Speaker design question-DIY/techies please help

What do the abbreviations Q, Qtc and SQL mean in subwoofer design? I built my first DIY subwoofer using a software program to design the box and I'm happy with the results, but I don't know what some of the terms mean.

Also, what does the following mean:

Low Q Sealed 0.55 ft³ sealed (Qtc=0.6)
Mid Q Sealed 0.35 ft³ sealed (Qtc=0.7)
High Q Sealed o.25 ft³ sealed (Qtc=0.8)
Small SQL 0.5 ft³ tuned to 35 Hz, Ø2" x 11.5"L

In the above, I know that they are refering to the size of the box and to the port diameter and length in the last line. I just don't know what low Q, Mid Q, high Q, Qtc and small SQL mean.

Also, please recommend a good book on speaker design. My primary interest is subwoofer design.

Pr: If just starting out, David B Weem's books are about the best as they are a lot simpler than most. Once you get a handle on that, Vance Dickason's books are very educational and full of actual test results and technical data. If you shoot me your mailing address, i can drop at least one of these books in the mail to you. That is, so long as you send them back in good shape in a reasonable amount of time : )

Other than that, the lower the Qtc ( Q in a closed box ), the better the transient response, the earlier that the speaker will begin to roll-off and the more controlled the bass will be at resonance. Having said that, even though the bass begins to roll-off at a higher frequency with a lower Q, the bass is actually slightly more extended in many cases.

On top of that, the Q of the system actually rises as one applies more drive to the speaker. When you start off with a Qtc of .5 ( transiently perfect response ), you'll actually end up with a Q of appr .53 - .65 if playing music with a good amount of bass at normal listening levels. Exactly how much the Q climbs will depend on the drive levels used and how well the driver itself can dissipate heat.

Having said that, many people find a very low static Q ( .5 ) to sound TOO tight and dry, even with the slight boost of Q that occurs during actual driven conditions. Raising the static Q will produce more apparent bass but it does so at a slight loss in transient response. The more that the static Q climbs, the more apparent bass that you have and the worse the transient response gets.

Depending on the speakers that you're mating the sub with, the size of your listening room, the type of bass reproduction and amount of extension you expect, any of the lower Q's will work quite well. By "lower Q", i'm talking about figures between .5 and .7 or so. Personally, i shoot for Qtc's of appr .5 - .6 or so knowing that it will climb slightly under load. Most people that aren't used to "accurate" bass with proper pitch and damping ( long time users of vented speakers ) should probably use something closer to a static Qtc of .65 or .70 or so. Going higher ( .8 or more ) will provide a bigger peak at resonance with less control, which tends to excite room nodes and gives you more "oomp". Sealed speakers with a static Q above .8 are kind of neglecting the benefits of a sealed and stuffed design, but different designers and end-users may have specific sonic attributes in mind. I say this because the net effect of using a higher static Qtc is similar to using a vent i.e. less control ( reduced quality ) in order to gain more output ( greater quantity ).

As a side note, the amount and type of damping material used in a sealed box not only effects the Q of the system, it also alters the extension ( both low and high ) and the over-all "flatness" of amplitude output. Depending on the design goals and the crossover frequency and slope used, some materials will work better than others.

Much of this is covered in Vance's book to a great extent. Interpreting the trade-off's involved in each approach becomes a matter of personal design decisions. Personally, i would rather sacrice a few Hz of extension in order to achieve a higher level of linearity and control over the operating spectrum, but that's just me. Then again, those that are worried about seeing spec's like the lowest -3 dB point aren't going to be looking at sealed and stuffed designs. They are more willing to sacrifice quality in order to get the quantity of output that they desire at the lowest frequencies. The fact that room reinforcement actually helps the sealed designs produce a flatter in-room response seems to be lost on these types of thinkers. When taking such factors into consideration, one can actually design a sub for excellent transient response and still achieve excellent extension due to boundary reinforcement. After all, once the transient response aka "control" is lost, do you really want "more" bass? *

This is a VERY complex subject and nothing that can really be summed up in one post or even a very long thread. Given the differences in design goals, room characteristics and personal preferences, these types of threads typically turn into mass chaos. I'll just say that if you're interested in reading such threads and seeing "explosive" differences in points of view, there is plenty of good reading on the subject over in the AA Speaker Asylum archive. I've debated several different manufacturers of vented designs there and the results have always been "interesting" to say the least. To be quite honest, i've received TONS of email over those threads with many people telling me that they learned more about loudspeaker design from those posts than they have in 20 - 30 years of reading "glossy rags". Since my goal is to try and help audiophiles become more educated consumers and happier end-users, those emails were "music to my ears" : )

Other than that, i'll let someone that is a proponent of vented systems answer your questions about that spectrum of speaker design. I will only add that some vented designs are much better than others and even the best vented designs lack the transient response and control of a "good" sealed and stuffed design. Much of this has to do with impedances and power transfer characteristics, which was another subject recently discussed in a very "controversial" loudspeaker cable based thread. Sean

* If one is building a sub for HT use, quantity is typically preferred over quality for sheer impact in terms of the sonics supporting the visuals on the screen. There's nothing like watching an earthquake while the couch that you're sitting on feels like it is going to shake apart beneath you : )


Thanks, I was hoping that you would respond. Much appreciated.

I'll order the books before I go any further. It helps to know the theory behind the math. BTW-I prefer quality to quantity.

Thanks again,
Glad i could help and hope this cleared things up a bit for you. If i can be of further assistance, throw up another post or drop me an email. Sean