hailing all experts...driver performance below f3 vs. fs (sealed)

[kramskoi]

hello...new poster here. I'm wondering about the distortion characteristics of two drivers with different values of Fs. This assuming that each sealed f3 is the same...say 20 Hz, for example. I have modeled a Tumult 15d2 vs. (2) 12inch tc7 drivers from tc sounds...the tumult's fs=16 Hz and the tc7's fs=23 Hz.

I'm wondering how they would perform against each other at the lower frequency (16 Hz). I'm having a hard time believing that these two systems would sound the same with the same L/T equalization curve.

It seems that there must be some apparent advantage to using the bigger driver. In the tumult's case, with a massive 33 xmax and Pe of 1kW, it models about 1dB higher (spl) against the dual tc7's. I'm wondering if it would also be cleaner below 20 Hz than the dual-tc7's.

I'm starting to think that maybe something is amiss in winisd pro. :roll: Thanks

[BobEllis]

Winisd pro does not concern itself with distortion. All it is telling you is that the two drivers can be equalized to produce the same frequency response.

When you say the Tumult shows 1 dB higher output, I assume that you have run the signal levels up to the point that you hit Xmax or the driver's power limit. Every driver behaves differently, although in general distortion will increase as excursion increases. So the driver producing the sound level with lower excursion relative to its xmax will likely show lower distortion. This is not always the case - see www.Zaphaudio.com where the Adire extremis showed slightly higher distortion than some other similar woofers at the test level. I believe Dan Wiggins said something like the extremis will hold up better to higher levels than the others, though.

The Linkwitz transform boosts the signal power - use the "Amplifier apparent load power (VA)" curve to see how hard you are pushing the 12s to get down to 20Hz. Again it would seem that the driver that can reach your goals unequalized would sound better than one that is equalized, but here there is another difference. Using the Linkwitz transform, you can get sealed system response. this means lower group delay, which may or may not be audible. On my to do list is plugging my subs ports and seeing if I like the L-T better than an equalized ported alignment.

Hope this helps

[Davey]

Using the published numbers for the Tumult...Sd=749, Xmax=33....and for the TC7..Sd=471, Xmax=24.4 you can compute the SPL capability differences. 20*log(Xmax1*Sd1/Xmax2*Sd2) (Times 2 for the TC7) That yields 0.63db higher capability for the Tumult versus two TC7's.

As Bob said, Winisd and the rest of the modeling programs don't have a clue about distortion levels. You would need some empirical data to make any judgements on that aspect.

The Linkwitz Transform is simply an equalization device which allows to move the cutoff frequency of the system lower and offset some underdamped or overdamped characteristics, but it can't change the ultimate physical capabilities of the drivers.

If you make the assumption that the distortion characteristics of these drivers are similar and you then use an LT circuit to equalize the response of the TC7's to be the same as the Tumult then these two systems should sound the same.

Cheers,

Davey.

[kramskoi]

Thanks Davey and Bob...under the advance parameters in winisd pro, the SPLmaxLF(gp@1m - 20Hz) gives spl values of 108.27 for the tumult and 101.7 for one tc7. The VA amp values are 324w for the tumult and 490w for the (2) tc7's. It seems that the two troughs on the graph would indicate the system resonance points of each system?

It seems that the single 15 is the better choice (albeit more expensive) here.

Thanks guys for the response. I've been on the fence with the dual-12 vs. 15 inch idea for quite a while now. I've always thought of 15 inch drivers as muddy and slow. But after reading reviews on the ACI Maestro and Revel B15, i've been slowly coming around to the idea. That Tumult driver is smash up! I think that i could get close to the maestro/revel B15 with a solid 1.5" mdf enclosure, Q=.577, L/T circuit and an adire ADA-1200 amp. :T ...good day!

[BobEllis]

resonances are indicated by peaks in the impedance curve. Are you seeing troughs in the max SPL curve? from the right, working left, the horizontal part of the trace is where you thermally limited - the driver has excursion to spare but the motor cannot take any more power. There is a knee where the system becomes excursion limited. A sealed box will drop off at 12 db/octave. A ported system will drop, then begin to rise again as you near the port resonance, which limits cone excursion. There may be a stretch in here where you are power limited. Below port resonance, excursion rises again to reach the limit and SPL drops off again.

[kramskoi]

resonances are indicated by peaks in the impedance curve. Are you seeing troughs in the max SPL curve? from the right, working left, the horizontal part of the trace is where you thermally limited - the driver has excursion to spare but the motor cannot take any more power. There is a knee where the system becomes excursion limited. A sealed box will drop off at 12 db/octave. A ported system will drop, then begin to rise again as you near the port resonance, which limits cone excursion. There may be a stretch in here where you are power limited. Below port resonance, excursion rises again to reach the limit and SPL drops off again.

The troughs i mention, are on the "VA amplifier apparent load" power graph that you noted. The two troughs seem to correlate with the system resonance of each sealed enclosure. The tumult's fsc is 29Hz and the tc7's is 34Hz. The lowest amp requirements seem to be at these two (fsc) frequencies.

[Carl V]

I went with the Tumult in a sealed 22" cube. Same ADA 1200 AMP
later on LT circuit. replaced two AV15 in 20" cubes with external amp
and BFD. Two Tumults in sealed 22" cubes is now the cats meow.

An aquantance bought the two AV15 and located two more.
We built two sealed M&K styled subwoofers. 18x22x24. ADA 600
plates on the back of each.. No A/B possible...all are big & heavy.
(3/4" BB & 3/4MDF with 3/4 BB Braces) However, I might have to
give the nod to the uber M&K clones. It's close. he spent less tho'.

[kramskoi]

I went with the Tumult in a sealed 22" cube. Same ADA 1200 AMP
later on LT circuit. replaced two AV15 in 20" cubes with external amp
and BFD. Two Tumults in sealed 22" cubes is now the cats meow.

An aquantance bought the two AV15 and located two more.
We built two sealed M&K styled subwoofers. 18x22x24. ADA 600
plates on the back of each.. No A/B possible...all are big & heavy.
(3/4" BB & 3/4MDF with 3/4 BB Braces) However, I might have to
give the nod to the uber M&K clones. It's close. he spent less tho'.

hello carl, and thanks...I only have a 1,700 cu.ft. room to fill and as a matter of fact i will be replacing an m&k dual 8 with, hopefully, a Tumult sealed unit.

Would you mind elaborating on your L/T values, box stuffing used, spl levels, room size, etc. etc.???

I was also aiming for a size of around 20-21" cubed with 1.5" mdf and cross bracing.

I recently read a post by Tom V. with SVS about subwoofers and headroom, and after reading it decided that i was going to get the best available 15inch driver...i think that is the tumult...if i have to put that thing in a shoe box and power it with batteries...i'm getting it!

I'm thinking about going through Acoustic Visions for the whole smash...as they can probably help with the whole L/T deal. I'm not sure yet how i'm going to do it. Just waiting on the tax refund... :T

Thanks again Carl...feel free to elaborate on the Tumults in any way...