QB and sealed boxes

bvbellomo

Member
2013-03-08 2:31 pm
A long time ago, someone posted how to calculate the Q (Qb) and frequency (Fb) of a box and suggested a Q between .6 and .71 was a reasonably good alignment between box size and driver:
http://www.diyaudio.com/forums/multi-way/26035-determining-drivers-sealed-box.html

What if I design a speaker with a Qb below .6? Obviously, I lose low end extension, but is that the only problem? Most people are trying to find the optimal box size for a given driver - so I understand they would not be thinking this way. I am trying to minimize the size of the box, and I understand there are trade-off doing that. Given I have an undersized box for the frequencies I want (trade off) I can get a smaller Fb with an oversized driver, at the expense of a larger Qb. If this is a bad idea, why?

I also understand there are variables more complicated than the above with more practical designs - I am just trying to understand the theory at this point.
 

bvbellomo

Member
2013-03-08 2:31 pm
Excellent thread and answers most of my questions, but does not seem to come to a conclusion. I would summarize this thread as:

Subjectively, people likely prefer a Q between .5 and 1, with the possibility of preferring a Q well outside this range - and base is "tighter" at one end of the range, but we aren't sure which end, and we aren't sure what "tightness" is.

BTW - what the 2nd thread calls Qtc (total Q) the first thread calls Qb (Q of the box) versus Qtc (Total Q of the driver only).
 
The tightest, fastest, most accurate bass would be nearer Qtc 0.5. Some small speakers use a higher Qtc to give a boost in the bass and to make the speaker sound bigger than it actually is. The Rogers Ls3/5a had a Qtc of 1.2, IIRC the Heybrook HB1 (8" two way) had a Qtc of 1.2, Acoustic research AR18 (also 8" two way) had a Qtc of 1. Good quality hifi subwoofers usually have a Qtc nearer 0.5.

In the 'Loudspeaker-design-cookbook' by Vance Dickason (Well regarded book in the diy community) Vance and his listening partner evaluate different stuffing materials and conclude that a Qtc of 0.92 sounds best with his two-way studio monitor design (6.5" Scan speak 18w/8545K00 woofer).