would Qts affect response in a TQWT?

[hacknet]

i`m intending to build a TWQT that was ment for a Fostex FE103E.

but becuase the FE103 is small, i`ve decided to try to replace it with a FE126E.

they differ mainy with the FE126 having a lower Fs and a much lower Qts. from 0.43 to 0.25. the Qms is lower but the Qes is higher.

would the FE126 work in a TWQT ment for a FE103.
i try figuring it out by using martin kings mathscad sheets but to no avail. after reading a couple tutorials, i don`t think it would make a large difference.. is that right?

[BillFitzmaurice]

It will make a lot of difference. TLs work best with high Qts drivers, ideally .5 to 1.0. The lower Fs is a help but still a .25 Qts is not well suited to a TL.

[hacknet]

what would happen? no bass?

i`ve seen FE166E in a TL. and the Qts of the 166 is 0.21. is there anything i can do to reduce the effects?

[BillFitzmaurice]

No bass is the result. A true TL mimics the response of a sealed box; an easy way to see whether a driver is suitable for a TL to the cutoff you want to achieve is to model the driver in an optimal size sealed box. If it won't work sealed it won't work TL. With a Qts of .3 or less you really have to go to a horn if you want strong bass response.

[hacknet]

oh okay. what type of cabinet would you recommend for a driver with Qts of 0.25? ported box? is there something with better bass?

[BillFitzmaurice]

oh okay. what type of cabinet would you recommend for a driver with Qts of 0.25? ported box? is there something with better bass?

A horn. That's also what Fostex recommends.

[GM]

i`m intending to build a TWQT that was ment for a Fostex FE103E.

but becuase the FE103 is small, i`ve decided to try to replace it with a FE126E.

they differ mainy with the FE126 having a lower Fs and a much lower Qts. from 0.43 to 0.25. the Qms is lower but the Qes is higher.

would the FE126 work in a TWQT ment for a FE103.
i try figuring it out by using martin kings mathscad sheets but to no avail. after reading a couple tutorials, i don`t think it would make a large difference.. is that right?

Yes, as already noted it would make a big difference, though you can use a series resistor to increase the effective Qts to match the 103's higher Qts, with the attendent loss of efficiency.

To calc a first approximation resistor value if actual specs are not known:

Qes' = Qes*(1+Rs/Rvc), where Rs is the series
resistance and Rvc is the driver's DC resistance, so:

Rs = (Qes'/Qes-1)*Rvc. Remember to include the series
resistances of your amplifier/cable/inductors in Rs,
so the actual resistor you buy may be somewhat
smaller than calculated value.

Qts' = Qes'*Qms/(Qes'+Qms)

Vas is unchanged

n0 = 9.6352*10^-10*Fs^3*Vas(liters)/Qes'

SPL = 112.018+10*Log(n0)

Unless Fs and Vas are a fairly close match to the 103 (couldn't find specs for the 126), it could be either an overdamped ('lean') or underdamped ('fat') sounding speaker, though experimenting with damping can lean out an underdamped design somewhat.

As noted though, a back horn design for this specific driver is the best cab to use if space and building complexity are not major issues.

GM

[Roy Lewis]

I am afraid I have to totally disagree with BillFitzmaurice on his opinion of low Qts drivers.

Over the years I have built a number of TL's for my self and friends, and apart from the first ones they all used low Q drivers.

My present speakers are a two-way using Seas CB17 RCY with a spec Qts of 0.25. By BillFm criteria I should have little or no bass.
WRONG, my speakers have a deep tight and well controlled bottom end. Even at low volumes they still provide excellent definition. I can not speak for Fostex but the drivers I have used work superbly in TL's.

A while ago I built a friend a pair of of TL's which used Audax 8in bass/mid with a Qts of 0.31 ( still low! ). He then aquired a pair of Scanspeak 8554 units with a spec Qts of 0.22, with a view to a new TL. As he used a simple first order XO on the bass we decided to dropin the 8554 to try it out.

The result proved a revelation. Not only was the mid clarity and imaging improved but the bass took on a greater level of depth and definition, not less !! This was further helped no dought, by the Scans have lower Fs and larger Vas, but still very low Qts

There are a lot of excllent TL's out there using low Qts drivers. One of the first was the oringinal Daline's desinged by Robert Fris, which used the Kef B110 which had a Q of 0.31. These little were capable of generating amazing bass in those smallish Tl's

Amoung the benifits of low Q drivers is they are less trobled by room placement . I and my friends have found the enclosures can placed close to a rear wall and still retain a controlled bass. A cause for domestic bliss !

I have always found that the so called classic TL with well stuffed lines can sap the life out of music, with low Q drivers you can reduce the amount of damping. David L Field expressed the same opinion as above ( though better written ) in his article in Speaker Builder 4/96.

In search for a better understanding of TL's, I had several emails from Martin King answering my questions. One reply stated " I also like the low Qts drivers and have found similar results. The higher Qts " 0.3- 0.4 !!, have a bigger bass but not as tight or controlled "

With my prefered drivers I have found I only need to line the walls with wool felt or profiled foam. The highly regarded PMC TL's use the same arrangment so I am in good company

AS I mentioned if you check out the net for TL's you will find that the majority use the lower 0.3-0.35 Q drivers. So I would try out those low Q Fosteks you might be pleasantly surprised

[BillFitzmaurice]

I am afraid I have to totally disagree with BillFitzmaurice on his opinion of low Qts drivers

My comments were based upon the groundbreaking work in TL design by George Augsperger, who has backed up his opinions with measured results that display the superiority of high Qts drivers in true transmision lines. The TLines I've built and measured cause me to agree with his conclusions. Keep in mind this refers to true 1/4 wavelength TLs, and not TL hybrids, which I have neither built nor measured.

with low Q drivers you can reduce the amount of damping.

Damping in a true TL is not there for the purpose of altering bass response, it's for damping midrange response bumps. In the case of Field's T-Rex there is a very good reason why he found it worked better not totally stuffed: it was not a transmission line. It was actually a hybrid TL/reflex, and therefore it didn't work as either a TL or a reflex, but as something in between.

[MJK]

Bill

You wrote :

My comments were based upon the groundbreaking work in TL design by George Augsperger, who has backed up his opinions with measured results that display the superiority of high Qts drivers in true transmision lines.

I just skimmed through George Augspurger's AES paper on TL theory and design. I could not find any claim that high Qts drivers were prefered in TLs. Where specifically did you find statements in the AES paper that high Qts drivers are superior? If I look at his alignment tables, he addresses Qts values between 0.31 and 0.58. It would seem to me that Roy's observations are consistent with the results obtained by Augspurger.

George Augspurger did some very good work on TL theory and design. In his paper he has selected a set of variables to optimize and arrive at a set of alignment tables consistent with these assumptions. But they are not the only set of alignment tables that could be generated. A different set of assumptions would result in different geometry alignments. If you are designing a BR box for a given driver, there are several different filter alignments that can be selected each with its own strengths and weaknesses and each yielding a different box size and port length. I consider George Augspurger's articles to be one piece of the TL theory puzzle but not all encompassing documents that have to be followed explicitly.