LT's vs low boost
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Basically an LT circuit is like a single band of parametric EQ with an additional circuit that allows for dialing in a target Qtc. It's used for small boxes where the Qtc is high and so is the Fs/Fc.
Here's John Murphy's webpage about them
If the box is big and the Qtc low all one needs is parametric EQ to boost the bottom end.
IB subwoofer FAQ page
"Complicated equipment and light reflectors and various other items of hardware are enough, to my mind, to prevent the birdie from coming out." ...... Henri Cartier-Bresson- Bottom
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Thomas,
Is there a limit to how low one should push the Qtc using a LT curcuit?
Also, what do you consider "big enough" and the "Qtc low enough"? Also, is this before or after stuffing?- Bottom
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It's after the damping is added
There's not really much point in using a LT circuit if one already has a (critically damped) Qtc of 0.5.
The benefit to LT's is in small sealed boxes with high a Qtc, when one has excess of amplifier power and driver excursion. Then the LT can be used to force the driver to function as if it were a bigger box.
IB subwoofer FAQ page
"Complicated equipment and light reflectors and various other items of hardware are enough, to my mind, to prevent the birdie from coming out." ...... Henri Cartier-Bresson- Bottom
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If you use the calculator on SL's site http://www.linkwitzlab.com/pz-eql.xls , you must keep K>0, which I guess is the limit on changing Qts.- Bottom
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even at a Qtc of .5, the box is still not performing to it's capabilities (vs. an LT circuit). For my application, we are talking 10 cu. ft. net(12-14 external volume) and f3=36 Hz. An "extremely" heavy and substantial enclosure.
I think a low f3 is the big thing with sealed boxes, given all the deep ported subs on the market. The biggest complaint against sealed subwoofers is frequency response. Everybody wants to know "what it does at 20 Hz".
The LT circuit has been defined as "surgical equalization", as it applies the perfect inverse curve of boost down to Fp, with the added benefit of lower group delay.
A parametric equalizer won't be as accurate and group delay will be higher.
Another thing, that i've slowly come to realize, is that implementing the LT circuit, based on box software, may not be the best approach.
On more than one occasion i've been told that i must find the "true" Fb of the enclosure. With an impedance analyzer, or some other comparable device, one can determine precisely where it occurs. Of course, this would be done after the box is stuffed and sealed. The best approach, however, may be a variable LT circuit like the Bassis. :T...our brains and nervous systems constitute a belief-generating machine, a system that evolved to assure not truth, logic, and reason, but survival...
2x15" TC Sounds .45Q sealed
M. Boutte HT
3x15" @ 10 Hz- Bottom
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I guess the point is why bother with a large box if one is going to use an LT?
And yes one needs to measure both the Fc and the Qtc to accurately determine the settings for the LT
IB subwoofer FAQ page
"Complicated equipment and light reflectors and various other items of hardware are enough, to my mind, to prevent the birdie from coming out." ...... Henri Cartier-Bresson- Bottom
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There are plenty of differing perspectives on EQ and sealed boxes. Personally I look at it more as the box volume adjusting the low frequency sensitivity. If you have flexibility in the EQ, adjust the box volume to what you think makes sense with the power you will have on tap, and then decide what sort of response you are after, noting the trade offs in program playback level as you add boost to the low end.
A LT or similarly executed EQ can certainly be used to make a "too small" box have the net frequency response of one that is larger, or it can be used to dial in the specific response curve you desire when driver and box volume combinations don't naturally allow it.Mark Seaton
"Make no little plans; they have no magic to stir men's blood..." - Daniel H. Burnham- Bottom
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The LT circuit has been defined as "surgical equalization", as it applies the perfect inverse curve of boost down to Fp, with the added benefit of lower group delay.
Another thing, that i've slowly come to realize, is that implementing the LT circuit, based on box software, may not be the best approach.- Bottom
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There are plenty of differing perspectives on EQ and sealed boxes. Personally I look at it more as the box volume adjusting the low frequency sensitivity. If you have flexibility in the EQ, adjust the box volume to what you think makes sense with the power you will have on tap, and then decide what sort of response you are after, noting the trade offs in program playback level as you add boost to the low end.
As a general thought, I think sometimes "LT" is used as a catch-all term for any response-extending eq.- Bottom
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