[murf]

Hello Audio Experts!
 I want to build a fancy sealed subwoofer box for a Titanic T-1200 (old style titanic).  This driver wants a pretty big box for a decent Qtc, such as (from WinISD):
145l (5.1cuft) = 0.70qtc
190l (6.7cuft) = 0.64qtc
I want a sub with little mush & boom; accurate & subtle may describe my goals better.  But it will be mostly for HT since my 10" woofers are fine for almost all music.

  So, how important is Qtc in a sub, and what should I aim for?   

TIA!  Murf

[Duke]

The following is my opinion.

The ears have relatively poor resolution in the time domain at low frequencies, such that by the time we hear bass notes, the room's acoustics have totally imposed themselves on the subwoofer's output.  In other words, we cannot hear the sub apart from the room it's in.

Most rooms tend to gently boost the bass as we go down in frequency, so in my opinion a sub with a correspondingly gentle rolloff will work better that a sub that's "flat" to a lower frequency.  I have a somewhat detailed post about that on my circle:

Room gain complimentary tuning

To get back to your question, a low Qtc box is imho preferable to a higher Qtc box if sound quality is the top priority, not because of superior transient response (as one might suppose), but because its inherent rolloff more closely approximates the inverse of typical room gain.  If your woofer has lots of excursion, you might even try a pressure-relief enclosure.  I don't know how to model a pressure-relief enclosure, unfortunately; when I used to build them, I had to tune them by ear (didn't have decent test equipment back then). 

[murf]

Thanks Duke,  With my main speakers I aimed for a Qtc of ~ 0.7.  So, in your opinion, what is a hi or low Qtc # for a subwoofer?  I have seen a few diy designs going down to a 0.50 or so.  Is this needed or desirable in a sub?

TIA!  Murf

[Duke]

In general I prefer a Qtc of .50 rather than .707 in a sealed sub, but it depends on the situation.  And it sounds like the box size is getting pretty big with the Qtc figures you're already looking at. 

Murf, can you post the T/S parameters of your woofer, anticipated crossover point, and a brief description of your room (tiny concrete bunker, open floorplan mansion, or something in between)?

Thanks.

Duke

[murf]

Driver Specifications:
Driver Dayton Titanic 1200 Original
Vas 280.280 litres Equivalent air compliance
Qts 0.407 Total driver Q at resonance
Qes 0.420 Driver electrical Q at resonance
Qms 8.22
Re  3.66
Le  1.96
Bl  16.8  (est)
Sd .047 (est)
fs 16.0 Hz Driver resonance frequency
Xmax 14.20 mm Maximum linear excursion
Dia. 30.48 cm Effective diameter
PEMax 350 watts Thermal power rating

Seems PE was recommending a box between 2-3 cuft, which gives a hi Qtc of .85-.9 or so.  I want to cross low from my Peerless "850146" 10' woofers (f3~45-50?), maybe 40-50hz?  Room is ~ 13'x21'x9' with 6' opening on small end.  1939 construction: thick plaster walls & ceiling, wood floors mostly carpeted, some sound absorption but not enough to be really dead.

Thanks!  Murf

[Duke]

Thanks for the info, Murf.  That 50 Hz crossover is going to work in our favor here.

The 6.7 cubic foot box you mentioned should work just great.  From your anticipated 50 Hz crossover down to the lower 20's, the modelled curve stays within 1 dB of our theoretically optimium 3 dB per octave rolloff.   You can probably get away with a little under 6 cubic feet by stuffing the box with about 1/2 to 3/4 pound per cubic foot of Polyfill or similar material.   

[murf]

OK Duke,
Now we're cooking with gas!

What program did you use for this (I used WinISD), and what Qtc did you get for the 6.7cuft box? 

Murf

[Duke]

I use a program that is no longer available for quick-n-dirty simulations like this one.  It said Qtc = .64 for the 6.7 cubic foot box.