[Anthony]

Kevin,
I have two of the "mistake" Tempest-X's (at least that's the name on the box).

The plan was for open baffle application and for the most part, they sound great.

However, I'm getting a nasty rumble during some movie soundtracks and a quick look shows that this is from max excursion.  It's happened twice, so now I've got the sub amp down 3 dB to prevent it from happening again.

It happens on both drivers, powered individually by an NHT SA-2 sub amplifier.  I put in an F-mod infrasonic filter (2nd order - 12 dB/oct - LR @ 20 Hz) and that did not help.  I need to do a very deliberate sweep to tell exactly which frequencies cause this, but I haven't gotten around to it yet.

You mentioned in the main thread on this that you had some calculations for dipole for those woofers, but I didn't see the results.

Any plots of Xmax versus frequency or advice on a rumble filter for these?

Thanks,
AC

[Kevin Haskins]

In an OB you can hit maximum excursion pretty easy.   Even with high output drivers your going to find that you are excursion limited, rather than power limited below about 50Hz.   You don't need to run a sweep to predict it.   You need exponentially more driver excursion the lower you go so the maximum output skyrockets as you drop in frequency.     

A 20Hz rumble filter may work fine with ported enclosures, that control cone motion down to the tuning frequency, but your going to need to move it higher to control overexcursion in an OB.    I'd keep adjusting it upward until you hit a level that keeps the drivers from being overdriven.   Its something your always going to have to keep an eye on if you ride the drivers at the bleeding edge.   I'd adjust it until you no longer are overdriving them and then move it up a couple extra Hz just for some safety margin.   

Oh... Linkwitz has a estimated SPL spreadsheet.   For the most part your going to have to figure it out empirically though.   Trail & error with your system in your room based upon what you listen to and @ what level is more meaningful than the spreadsheet.   

[Anthony]

do the IB folks run into the same problems?  I would imagine they'd have to.

I have a Behringer unit.  I wonder if adding a filter would help.

Also, this problem has only maifested when I was driving 75 dB reference through the one driver (actual output is higher, but I calibrated pink noise at reference to 75 dB).  If I ran both together, I would be getting that output with less excursion on each driver.  I'd probably still need the filter, though.

Thanks for the help.

[Kevin Haskins]

do the IB folks run into the same problems?  I would imagine they'd have to.

I have a Behringer unit.  I wonder if adding a filter would help.

Also, this problem has only maifested when I was driving 75 dB reference through the one driver (actual output is higher, but I calibrated pink noise at reference to 75 dB).  If I ran both together, I would be getting that output with less excursion on each driver.  I'd probably still need the filter, though.

Thanks for the help.

Yes... the IB folks do but they get more output from the driver than you do on an open baffle.   In OB operation much of your output is wasted by the off-axis cancellation.   IBs at least separate the front wave from the rear wave so you get more output from an IB.   In IB operation, most people still use anywhere from 2-8 drivers because they are giving up output high up in the bandwidth for the extension and low roll-off rate of the IB.    Even then they have to be careful about overdriving woofers because if you get a large low-frequency signal level, and you have the power on tap, you will pop the drivers out of the gap.   

The bottom line is that you can overdrive even the most extreme high output drivers and an OB is the least effective way to achieve high-output at low frequencies.   You will be much happier if you limit yourself to about 30Hz as your goal for OB playback.

[poseidonsvoice]

Even if you need more than 2 drivers, with Kevin's prices, I still feel its quite affordable. And...IB bass sounds the best to my ears. Always clean.

Best,
Anand.

[Anthony]

Thanks all.

I hooked up the two woofers together (parallel VC's in series together) and the problem is much better, but I haven't stressed it yet. 

Another thing I may try is running them in resistively damped operation to get the Qtc up over 1.  That should help a bit at the expense of power handling, but I definitely have power to spare.

But now that each woofer is only producing half the bass, they aren't very stressed, so that may fix the problem.  I'll watch a nice boomy movie tomorrow night and see if the problem manifests again.

Thanks again.

AC