Question for Jon Marsh re: dual Avalance 18 LT design

[Greybeard191]

Jon Marsh mentioned in another thread:

" I finished a dual 18" Avalanche LT design that was previously requested today. There is FRD consortium workshop XL sheet, but it's not exactly user friendly."

I've been trying to use this same spreadsheet to do exactly the same thing... come up with a design for dual Avalance 18's in a smallish (6-8 cu ft box) design. I'm looking specifically at using the adire ada-1200 whcih would deliver 600W in the 8 ohm load. My reasoning for doing this was that the ada-1200 appear unable to drive the avalanche 18's into overexcursion even at full power under 10hz (I'm more concerned about protecting the drivers from overexcursion than driving them to max at more reasonable frequencies, 30hz and up)

so my question... it appears from the spreadsheet (dual 18's, 8 cu ft, Q=.58, 600 W into 8 ohms) fc=14hz, f3=17.7) that the combination puts out about 112 dBspl at 100 watts input.

It also appears that the amp runs out of juice attempting to maintain 112dBspl (at whatever input level drove it to 100 watts at say, 40hz) at around 21-22 hz. It also appears that excursion is around max at 600 watts at around the same frequency.

So my questions:

1) Am I reading the spreadsheet right? Is the ada-1200 (600w into 8 ohms) a "safe" amp for 2 avalanche 18's, in terms of over-excursion?

2) Are there any more specifics on your dual 18 LT design that youre willing to share?

A suggestion... is this is a viable design, Chad should post it on his site.

I registered just to ask this question BTW... happy to be here.

thanks in advance for any answers you can give

David.

[cjd]

An ADA1200 should not be able to overdrive a pair of Avalanche 18's wired in series. It will barely allow a single Avalanche 15 to hit XMax limits in a 2.5cu/ft sealed box. If you use the LT and/or EQ circuits on this amp, I would be more concerned with the possibility that it might not be enough. Except that you're looking at stupid silly output levels.

IIRC, a single AAA18 likes 5-6cu/ft. Two in 4-6?!

C

[ThomasW]

Jon optimized a dual Ave 18" design for Adam based on him using a Crown K2. That design was 150L (net internal volume) per 18", each 18" in it's own box..

The amp will run out of gas (800 watts/driver) just before the driver runs of of excursion.

For dual 18"'s in a single box just use 300L net internal volume.

[JonMarsh]

What ThomasW said...

Some other points:

A truly flexible LT design which can trade off amplifier power against enclosure volume requires some specific characteristics in the driver, which can be summarized as a large motor coupled to a relatively small cone. Drivers that meet that criteria tend to be over magneted 12's, like my BPD1203's, or the occasional 15 with a large motor system, like the Tumult. Avalanche 12's should work quite well in that regard, also.

The dual 18" avalanche will still benefit from the Bassis as an equalizer, but other equalizer's could be used, with nearly the same results, as long as you understand how to setup the curve.

In general, my own preference is to figure out what the available power is, suited to the driver thermal capability, then match that to an enclosure volume which just allows you to hit Xmax in the LF region with that power (smaller box and you won't reach driver mechanical potential; larger box, and well, it's larger! the whole exercise is about getting the box small!). Then combine with an LT equalizer to tune the frequency response to the desired extension and Q.

But hey, I'm NOT the subwoofer guru around here- that's strictly an occasional side line. I'm Mr. Full Range speakers, remember?

[Greybeard191]

Jon optimized a dual Ave 18" design for Adam based on him using a Crown K2. That design was 150L (net internal volume) per 18", each 18" in it's own box..

The amp will run out of gas (800 watts/driver) just before the driver runs of of excursion.

Well you guys are losing me...

I think what I'm struggling with is how to interpet the spreadsheet. If I plug a single Ave18 @ 5.2 cu ft (150 liters), fsc=14hz, and q=.5 and drive it with 600 watts into 4 ohms (an ada-600), the excursion graph says it reaches Xmax at abut 13-14 hz. At least I think that's what it says.

Realizing that the LT setup (fsc, q) might be different for Jon's design, I'm wondering why Thomas says a K2 runs out of power before the Ave-18 runs out of excursion.

Or maybe this is the problem... the spreadsheet gives you a spot for power inputted (prior to equalization, in this case set to 100W), and also max power (in this case 600W). The frequency vs power graph is fairly easy to interpet... it seems to indicate that below a certain frequency (in this case about 17 hz) the ada-600 would run out of juice attempting to maintain the spl level generated at whatever input caused it to output 100W at 40-50 Hz.

The frequency vs. excursion graph is the harder one, at least for me. It indicated the Ave-18 reaches overexcursion at 13-14 Hz. What I can't figure out is if that curve/calculation assumes that there is "infinite" power, as related to the 100W input prior to equalization.

To rephrase the question, does the fact that the freq vs power curve runs out of juice at a higher frequency than the freq vs excursion curve mean that the Ave-18 will not be overdriven. This scenario matches (probably) with what Thomas said, as 800W applied on the freq vs. power curve pushes the "out-of-juice" point down to 14-15 Hz, just above the over excursion point.

Hope these rambling make sense.

David.

[hayden]

David,
I think that you are worrying a little to much about this design I've used the old Blueprint 1803 (inferior, but similar to the Avalanche 18 - Chad at Ascendant will verify this) in a box of about 5.5 cu.ft with a custom LT circuit. I drove it with a QSC MX1500a (at least 1,500 watts into 4 ohms) and the amp clips just as the BP is reaching its limits, at least I think so - I've never bottomed it. As a side note, remember that when the driver/amp combo reaches these levels the SPL is almost unbearable in normal rooms. When you see an 18" driver moving over 2" peak to peak, you will understand.
Just my .02,
JP