed. You get 105db at 35Hz with 100 watts.

[MJL21193]

Here's the drivers excursion for the above:

[MJL21193]

Here's a vented box. 140 litres. 110db at 35Hz with 150 watts.