What Is The Problem With Low Qts Drivers in QWTLs?

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I've read so much on this but I wish I could still translate it to plain English...or Spanish for that matter... :D

I've been staring at some totally kicka@* B&C drivers for a TL sub project but unfortunately their Qts is like .2 and even lower.

Can someone water down the science a little so I can undesrtand it?
from http://www.t-linespeakers.org/design/MJK-for-dummies/index.html
” Qts -- I use a Qts value of 0.35 to distinguish between high and low Qts. You might have another definition. Drivers with a high Qts perform better than driver with low Qts in a quarter wave design. On the other hand, the lower Qts driver seems to require less volume, thereby resulting in a smaller enclosure. , and drivers with a moderate or high Qts will often result in the best low end performance. But this is also a matter of taste and preferences. "Good sound" is defined by you only. The output from the opening is broader with high Qts drivers, compared to low Qts drivers that have a narrower output from the opening. Drivers with a Qts lower than 0.30 seem to be difficult to control in a TL.”

It's not clear to me what 'narrow' & 'broad' output is....

Re: "Low Qts drivers have a roll off that starts earlier than high Qts drivers" - perhaps this is because Low Q drivers have less stored energy to be reinforced by the TL resonance?


Joined 2003
PeteMcK said:

It's not clear to me what 'narrow' & 'broad' output is....

How wide its gain BW is. The object of a driver cab is to boost the driver's BW below its mass corner, i.e. its point source BW or where the driver's response starts to roll off (Fhm). The lower its Qts (stronger motor), the higher in frequency (Fhm) the roll off occurs, ergo the more gain BW the cab must handle for a given Fb (Fp, Fc).

Since short of a large compression horn there can only be so much cab efficiency, it stands to reason that as Fhm rises, so also Fb must rise for a given response curve and why low Qts drivers tend to require small cabs tuned high for a T/S max flat alignment with high Qts drivers needing large cabs tuned low.

Anyway, TL loading causes 'ripple' in the passband, so must be damped if used for more than two octaves to smooth it out, negating much of its gain, so a high Qts driver is required if you want a ~flat response over a wide BW since its response will ~mirror the driver's IB response down to Fs if tuned this low.

The TQWT (reverse tapered horn) can load the driver more, so can be shorter for a given tuning (Fp) and here the low Qts driver's stronger motor is preferred with the TQWT's aspect ratio increasing with decreasing Qts. Another advantage of these is that you can fine tune them by adding straight vent sections to the terminus to keep net Vb from getting to far out of hand with a really low Qts such as 0.2.

Thanks...so in the design of the less ambitious subwoofer...something only running between 30 HZ to 150 Hz, would this matter...this higher rolloff of low Qts driver?

The goal to the design for me is as little a box as possible with a driver of Qts of .2...a B&C 12NDL76

B&C 12NDL76

Fs 50 Hz
Re 5.3 Ohm
Qes 0.21
Qms 4.2
Qts 0.20
Vas 73 (2.5) dm3 (ft3)
Sd 522 (80.9) cm2 (in2)
Eta Zero 4.3 %
Xmax +/- 6.5 mm
Xvar +/- 6.5 mm
Mms 53 g
Bl 20.1 Txm
Le 1 mH

What would happen in a front loaded QW folded box?
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