Questions about Tumlt/ADA-1200 combo (1 Viewer)

[Brian Bunge]

Mark,

I hope I haven't given you any indication that I think that. The beauty of the LT circuit is that the initial Qtc and Fc can be "transformed" to whatever you like. All I care about is that the driver has enough excursion and the amp enough power to handle the EQ applied by the LT.

 

[Jack Gilvey]

Some interesting comments on the LT here:

http://www.rythmikaudio.com/lt.htm

 

[dave alan]

Jack,

Very cool. Thanks for the link.

From the link you provided:

"So far so good. Is there any limitation one LT circuit? Yes. The amount of fs and Qts one can change with LT turns out to be interdependent. That is, if one change the fs by a factor of X (that is old fs over new fs), one can only change Qts by a factor between X and 1/X. For instance, if one change the fs from 40hz to 30hz, one can change the Q value by a factor between 3/4 to 4/3. Therefore if one does not want to change fs by much, there is not a wide range that one can change Q value."

It makes sense that the L/T formula would have a numerator and a denominator involving the existing Fs and Qs and the desired new Fs and Qs, which would make Fs and Qs interdependent.

Also, since we're talking about the sound of a sub, which means in-room response, as you lower the existing Fs, the Qs will change with room gain and will be less of a factor in reproducing music program.

Basically, what that means in the real world app is that the more boost you apply (smallest box/largest amp that allows for optimal use of a given driver's excursion capabilities), the more control you have over the final Qs...no?

Of course, I'm leaving out the driver's thermal capability, assuming no more than 12 dB boost will be required and short duration content in that area of program, which Brian found first hand does matter in extreme case useage.

Whaddaya think?

 

[Jack Gilvey]

Agreed. Since the LT circuit doesn't raise Qtc...starting with the highest potentially-desired Qs/smallest box allows use of the entire range below. My recommendation for a larger box is based purely on pouring less power into the driver.

 

[David_P]

does the spreadsheet linked above relate to unstuffed boxes?

David

 

[RichardHOS]

There's something really odd about that LT spreadsheet (linktranadv29.13.xls) linked to earlier. Opening it up with the Tumult driver specs already entered, I tried setting Vb to 100000 (to simulate an open baffle) and the target Fsc and Qts to essentially the raw driver Fsc and Qts (Qts to .358 and Fsc to 18.9 to keep "k" positive). This should represent a driver in free air, or open or infinite baffle. If you look at the LT filter response with these settings, you see, as expected, essentially 0 filter gain across the entire frequency range.

It is my understanding that as the frequency drops, the power required to reach Xmax also drops (drastically). i.e., just a few watts or so might bottom many drivers in single digit Hz in free air.

However, if you look at the power vs. frequency graph on the bottom of the "information" sheet, it is indicating (if I am reading things correctly) that approx. 1400 watts input are required to bottom the driver at 10hz.

Something here is exceptionally wrong. I began to wonder about this as I found that it was essentially impossible to bottom the Tumult (or any other high excursion driver I tried to model) in virtually any size enclosure with any amplifier power realistically achievable.

What am I missing here? My only thought is that this spreadsheet is not calculating excursion vs. power properly, but I haven't dug into its guts yet to try and find out.

Any thoughts?

 

[David_P]

I never tried that, but I remember someone posting (on the Infinity Baffled forum IIRC) that it takes about 300 watts to bottom a Tumult at 10hz in an IB.

Also based on memory, I think in Unibox, you set the VB to something really big, and set desired Qtc to match the driver Qts... in this case, Unibox models max power as about 135 watts.

Something's not adding up.

David

 

[dave alan]

The only thought I had is actually a question.

Does the spreadsheeet you refer to have a HPF feature in the L/T circuit, and, if so, is it disabled in your simulation?

 

[David_P]

What is this exactly? I don't see it in the spreadsheet.

David

 

[dave alan]

They might call it a subsonic filter. It's a high pass filter that's set to a low frequency to filter out signal below that frequency.

 

[RichardHOS]

Ah.

Whoever input the Tumult properties into the file linked to (well, the link was in the other HTF thread linked to above) used 68mm for peak excursion. The program wants one way excursion, not peak-peak. Setting this properly yields ~350 watts to bottom the driver in an IB at 10Hz, which is consistent with the real-world experience mentioned above. I also verified this with Unibox, which gives very similar results for IB Tumult w/ 350 watts.

Now things are beginning to look better for my other sims. I assumed when I opened the file that, since the Tumult had 68mm entered for peak excursion the program was set up for peak-peak entry. Man, that was giving me a headache with my other sims, trying to figure out why I needed 100,000 watts of power.

Anyway, David_P, I'd suggest using the proper 34mm one-way linear excursion in the LT spreadsheet and redoing your sims. You might find out you can stuff that thing in a smaller box than you thought.

 

[RichardHOS]

I found another problem... this one apparently one with the spreadsheet calculations.

When you use two drivers and two amplifier channels (one per driver), and double the box volume, the raw response and required EQ filter match exactly the single driver configuration (i.e., doubling everything keeps the calculated filter the same, which is what one would expect).

However, the excursion vs. frequency is not the same as for the case of a single driver in half the box size. Nor is it half, or double. I'm not sure what is wrong, but using two drivers and two amp channels the program is definitely miscalculating the excursion requirements.

Perhaps I'll shoot FRD an email.

 

[David_P]

I'm not sure that this is correct...

Brian Bunge built a 2 cu ft Tumult, sealed/LT'd (unsure of Qtc), and is well renowned for having cooked one voice coil using a bridged K2 (2400watts). Never bottomed it, cooked it.

Under the spreadsheet, he should have easily been able to bottom it at anything close to 10hz using that kind of power. And he claimed it was well protected from over excursion.

Maybe Brian can chime in here and clarify?

David

 

[Brian Bunge]

David,

Well, it is actually 2ft^3 (55L) heavily stuffed with a resultant Qtc of .6 after LT. I can't verify exactly how much power was going into the Tumult when I fried the VC so it could have easily reached it's thermal limits before it got anywhere near Xmax.

According to Unibox, using a Tumult in a 55L enclosure heavily stuffed and 2400W of power, I should excede Xmax at 24Hz. With "only" 800W of input power I should never excede 25mm at 10Hz or above.

 

[dave alan]

Unibox is wrong.

There's no way you'll bottom a Tumult in 2 cu ft using 800 watts or 2400 watts.

As Brian found out, the VCs will cook but the driver will not bottom.

In an IB, 350 watts won't bottom a Tumult either until below 5 Hz.

It's easy enough to verify if you have several sizes of sealed boxes built and a few Tumults on hand, can quick clamp them and apply gobs of power for short bursts.

 

[Brian Bunge]

Dave,

To quote myself: "With "only" 800W of input power I should never excede 25mm at 10Hz or above." Considering the Tumult has a 34mm Xmax, not being able to bottom a Tumult with 800W is a given.

If you'll send the Tumults, I'll build the boxes, bridge the K2, and remove the LT and and BFD from the equation for testing.

 

[dave alan]

Sorry I missed that...thanks for pointing it out.

I'll have to think about the offer, though.

Dave