Closed box roll off, a question

[SSassen]

DIY-ers,

I'm modelling a few enclosures for a sub, and it has been several years since I've been doing anything with loudspeakers so things are a bit rusty, could you help out a little?

XLS10 in a 30-liter closed box
F3 = 78Hz
Qtc = 0.30

If my mind is not playing tricks on me below F3 the slope should be 12db/oct, hence with a reduction in frequency by 1/2 the output should be -12db lower right? Now look at the below graphs.





BassBox 6 Pro tells me I'll only see a -6db reduction, I'm sure I'm missing something here, but I just don't see it. Anybody care to enlighten me?

Thanks,

Sander Sassen
http://www.hardwareanalysis.com

[Bill Fitzpatrick]

Above about F20 the rate of roll-off will vary with Q.

[Dave Jones]

Originally posted by Bill Fitzpatrick
Above about F20 the rate of roll-off will vary with Q.

What he said. Look between 5 and 10Hz.

Why would you model an XLS10 in a closed box? :-)

[SSassen]

Never mind, I overlooked the influence of Q, which is set far too low, as per the below explanation:

Qtc (closed boxes only) The system total Q. Usually Qtc should not be less than the total Q (Qts) of the driver. Many designers consider the optimum value for Qtc to be 0.707. Some commonly used Qtc values are listed below along with a description:

Qtc = 0.5 produces a “Critically Damped” closed-box response.
Qtc = 0.577 produces a “Bessel” or D2 closed-box response.
Qtc = 0.707 produces a “Butterworth” or B2 closed-box response.

The Qtc value is inversely proportional to the system damping. In other words, as the Qtc value is reduced, the system damping increases. Closed boxes with a lower Qtc value produce a “tighter” sound because they are damped more. You might conclude from this that a low Qtc (such as the critically damped 0.5 value) would often be preferred. However, this is not the case. To some listeners a low Qtc sounds a bit “thin” and they prefer a higher value—even as high as 1.0 or higher. Although Qtc values above 0.707 will produce a response bump at the system resonance frequency, the increased level produces a “warmer” sound which some prefer.

Best regards,

Sander Sassen
http://www.hardwareanalysis.com

[Ouroboros]

I'd certainly disagree that a Q of 0.707 (or even higher) is to be prefered. I agree with Martin Colloms that Qtc of 0.5 to 0.6 gives a much more realistic LF end, and is likely to be a much better interface with the bass enhancement in the listening room due to the 'room modes'.

High Q speakers aways remind me of a teenager showing off his car sound system; impressive but hardly qualifying to be called 'faithful to the original recording'!

[mike.e]

http://www.geocities.com/kreskovs/Box-Q.html