LT's vs low boost

[Bent]

https://www.htguide.com/forum/showpost.php?p=235469&postcount=4

Thomas, the above post got me wondering, if a sealed emclosure is big enough, and the driver still doesn't like to perform in the deeper regions like one would wish, is there a differemce between a custom L-T cct compared to a parametric EQ?

[ThomasW]

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

http://www.trueaudio.com/st_lkxfm.htm

If the box is big and the Qtc low all one needs is parametric EQ to boost the bottom end.

[BobEllis]

Although according to WinISD, the LT offers much better group delay than a 6th order (ported+peaking 2nd order HP) with the same box size and target extension.

[---k---]

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?

[ThomasW]

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.

[BobEllis]

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.

[kramskoi]

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

[ThomasW]

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

[Mark Seaton]

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.

[steve nn]

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.

I had a little expectation of GD going up (discernible anyway) but have not been able to detect the effect at all.

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.

Thank goodness I don't have (or take) the time to learn all the fancy programs out there. Sure some will be good and a big help, but then others will be like some druid telling me how to do my job with no practical experience.

[Jack Gilvey]

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.

Since Mark taught me many years ago to think in terms of the relationship between Vb and LF sensitivity, it won't be surprizing that I agree.

As a general thought, I think sometimes "LT" is used as a catch-all term for any response-extending eq.